Influence of Aircraft Power Electronics Processing on Backup VHF Radio

electronics

Article Inﬂuence of Aircraft Power Electronics Processing on Backup VHF Radio Systems

Jan Leuchter 1,* , Radim Bloudicek 1, Jan Boril 1, Josef Bajer 1 and Erik Blasch 2

1 Faculty of Military Technology, University of Defence, 66210 Brno, Czech Republic; [email protected] (R.B.); [email protected] (J.B.); [email protected] (J.B.) 2 MOVEJ Analytics, Dayton, OH 45433, USA; [email protected] * Correspondence: [email protected]

Abstract: The paper describes the influence of power electronics, energy processing, and emergency radio systems (ERS) immunity testing on onboard aircraft equipment and ground stations providing air traffic services. The implementation of next-generation power electronics introduces potential hazards for the safety and reliability of aircraft systems, especially the interferences from power electronics with high-power processing. The paper focuses on clearly identifying, experimentally ver- ifying, and quantifiably measuring the effects of power electronics processing using switching modes versus the electromagnetic compatibility (EMC) of emergency radio systems with electromagnetic interference (EMI). EMI can be very critical when switching power radios utilize backup receivers, which are used as aircraft backup systems or airport last-resort systems. The switching power electronics process produces interfering electromagnetic energy to create problems with onboard aircraft radios or instrument landing system (ILS) avionics services. Analyses demonstrate significant

threats and risks resulting from interferences between radio and power electronics in airborne systems. Results demonstrate the impact of interferences on intermediate-frequency processing, namely,

Citation: Leuchter, J.; Bloudicek, R.; for very high frequency (VHF) radios. The paper also describes the methodology of testing radio Boril, J.; Bajer, J.; Blasch, E. Inﬂuence immunity against both weak and strong signals in accordance with recent aviation standards and of Aircraft Power Electronics guidance for military radio communication systems in the VHF band. Processing on Backup VHF Radio Systems. Electronics 2021, 10, 777. Keywords: aircraft electronics; radio navigation; electromagnetic compatibility; electromagnetic https://doi.org/10.3390/electronics interference; radio interference; power electronics 10070777

Academic Editor: Mohd. Hasan Ali 1. Introduction Received: 1 March 2021 Safety-critical avionics situations arise when a radio signal is unavailable for pilots Accepted: 20 March 2021 Published: 25 March 2021 who need commands and information for their flight as well as for air traffic controllers (ATC) who provide air traffic services in their airspace. The circumstances can be more

Publisher’s Note: MDPI stays neutral serious during an approach to landing or in critical conditions when pilots and ATCs must with regard to jurisdictional claims in reliably communicate beyond standard phraseology. Such a radio signal degradation from published maps and institutional affil- avionics equipment, known as electromagnetic interference (EMI), can directly decrease safety iations. in air traffic transport, surveillance, and travel. Current avionics power systems implement power electronics devices for energy generation, conversion, and distribution over electrical networks. Power electronics switching circuits can improve the efficiency performance of modern airborne power systems. The number of switched power elements in airborne systems is expected to steeply rise in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the next decade, where every subsystem or device working as a part of an electrical or This article is an open access article electronic aircraft system already contains or will contain power electronics. An important distributed under the terms and recent power technology is switching converters based on pulse-width modulation (PWM) conditions of the Creative Commons switching. These PWM converters are one of the fastest developing technologies that are Attribution (CC BY) license (https:// used in the modern aircraft using electric drives. Power electronics circuits for electric creativecommons.org/licenses/by/ drives allow the control of the directional flow of both data and power between sources 4.0/). and consumers backward and forward in an entire aircraft power system. Two main factors

Electronics 2021, 10, 777. https://doi.org/10.3390/electronics10070777 https://www.mdpi.com/journal/electronics Electronics 2021, 10, x FOR PEER REVIEW 2 of 27

Electronics 2021, 10, 777 2 of 26 sources and consumers backward and forward in an entire aircraft power system. Two main factors participate in the power switching improvement: (1) recent high-voltage de- participatevices based in on the the power SiC (silicon switching carbide) improvement: technology (1) permit recent more high-voltage dynamical devices and reliable based oncontrol the SiC of the (silicon energy carbide) flow; technologyand (2) significantly permit more more dynamical accurate real and-time reliable monitoring control of of the the energyaircraft flow; system and power (2) significantly use [1,2]. more accurate real-time monitoring of the aircraft system powerThe use implementation [1,2]. of power electronics, including the power switching sources in real Theaeronautical implementation systems, ofneeds power to comply electronics, with includingcurrent aeronautical the power standards switching and sources their inimplement real aeronauticalation directives. systems, The needs probability to comply of occurrence with current of aeronauticalan electromagnetic standards compati- and theirbilityimplementation (EMC) hazard is directives. very low, but The the probability requirement of occurrencehighlights a of risk an assessment electromagnetic of in- compatibilityduced signal (EMC)susceptibility hazard issuch very as low, category but the Z requirementof 1800 volts highlights × meters a[3 risk]. The assessment RTCA DO of- induced160 standard signal contains susceptibility basic suchEMC asrequireme categorynts Z of and 1800 EMC volts testing× meters on aircraft [3]. The and RTCA general DO- 160test standard procedures. contains Hence, basic this EMC paper requirements brings to and attention EMC testing several on aircraft potential and EMC general threats test procedures.caused by a Hence,switching this power paper bringssupply to and attention their effects several on potential the reliability EMC threatsof, for example, caused by th ae switchingaeronautical power backup supply system and theirwith effectsvery high on the frequency reliability (VHF of, for) services example,, which the aeronautical are not cur- backuprently well system described with veryin aeronautical high frequency standards (VHF) [3] services,. which are not currently well described in aeronautical standards [3]. 1.1. Overview of Aeronautical Radio Backup Systems 1.1. Overview of Aeronautical Radio Backup Systems As backup power systems are inseparable components in many onboard and off- As backup power systems are inseparable components in many onboard and off-board board aircraft technologies, there is a need to understand the impacts on system perfor- aircraft technologies, there is a need to understand the impacts on system performance. mance. Some backup sources are similar to the main systems, but the onboard systems Some backup sources are similar to the main systems, but the onboard systems are subject are subject to placement and weight restrictions. to placement and weight restrictions. As for radio equipment, there is the main communication very high frequency As for radio equipment, there is the main communication very high frequency (COM- (COM-VHF) radio (see Figure 1), working in the bandwidth of 117–137 (144) MHz. The VHF) radio (see Figure1), working in the bandwidth of 117–137 (144) MHz. The com- communication signal is typically designed using software-defined technology, where ra- munication signal is typically designed using software-defined technology, where radio dio frequency (RF) blocks are efficiently shielded from interferences coming from other frequency (RF) blocks are efficiently shielded from interferences coming from other elec- electronic devices; however, such shielding can increase weight, size, and other power tronic devices; however, such shielding can increase weight, size, and other power supply supply requirements (filtering, signal processing, etc.), in comparison to electronic devices requirements (filtering, signal processing, etc.), in comparison to electronic devices working inworking environments in environments without anywithout interferences. any interferences. A backup A VHFbackup communication VHF communication receiver re- is typicallyceiver is typically built as a built simple as a or simple double-heterodyne or double-heterodyne receiver inreceiver a low-dimension in a low-dimension RF design. RF Indesign. addition, In addition, there are there emergency are emergency radio devices radio devices in military in military aviation aviation as a part as of a apart pilot’s of a survivalpilot’s survival kit. These kit. These radio radio receivers receivers are based are based on regenerative on regenerative reception, reception, having having simple sim- designsple designs and and compact compact dimensions. dimensions. Likewise, Likewise, the the radio radio receivers receivers must must survive survive a a pilot’s pilot’s ejection,ejection, carrycarry theirtheir ownown powerpower source,source, andand operateoperate inin veryvery extreme extreme conditions conditions [ 2[2].].

FigureFigure 1.1. VeryVery highhigh frequencyfrequency (VHF) (VHF) radio radio systems systems in in the the cockpit cockpit of of an an L-159 L-159 aircraft. aircraft.

InIn thisthis paper,paper, anan example example ofof electromagnetic electromagnetic interferenceinterference (EMI)(EMI) producedproduced byby powerpower electronicselectronics versusversus the VHF radio radio systems systems is is presented presented.. The The airborne airborne VHF/ VHF/ultra-highultra-high fre- frequencyquency (U (UHF)HF) main main backup backup COM COM radios radios LUN LUN-3520-3520 and and LUN LUN-3524-3524 were were tested. tested. Both Both ra- radiosdios are are based based on on the the same same technology. technology. In In addition, addition, two other devices were experimen-experimen- tallytally veriﬁedverified inin thethe instrumentinstrument landinglanding systemsystem (ILS)(ILS) receiverreceiver thatthat areare alsoalso basedbased onon thethe VHF/UHF frequency bandwidth: (1) VHF localizer from 108.10 MHz to 111.95 MHz, and (2) the UHF glideslope signals are in the 326–335 MHz range that provide the pilot with

Electronics 2021, 10, 777 3 of 26

course guidance to the runway centerline. In aviation, the localizer and glideslope beacons operate with two signals that are transmitted at 40 ILS channels with two modulation signals of 90 Hz and 150 Hz. The typical VHF radio systems are shown in Figure1. The receiver LUN3520 is built as a heterodyne receiver with an intermediate frequency of 21 MHz in the VHF bandwidth and a double-heterodyne receiver in the UHF bandwidth, with two intermediate frequencies (IF) of 56 MHz and 21 MHz. The reason why the double- heterodyne is utilized is the suppression of mirror frequencies. For example, the received signal frequency is let through by normal processing, but an amplitude modulation (AM) demodulator extracts the signal from the carrier wave and the low-frequency (LF) power amplifier [2,4]. In aeronautics, another example is air traffic controller (ATC) radio systems at the air traffic service unit (ATSU). There is the communication system providing aeronautical mobile services, consisting of VHF/UHF transmitters, receivers, backup radios, and last-resort emergency systems. The composition is given by standards and regulations published by an authorized aviation authority [4–6]. Generally, ATSU communication systems are spatially divided into transmitting, receiving, and last-resort modes. When in receiving mode, the radio is not placed in technical rooms at the ATSU but rather stands alone so as not to be disturbed by other ATSU systems and consequently decrease the sensitivity of the radios. With the isolation conditions, the receivers can reach the sensitivity of −93 dBm (measured in accordance with [5,6]). In the ATSU communication system, there are two levels of backup. The first is the “N + 1” backup, where the transmitter and receiver have added blocks, which are the same as the main radios. The second level of backup is the last-resort system (LRS) (i.e., direct mode operations or repeater talk-around), which is completely independent from the main power supply, radio devices, and radio management. The LRS system is used when all main ATSU systems, including the power supply, collapse. Although LRS radios are typically based on the same software-defined radio (SDR) technology as the main radio system, LRS radios have common receiving and transmitting components and all the radios are placed in the technical room with other ATC technologies—servers, power electronic devices, universal power supplies (UPSs), pulse sources, etc. Hence, such LRS radios can operate in environments with high levels of EMI [3,5–7].

1.2. Overview of Airborne Power Electronics and Power Processing Aircraft power electronics uses power switching technologies to control the flow of power. Power electronics represents one of the fastest developing technologies towards the integration of electric drives in aircraft systems. They have enormous potential for energy conversion applications to reduce the power density of the electric equipment on board modern aircraft. In addition, high-speed electric drive research may significantly contribute to increasing the power density of electromechanical systems [8,9]. Power electronics on board aircraft uses switching technologies with power devices based on the modern metal–oxide–semiconductor field-effect transistor (MOSFETs) and insulated-gate bipolar transistor (IGBTs) devices based on the silicon carbide (SiC) or gallium nitride (GaN) technology to increase the frequency of power switching. High-speed switching requirements are closely connected to the power density of the power circuits and power losses. The power electronic directly controls the dynamic behavior of the drive, employing typical vector control or direct torque control. However, there is usually no active control of the switching behavior of the power electronic elements implemented in the system, such as the adaptive dead-time controlled by the switched current/voltage or as per EMC requirements. As the power outputs and maximal speeds of electrical drives increase, controlling these parameters is gaining in importance. Moreover, much higher switching frequencies have been made possible by introducing novel power electronic elements based on SiC organ technologies, which makes the problem even more difficult [7,8]. For example, it is well known that the motor electrical losses increase with a higher content of current harmonics. If a typical three-phase alternating current (AC) converter Electronics 2021, 10, x FOR PEER REVIEW 4 of 27

Electronics 2021, 10, 777 4 of 26 difficult [7,8]. For example, it is well known that the motor electrical losses increase with a higher content of current harmonics. If a typical three-phase alternating current (AC) converter is used instead of the sinusoidal voltage supply, the total motor losses may in- creaseis used by instead 5% to of10%. the Hence, sinusoidal the effects voltage of supply,EMC compliance the total motor are related losses to may both increase losses and by power5% to 10%.quality Hence, issues. the The effects power of quali EMCty compliance depends on are the relatedcontent to ofboth harmonic losses components and power inquality the voltage issues. produced The power by quality PWM depends AC converters on the contentemploying of harmonic various modulation components strate- in the gies,voltage power produced electronics by PWM technologies, AC converters and employinglayout topologies. various modulationPower electronics strategies, switching power produceselectronics EMI, technologies, which becomes and layout a major topologies. problem Power and makes electronics EMC switching analysis producesin various EMI, op- eratingwhich becomesmodes essential a major [1,2,8,9] problem. and makes EMC analysis in various operating modes essentialAn electrical [1,2,8,9]. power system is shown in Figure 2, which is a simplified representation of theAn aircraft electrical electrical power power system system. is shown The in primary Figure2 AC, which system is a uses simpliﬁed integrated representation drive gen- eratoof thers aircraft(IDG) from electrical the aircraft power engine system. and The gearbox primary to ACget systemthe high uses speeds integrated required drive for aircraftgenerators use. (IDG) Modern from AC the generators aircraft engine produce and gearboxpower up to to get 100 the kVA high based speeds on required the three for- phaseaircraft concepts use. Modern to reduce AC generators the power producedensity of power AC generators. up to 100 kVA The based output on of the the three-phase generator musconceptst be constant to reduce at the 115 power VAC density 400 Hz, of but AC p generators.ermanent m Theagnet output generators of the generator have become must prominentbe constant because at 115 VAC the 400maximum Hz, but permanentefficiency is magnet up to generators90%, but the have output become (voltage, prominent frequency)because theis variable maximum and efﬁciency must be is stabilized up to 90%, by but power the output electronics (voltage, AC frequency)-to-AC conversions is variable [and8]. must be stabilized by power electronics AC-to-AC conversions [8].

FiguFigurere 2. Power processing of onboard a aircraftircraft electronics.

Bus power control units may also be used as power converters to set a constant bus voltage of 115 VAC (volts(volts alternatingalternating current) at 400400 Hz.Hz. DC bus systems frequently use the method of power conversio conversionn by use of transformertransformer rectiﬁerrectifier unitsunits (TRUs).(TRUs). TRUs use a transformer to to provide the desired 115 VAC toto 2828 VDCVDC conversions.conversions. TheseThese transformerstransformers are very heavy and and bulky, bulky, and therefore high high-frequency-frequency switching transformers (HFT) are used to improve the powe powerr density. These HFTs require power electronics to increase the switching frequency up to 20 kHz. In addition, power electronics is necessary for AC or DC electrical drives drives,, as can be seen in Figure2 2.. ForFor high-powerhigh-power switching,switching, frequenciesfrequencies ofof ~3~3 k kHzHz are used.used. It is evident that the main power processing switching is around 3 kHz power electronic switching [[77–9]. Power electronics electronics improve improvess the the performance performance of ofmodern modern airborne airborne power power systems. systems. Im- plementationImplementation of ofpower power electronics electronics including including a a“s “smartmart grid“ grid“ o orr Intern Internetet of of Things Things (IoT) technology in in a a real real airborne system system needs needs to to comply comply with with current current regulatory regulatory standards. standards. Power electronics standards for onboard aircraft systems address safety and reliability hazards from interferences induced by power electronics with a high voltage [8,9]. Current regulations are being carefully considered, and the critical aspects of the implementation have to be clearly identiﬁed and analyzed. An example of EMI is presented in Figure3. The power electronic—DC-to-AC converter with a transformer, which converts a DC

Electronics 2021, 10, x FOR PEER REVIEW 5 of 27

Electronics 2021, 10, 777 hazards from interferences induced by power electronics with a high voltage5 of 26 [8,9]. Cur- rent regulations are being carefully considered, and the critical aspects of the implementation have to be clearly identified and analyzed. An example of EMI is presented in Fig- ure 3. The power electronic—DC-to-AC converter with a transformer, which converts a voltage of 28 VDCDC voltage to 115 VACof 28 atVDC 400 to Hz—generates 115 VAC at 400 unwantedHz—generates harmonics unwanted at multiples harmonics of at multiples the frequency ofof thethe switchingfrequency signal.of the switching These unwanted signal. These signals unwanted then propagate signals then through propagate the through conductive connectionsthe conductive (Figure connections3a) or are radiated(Figure 3a) (Figure or are3 b)radiated by the (Figure conductors. 3b) by Figure the conductors.3 Fig- presents a clearure example 3 presents of interferences a clear example caused of interferences by power electronics, caused by which power can electronics, seriously which can compromise theseriously functionality compromise and reliability the functionality of the avionicand reliability devices. of the Figure avionic3 also devices. shows Figure 3 also the wideband charactershows the of wideband interfering character power of signals, interferi whichng power can be signals, up to MHzwhich frequencies can be up to MHz fre- and the interferencesquencies can and decrease the interfere the reliabilitynces can decrease of onboard the reliability aircraft systems, of onboard as it aircraft will be systems, as shown in the restit will of thisbe shown paper. in the rest of this paper.

fs = 20 kHz

(a) (b)

FigureFigure 3. Power 3. Power electronics electronics electromagnetic electromagnetic interference interference (EMI): (EMI) (: a(a)) conductive; conductive; ((bb)) radiative.radiative.

Every individualEvery design individual of the powerdesign electronicof the power system electronic requires system a unique requires analysis a unique of analysis of the EMI amongthe power EMI among supplies, power distribution supplies, ofdistribution networks, of and networks, intended and consumers—i.e., intended consumers—i.e., avionics and radioavionics systems. and The radio EMI systems. analysis The has EMI to deﬁne analysis critical has to frequencies define critical or frequency frequencies or fre- bands and to setquency safe limits bands of and the to interference set safe limits levels of the at interference those sensitive levels frequencies at those sensitive for such frequencies susceptible devicesfor such as RFsusceptible systems. devices Practical as RF experiences systems. Practical with a modernexperiences avionic with systema modern avionic emphasize thesystem importance emphasize of monitoring the importance EMC problemsof monitoring [1,3, 9EMC]. problems [1,3,9]. It is evident thatIt is multiple evident powerthat multiple electronic power converters electronic are converters necessary are in necessary future aircraft in future aircraft electronics. Theelectronics. trend has The two trend important has two downsides: important downsides: signiﬁcantly significantly lowering lowering the quality the quality of of delivered electricdelivered energy electric and energy increasing and increasing unwanted unwanted electromagnetic electromagnetic radiation. radiation. The The proba- probability of occurrencebility of occurrence of an EMC of an hazardEMC hazard is very is very low. low. However, However, this this paper paper brings brings to to attention attention severalseveral potential potential threats threats caused caused by a switchingby a switching power power supply supply such assuch signal as signal noise noise and and frequencyfrequency disruption. disruption. The signal The disruptionsignal disruption has the has potential the potential for onboardfor onboard aircraft aircraft systems systems safetysafety and reliability and reliability hazards, hazards especially, especially interferences interferences with thewith power the power supply. supply The . The next next section providessection theprovides critical the aspects critical of aspects the power of the electronics power electronics of aircraft of aircraft systems. systems.

2. Analysis of Interference2. Analysis of Sources Interference on Airborne Sources on VHF Airborne Systems VHF Systems Current and futureCurrent airborne and future power airborne systems power are systems and will are and be basedwill be onbased switching on switching com- components. Theponents power. The electronics power electronics on board on aircraft board isaircraft currently is currently based onbased Si technology, on Si technology, and and the near futurethe near will future bring will SiC bring technology SiC technology into onboard into onboard power electronics. power electronics The basic. The basic difference between these technologies is not only the difference in switching frequencies but difference between these technologies is not only the difference in switching frequencies but also in the possible range of interferences with the airborne radio systems. As Si technol- also in the possible range of interferences with the airborne radio systems. As Si technology ogy mostly influences VHF and the lower part of UHF (ultra-high frequency) radio sys- mostly influences VHF and the lower part of UHF (ultra-high frequency) radio systems, tems, SiC technology will influence UHF navigation and surveillance systems in aviation SiC technology will influence UHF navigation and surveillance systems in aviation [7,10]. [7,10]. A solution to mitigate the interference is to efficiently suppress the mirror frequencies, for signal reception, but there are also parasitic reception channels. The main signal reception is on intermediate frequencies (IF), their multiples, and partial harmonics plus the sum and difference signals of the input frequencies, as all the amplifiers have non- linearity [10]. Reception on such IF frequencies can efficiently suppress not only the strict linear characteristics of used amplifiers but also EMI shielding measurements. How- ever, such measurements require more device space as well as an increase in the device Electronics 2021, 10, x FOR PEER REVIEW 6 of 27

A solution to mitigate the interference is to efficiently suppress the mirror frequencies, for signal reception, but there are also parasitic reception channels. The main signal reception is on intermediate frequencies (IF), their multiples, and partial harmonics plus the sum and difference signals of the input frequencies, as all the amplifiers have non- Electronics 2021, 10, 777 linearity [10]. Reception on such IF frequencies can efficiently suppress not only the strict6 of 26 linear characteristics of used amplifiers but also EMI shielding measurements. However, such measurements require more device space as well as an increase in the device weight. Although shielding materials can bring a high level of suppression of unwanted signals— 41weight. dB or more Although [11,12] shielding—the shielding materials spacing can bring in a adevice high levelconsequently of suppression increases of unwanted the de- signals—41 dB or more [11,12]—the shielding spacing in a device consequently increases vice’s dimensions. In Figure 4, there is a comparison between two technologically identi- the device’s dimensions. In Figure4, there is a comparison between two technologically cal devices. The first one is the solution for the main airborne radio (Figure 4a), where the identical devices. The ﬁrst one is the solution for the main airborne radio (Figure4a), where RF part is completely shielded and also the device is more robust, and the second is an the RF part is completely shielded and also the device is more robust, and the second is an airborne backup radio (Figure 4b), where the shielding is simpler and the complex solu- airborne backup radio (Figure4b), where the shielding is simpler and the complex solution tion is thinner. Despite the different mechanical dimensions, the main electrical parame- is thinner. Despite the different mechanical dimensions, the main electrical parameters, ters, except EM immunity, are fully comparable. except EM immunity, are fully comparable.

(a) (b)

FigureFigure 4. 4.ShieldingShielding of ofradio radio frequency frequency (RF (RF)) blocks blocks in inLUN LUN-3520-3520 (a) ( aand) and LUN LUN-3524-3524 (b ()b radios.) radios.

2.1. Possible Sources of Electromagnetic Interferences 2.1. Possible Sources of Electromagnetic Interferences As receivers of airborne radios can operate in very harsh conditions—temperature ranges Asfrom receivers −50 C to of 50 airborne C—there radios are canwide operate pressure in veryranges harsh corresponding conditions—temperature to pressures − ◦ ◦ at rangesthe sea fromlevel to50 pressuresC to 50 atC—there heights of are more wide than pressure ten kilometers ranges corresponding (e.g., flight level to— pressuresFL430 orat 43,000 the sea ft) level[13,14], to from pressures which at receivers heights ofmust more also than operate ten kilometers in environments (e.g., flight with level—high FL430 or 43,000 ft) [13,14], from which receivers must also operate in environments with levels of interferences. From the EMI point of view, receiver electronics of airborne radios high levels of interferences. From the EMI point of view, receiver electronics of airborne are very close to possible electromagnetic disturbance, where the possible sources of such radios are very close to possible electromagnetic disturbance, where the possible sources disturbance can be the power electronics in aircraft systems working with high levels of of such disturbance can be the power electronics in aircraft systems working with high power transmission. Although the voltage level in aircraft systems does not exceed the levels of power transmission. Although the voltage level in aircraft systems does not typical voltage of airborne systems, 115 V/400 Hz—generators, converters, etc.—typical exceed the typical voltage of airborne systems, 115 V/400 Hz—generators, converters, etc.— currents can easily reach the values of hundreds of amperes. Secondly, nowadays, typical currents can easily reach the values of hundreds of amperes. Secondly, nowadays, onboard electronics work with 28 volts, containing power electronics converters and sim- onboard electronics work with 28 volts, containing power electronics converters and similar ilar circuits using pulses, which consequently generate wide-spectrum signals. Both these circuits using pulses, which consequently generate wide-spectrum signals. Both these sources and the spatial neighborhood of the aircraft systems can cause two basic types of sources and the spatial neighborhood of the aircraft systems can cause two basic types EMI in the VHF bandwidth. The first one is the increased level of EMI sensor noise, where of EMI in the VHF bandwidth. The first one is the increased level of EMI sensor noise, suchwhere noise such can noise cause can the cause perceptibl the perceptiblee degradation degradation of sensitivity of sensitivity or squelch or squelch activation. activation. As forAs navigation for navigation receivers, receivers, where v whereery high very freque highncy frequency omni-directional omni-directional range (VOR), range instru- (VOR), ment landing system-localizer (ILS/LOC), and marker (MKR) beacon receivers work in instrument landing system-localizer ( ILS/LOC), and marker (MKR) beacon receivers the VHF bandwidth, such interference can cause an increased error in navigation meas- work in the VHF bandwidth, such interference can cause an increased error in navigation urement or unintended signalization (MKR optical signalization). The second way of dis- measurement or unintended signalization (MKR optical signalization). The second way of ruption is EMI on a specific frequency. An airborne radio has many unwanted receiving disruption is EMI on a specific frequency. An airborne radio has many unwanted receiving effects resulting from imperfect shielding, non-linear characteristics of amplifiers, use of IF frequencies, and solutions for squelch circuits. An example shape of a disturbing signal fully corresponds to the standards [3,12]. A complex overview of the disturbing signal affecting the VHF backup communication or navigation system is shown in Figure5. The figure shows the interference with an 822 Hz disturbing signal. Electronics 2021, 10, x FOR PEER REVIEW 7 of 27

effects resulting from imperfect shielding, non-linear characteristics of amplifiers, use of IF frequencies, and solutions for squelch circuits. An example shape of a disturbing signal fully corresponds to the standards [3,12]. A complex overview of the disturbing signal affecting the VHF backup communication or Electronics 2021, 10, 777 navigation system is shown in Figure 5. The figure shows the interference with an 8227of Hz 26 disturbing signal.

FigureFigure 5. 5. CharacteristicsCharacteristics of of burst burst signal signal produced produced by by power power electronics. electronics. Figure5 shows the example of an EMI with the burst signal, disturbing an airborne Figure 5 shows the example of an EMI with the burst signal, disturbing an airborne backup radio. The burst signal generator simulates airborne power electronics systems on backup radio. The burst signal generator simulates airborne power electronics systems on board the aircraft. Real onboard power electronics contains power generators and various board the aircraft. Real onboard power electronics contains power generators and various converters: AC-to-AC, AC-to-DC, and motor drivers. Most of them are based on pulse converters: AC-to-AC, AC-to-DC, and motor drivers. Most of them are based on pulse switching, currently mostly using Si technology, and expanding to SiC technology, where switching, currently mostly using Si technology, and expanding to SiC technology, where the pulse width in SiC systems can be up to four times shorter in comparison to traditional the pulse width in SiC systems can be up to four times shorter in comparison to traditional Si systems [7,9]. Si systems [7,9]. 2.1.1. Interferences Caused by the Increased Level of Noise 2.1.1. Interferences Caused by the Increased Level of Noise As true absolute zero (0 K) should be impossible due to quantum ﬂuctuations, every As true absolute zero (0 K) should be impossible due to quantum fluctuations, every electronic circuit generates thermic noise whose power, Pnoise, is given by the basic equation electronic circuit generates thermic noise whose power, Pnoise, is given by the basic equation Pnoise = kTB, (1)

where k is the Boltzmann constant, T is푃 the푛표푖푠푒 temperature= 푘푇퐵, in Kelvins, and B is the operation(1) wbandwidth.here k is the Normally, Boltzmann the constant, level of thermicT is the temperature noise limits thein Kelvins, receiver and sensitivity B is the whereoperation the bandwidth.sensitivity parameterNormally, isthe derived level of from thermic three noise times limits the signal-to-noisethe receiver sensitivity ratio (SNR) where value the sensitivity[2,4,5]. In theparameter case of is EMI, derived the interference from three times noise powerthe signal is added-to-noise to theratio thermal (SNR) value noise; [2,4,5]however,. In thetheinterference case of EMI, signal the mustinterference have similar noise characteristics power is added to white to the noise—a thermal random noise; howesignalver, with the a uniforminterference power signal spectral must density. have similar The burst characteristics signal can have to white such characteristicsnoise—a ran- domin a speciﬁcsignal with narrow a uniform bandwidth, power such spectral as 25 kHz density. or 8.33 The kHz burst channel signal spacing can have corresponding such char- acterto suchistics characteristics. in a specific narrow In these bandwidth, conditions, such the interferenceas 25 kHz or burst 8.33 signalkHz channel corresponds spacing to cwhiteorresponding noise as itsto pulsesuch characteristics. parameters have In a these rise time conditions, of less than the 2interference ns and a tail burst time ofsignal less correspondsthan 10 ns. Theseto white values noise correspond as its pulse to parameters VHF frequencies have a rise which time are of usedless than in aeronautics 2 ns and a tailfor time communication of less than and10 ns. navigation These values services: correspond radio frequencies to VHF frequencies of 108–137 wh MHzich are aircraft used insystems, aeronautics where for the communication frequencies 108–112 and navigation MHz are services: used for r navigationadio frequencies system of VOR 108– and137 for the localizer (LOC) part of ILS systems. Typical bandwidths are 50 kHz for navigation MHz aircraft systems, where the frequencies 108–112 MHz are used for navigation system system VOR and ILS/LOC, and 8.33 kHz for radio communication systems [4,5,10]. The VOR and for the localizer (LOC) part of ILS systems. Typical bandwidths are 50 kHz for airborne power electronics inﬂuences these frequencies with the increasing white noise navigation system VOR and ILS/LOC, and 8.33 kHz for radio communication systems directly intruding the RF parts of the receivers. Total noise P in the RF parts consists of [4,5,10]. The airborne power electronics influences these frequenciesnoise with the increasing thermal noise Pnoise and generated noise PG. white noise directly intruding the RF parts of the receivers. Total noise Pnoise in the RF parts

consists of thermal noise Pnoise and generated noise PG. PT = kTB + PG (2)

As the total level of noise steeply increases with the power electronics effect, the sensitivity of radios decreases as the SNR decreases. An example is shown in Figure6, where the typical amplitude-modulated double-sideband ( AM-DSB) signal in the normal noise environment is used in aeronautical communication services. The comparison between Figures6b and7b presents the inﬂuence of interference with the change in the SNR parameter and, consequently, the decrease in reception quality of an airborne radio. Electronics 2021, 10, x FOR PEER REVIEW 8 of 27

푃푇 = 푘푇퐵 + 푃퐺 (2) As the total level of noise steeply increases with the power electronics effect, the sensitivity of radios decreases as the SNR decreases. An example is shown in Figure 6, where the typical amplitude-modulated double-sideband (AM-DSB) signal in the normal noise Electronics 2021, 10, x FOR PEER REVIEW 8 of 27 environment is used in aeronautical communication services. The comparison between Figure 6b and Figure 7b presents the influence of interference with the change in the SNR parameter and, consequently, the decrease in reception quality of an airborne radio.

푃푇 = 푘푇퐵 + 푃퐺 (2) As the total level of noise steeply increases with the power electronics effect, the sensitivity of radios decreases as the SNR decreases. An example is shown in Figure 6, where the typical amplitude-modulated double-sideband (AM-DSB) signal in the normal noise environment is used in aeronautical communication services. The comparison between Electronics 2021, 10, 777 Figure 6b and Figure 7b presents the influence of interference with the change in the8 SNR of 26 parameter and, consequently, the decrease in reception quality of an airborne radio.

(a) (b) Figure 6. Test signal in the VHF bandwidth for aeronautical mobile services, (a) signal in the RF spectrum and the low- frequency (LF) output signal (b), in time domain and its spectrum.

well as the squelch opening. By the disturbing signal, the level of noise steeply increases (a) (b) as it is shown in Figure 7. Figure 7 shows a clear effect of interference of power electronics Figure 6. TestTest signal signal in in the the VHFcircuits VHF bandwidth bandwidth at the frequency for for aeronautical aeronautical of 822 mobile Hz, mobile which services, services, intrudes (a) (signala) signalinto in radiothe in RF the circuits spectrum RF spectrum at andthe thefrequenc and low the- y of low-frequencyfrequency (LF) (LF)output output signal signalthe (b )t, unedin (b ),time in radio, time domain domain e.g. and, 118 andits MHz spectrum. its spectrum. with 1 kHz AM-DSB modulation [5].

Figure 6 shows that the 118 MHz carrier and both side bands carrying voice information are represented by a 2 kHz audio signal. The total signal has a level corresponding to the normal sensitivity of airborne VHF radios, where the reception is guaranteed as well as the squelch opening. By the disturbing signal, the level of noise steeply increases as it is shown in Figure 7. Figure 7 shows a clear effect of interference of power electronics circuits at the frequency of 822 Hz, which intrudes into radio circuits at the frequency of the tuned radio, e.g., 118 MHz with 1 kHz AM-DSB modulation [5].

(a) (b)

FigureFigure 7.7. VHFVHF receiverreceiver circuitscircuits inin thethe disturbingdisturbing environment,environment, signalsignal inin thethe RFRF spectrumspectrum ((aa)) andand LFLF outputoutput signal,signal, inin timetime domaindomain andand itsits spectrumspectrum ((bb).).

FigureThe disturbing6 shows that signa thel fully 118 MHz intrudes carrier through and both the sideshielding bands of carrying the RF parts voice of informa- the air- tionborne are radio, represented which bycreates a 2 kHz the audiodifferen signal.t noise The level total from signal the has normal a level thermal corresponding noise. The to thedifferent normal noise sensitivity level degrades of airborne the VHF normal radios, radio where sensitivity the reception more isthan guaranteed three times. as well With as

the squelch opening. By the disturbing signal, the level of noise steeply increases as it is (a) (b) shown in Figure7. Figure7 shows a clear effect of interference of power electronics circuits Figure 7. VHF receiver circuitsat the in the frequency disturbing of 822environment, Hz, which signal intrudes in theinto RF spectrum radio circuits (a) and at LF the output frequency signal, of in the time tuned

domain and its spectrum (bradio,). e.g., 118 MHz with 1 kHz AM-DSB modulation [5]. The disturbing signal fully intrudes through the shielding of the RF parts of the airborneThe radio,disturbing which signa createsl fully the intrudes different through noise level the fromshielding the normal of the RF thermal parts noise.of the Theair- differentborne radio, noise which level degradescreates the the differen normalt radionoise sensitivitylevel from more the normal than three thermal times. noise. With this,The squelchdifferent circuits noise level of the degrades radio can the be normal in a completely radio sensitivity random state—ON/OFFmore than three ortimes. balancing With between them. In addition, the disruption can not only inﬂuence the communication radio station but it can also completely degrade the navigation measurement as the localizer of the ILS system operates in the VHF bandwidth and works on the amplitude methods [5,10,15].

2.1.2. Interferences Caused by the Direct Disruption Disturbing signals produced by the airborne power electronics can also inﬂuence other RF components of radio systems. Such disturbing signals can be directly induced into the IF or LF parts of radio systems where the level of interference depends on the magnetic protection of the onboard radio systems [16]. Electronics 2021, 10, x FOR PEER REVIEW 9 of 27

this, squelch circuits of the radio can be in a completely random state—ON/OFF or balancing between them. In addition, the disruption can not only influence the communication radio station but it can also completely degrade the navigation measurement as the localizer of the ILS system operates in the VHF bandwidth and works on the amplitude methods [5,10,15].

2.1.2. Interferences Caused by the Direct Disruption

Electronics 2021, 10, 777 Disturbing signals produced by the airborne power electronics can also influence9 of 26 other RF components of radio systems. Such disturbing signals can be directly induced into the IF or LF parts of radio systems where the level of interference depends on the magnetic protection of the onboard radio systems [16]. The burst generator simulates a typical source of EMI. The disturbing signal also simulates aa typicaltypical transienttransient behavior behavior of of typical typical switching switching components components based based on on Si technology,Si technol- asogy, shown as shown in Figure in Figure8. 8.

Figure 8. DetailDetailss of of simulated simulated switching switching process processinging of SiC of SiCpower power component component used usedin aircraft in aircraft powerpower electronics.electronics.

ThereThere isis thethe simulationsimulation ofof thethe typicaltypical behaviorbehavior ofof thethe SiSi switchingswitching component.component. WhenWhen thethe componentcomponent is switchedswitched on,on, thethe transienttransient capacitycapacity mostly influencesinfluences the time characteristics,teristics, andand withwith switchingswitching off,off, typicaltypical oscillationsoscillations occur.occur. InIn thethe exampleexample inin FigureFigure8 8,, the the oscillations havehave thethe frequency frequency of of 2.3 2.3 kHz, kHz influencing, influencing audio audio circuits circuits of theof the communication communica- station.tion station. Figure Figure8 shows 8 shows the influence the influence of the of switching the switching quality quality of power of power electronics electronics that causesthat cause thes interferencesthe interferences in the in the radio radio spectrum. spectrum. Primarily, Primarily the, the influence influence of of inductances,inductances, capacitancescapacitances that circuits of power electronics normally have, and thus resonant charac- teristicsteristics isis typicallytypically hiddenhidden inin thethe switchingswitching processprocess duringduring fastfast switchingswitching andand cannotcannot bebe easily suppressed.suppressed. Such resonant behaviorbehavior isis shownshown inin FigureFigure8 8.. InIn thethe FigureFigure8 8 example, example, the the simulated simulated power power electronics electronics disturbing disturbing signals signals contain contain frequenciesfrequencies which which directly directly correspond correspond to to audio audio frequencies frequencies of ofthe the airborne airborne radio. radio. In Fig- In Figuresure 5 and5 and Figure8, shown 8, shown are several are several signals signals which which can directly can directly intrude intrude the LF the components LF compo- ofnents the of radio the systemradio system by direct by induction.direct induction. The first The one first is one the 822is the Hz 822 signal Hz signal representing represent- the repetitioning the repetition frequency frequency of the disturbing of the disturbing pulses, and pulses, the secondand the signal second comes signal from comes the from tran- sientthe transient effect of effect the switching of the switching components components where the where 2.3 kHz the signal 2.3 kHz also si directlygnal also influences directly the LF components of the airborne radio by magnetic induction and causes a disruption in influences the LF components of the airborne radio by magnetic induction and causes a the audio signal as well as influencing the squelch circuits which can cause a random state. disruption in the audio signal as well as influencing the squelch circuits which can cause 2.2.a random Influence state. of Onboard Power Electronics on the Critical Navigation Systems As some of the aeronautical navigation systems, mainly the marker beacon system and 2.2. Influence of Onboard Power Electronics on the Critical Navigation Systems especially the localizer (LOC) channel of the instrument landing system (ILS), operate in the VHFAs some bandwidth of the andaeronautical use the same navigation reception systems, technology mainly as VHF the m communicationarker beacon system radios, aand similar especially disturbing the localizer effect acts(LOC) on channel the LOC of and the i MKRnstrument receivers. landing Both system of them (ILS), are operate based onin the AM VHF navigation bandwidth methods; and use however, the same they reception do not gain technology the navigation as VHF quantities communication in the sameradios way, a similar and these disturbing quantities effect have acts different on the characters.LOC and MKR In addition, receivers. a power Both of electronics them are disruptionbased on AM may navigation have a different methods; impact however, on the they measurement do not gain errors the navigation as well as onquantities the result in and, consequently, the safety of air traffic as the ILS whole system (LOC and MKRs) is primarily used for approach to landing and landing [5,10,17,18]. The ILS system is the standard navigation system belonging to the group of non- autonomous onboard aircraft equipment. The ILS system operates with one-way transmission (from the ground part to the airborne part), on VHF frequencies (localizer—LOC, markers) and UHF frequencies (glide path—GP). All parts of the ILS system use typical amplitude modulation in AM-DSB mode, where LOC and GP modulation signals have frequencies of 90 Hz and 150 Hz for navigation measurement and 1020 Hz for identification. Markers use modulation frequencies of 400 Hz, 1300 Hz, and 3000 Hz for the outer (OM), middle (MM), and inner (IM) marker differentiations. Electronics 2021, 10, 777 10 of 26

As for the RF signal level, there is the strict determination in [5,17,19] and there are three levels of intensities of electrical components of the electromagnetic field: • In all parts of ILS coverage, the intensity of the electrical field must not be lower than 40 mV.m−1 (equivalent to power density of −114 dBWm−2); • For ILS in CAT I operation, the intensity of the electrical field must not be lower than 90 mV.m−1, on the approach path at distances from 18.5 km to the distance corresponding to the decision height; • For ILS in CAT II/III operation, the intensity of the electrical field must not be lower than 100 mV.m−1 in the course sector, on the glide path, for the distance of 18.5 km increasing to 200 mV.m−1 at the height of 15 m above the runway level. It is necessary to analyze the ILS parameters for the determination of errors, which are caused by onboard power electronics. Difference in depth of modulation (DDM) is the navigation quantity for course analysis as well as the glide slope channel, and both depend on antenna patterns, beam power output, and the depths of amplitude modulations [5,10,17]. The basic equation showing all elements is

m1E1nFL(ϑ) − m2E2nFP(ϑ) DDM = (3) E1nFL(ϑ) + E2nFP(ϑ)

where m1, m2 are the depths of amplitude modulation; E1n, E2n present the intensities of the electrical ﬁelds from both beams that correspond to the transmitting power of the LOC (GP) beacon; and FP(ϑ), FL(ϑ) are LOC and GP antenna patterns, respectively. The equation shows the situation where DDM is given only in the equisignal zone, which is given by two antenna patterns transmitting electromagnetic energy with amplitude modulation. Such solutions of LOC (GP) beacons suffer from many additional problems where beacon transmitters must secure a high level of equality in the depth of amplitude modulation as well as the transmitting power of each beam forming the equisignal zone of the localizer (glide path) beacon. Only the case where the DDM is used as a navigation quantity, which depends only on the measured angle, results in little error. In other cases, the DDM has an additional error caused by non-equalities in the transmitter electronics. A typical LOC (GP) beacon works on the principle of reference zero, where the maximum and minimum directing principles are combined to obtain the equisignal zone as shown in Equation (3). Although an onboard receiver compares amplitudes of navigation signals in all cases, reference zero beacons secure the equality of needed signals by their own principle of work, as all signals are generated in one device and, afterwards, are divided in bridge-type circuits. Considering the reference zero operating principle of the LOC (GP) beacon, the DDM parameter is determined as

mE Fb(ϑ) DDM = 2n × (4) E1n FC(ϑ)

where m is the depth of amplitude modulation; E1n, E2n are amplitudes of emitted signals —maximum and minimum directing; and Fb(ϑ), Fc(ϑ) are antenna patterns. Despite the different principles of the working ground beacons, the onboard ILS receiver does not recognize ground parameters. As for the technical solution of the ILS receiver, there are two basic types. The ﬁrst is a wideband SDR receiver, which receives most of the aeronautical communication, navigation, and signals, as well as surveillance services and anti-collision systems. The second is the traditional VHF/UHF receiver, based on the heterodyne principle. The ILS/LOC is typically combined with the VOR receiver as they use the same frequency bandwidth. The processing in an ILS receiver is the same for LOC and GP channels. Both the LOC and GP compute the DDM parameter in the same way, but there is only the one difference—the used basic depth in modulation. A typical block diagram of an ILS receiver is shown in Figure9[5,17]. Electronics 2021, 10, x FOR PEER REVIEW 11 of 27

where m is the depth of amplitude modulation; E1n, E2n are amplitudes of emitted signals— maximum and minimum directing; and Fb(), Fc() are antenna patterns. Despite the different principles of the working ground beacons, the onboard ILS receiver does not recognize ground parameters. As for the technical solution of the ILS receiver, there are two basic types. The first is a wideband SDR receiver, which receives most of the aeronautical communication, navigation, and signals, as well as surveillance services and anti-collision systems. The second is the traditional VHF/UHF receiver, based on the heterodyne principle. The ILS/LOC is typically combined with the VOR receiver as they use the same frequency bandwidth. The processing in an ILS receiver is the same for LOC and GP channels. Both the LOC and GP compute the DDM parameter in the same way, but there is only the one Electronics 2021, 10, 777 11 of 26 difference—the used basic depth in modulation. A typical block diagram of an ILS receiver is shown in Figure 9 [5,17].

FigureFigure 9. 9. InstrumentInstrument landing landing system system ( (ILS)ILS) receiver receiver block block diagram. diagram.

FigureFigure 9 illustrates two two dipole dipole antennas antennas,, one one for for the the LOC LOC channel, channel, and and a second a second for thefor GP the channel, GP channel, which which receive receive the ILS the signals. ILS signals. After the After RF and the RFIF processing and IF processing and demod- and ulation,demodulation, the signal the is signal let through is let through filtering ﬁltering to gain to gainthe amplitude the amplitude of navigation of navigation signals signals 90 and90 and 150150 Hz Hz[5,14,15] [5,14,.15 The]. The amplitude amplitude of the of the90 Hz 90 Hznavigation navigation signal signal is given is given by by " # mm EEF2nFL(ϑ) u1(t) = 1 − 2nL ( ) × sin F90Hzt (5) u1(t )=2 1 −E1nFP(ϑ)  sin F 90 Hz t (5) 2 EF1nP ( ) and the 150 Hz navigation signal is given by and the 150 Hz navigation signal is given by " # m E2n.FP(ϑ) u2(t) = m1 + EF2nP. ( ) × sin F150Hzt (6) u2(t )=2 1 +E1n.FL(ϑ)  sin F 150 Hz t (6) 2.EF1nL ( ) If the amplitudes of signals u1(t) and u2(t) are extracted: If the amplitudes of signals u1(t) and u2(t) are extracted: " # " # m E2n × FL(ϑ) m E2n × FP(ϑ) U90Hz = 1 − and U150Hz = 1 − (7) 2 E1n × FP(ϑ) 2 E1n × FL(ϑ)

where the DDM [–] parameter is determined by the equation

U − U DDM = 90Hz 150Hz (8) U90Hz + U150Hz In Figure9, there is an example of the normal functionality of the glide path (GP) channel of the backup ILS receiver and its cross pointer indicator (CPI). The GP indicator does not indicate any deviation, and the hidden ﬂag informs the pilot about the normal functionality of the whole GP channel. The ILS receiver operates on the 13th channel, which means 109.30 MHz for the localizer and 332.00 MHz for the GP. The marker indicates the signals carried on the frequency of 75.00 MHz in all cases. There is a normal behavior of the ILS/GP receiver in normal conditions in Figure 10, where the GP receiver processes Electronics 2021, 10, x FOR PEER REVIEW 12 of 27

m EF2nL ( ) m EF2nP ( ) U90Hz =−1 and U150Hz =−1 (7) 2 EF1nP ( ) 2 EF1nL ( ) where the DDM [–] parameter is determined by the equation

UU90Hz− 150 Hz DDM = (8) UU90Hz+ 150 Hz In Figure 9, there is an example of the normal functionality of the glide path (GP) channel of the backup ILS receiver and its cross pointer indicator (CPI). The GP indicator does not indicate any deviation, and the hidden flag informs the pilot about the normal functionality of the whole GP channel. The ILS receiver operates on the 13th channel, Electronics 2021, 10, 777 which means 109.30 MHz for the localizer and 332.00 MHz for the GP. The marker12 indi- of 26 cates the signals carried on the frequency of 75.00 MHz in all cases. There is a normal behavior of the ILS/GP receiver in normal conditions in Figure 10, where the GP receiver processes the normal ILS/GP signal and the control flag is hidden. For pilots, the flag themeans normal full ILS/GP and reliable signal and ope therability control of ﬂagthe isGP hidden. receiver For pilots, as well the as ﬂag the means CPI full display and reliable[5,10,17,19] operability. of the GP receiver as well as the CPI display [5,10,17,19].

FigureFigure 10.10. NormalNormal ILS/glideILS/glide path ( (GP)GP) indication.

TheThe analysisanalysis in Eq Equationsuations (6 (6)–(8))–(8) shows shows the the potential potential ways ways the the disturbing disturbing signal signal in- influencesfluences the the ILS ILS receiver receiver navigation navigation measurement. measurement. Al Althoughthough disturbing signalssignals generatedgenerated byby powerpower electronicselectronics dodo notnot directlydirectly influenceinfluence thethe ILSILS signals,signals, theirtheir receptionreception abilityability cancan bebe dramaticallydramatically loweredlowered asas disturbingdisturbing signalssignals degradedegradethe thereceiver receiversensitivity sensitivityin inthe theVHF VHF bandwidth,bandwidth, asas shownshown inin FigureFigure7 .7. However, However, the the disturbing disturbing signals signals do do not not have have an an impact impact on amplitudes of navigation signals and consequently the computed DDM. On the other on amplitudes of navigation signals and consequently the computed DDM. On the other hand, power switching components, based on traditional Si technology, can easily generate hand, power switching components, based on traditional Si technology, can easily gener- signals that may be similar to navigation tones 90 and 150 Hz. In the examples shown in ate signals that may be similar to navigation tones 90 and 150 Hz. In the examples shown Figure8, there is the 822 Hz disturbing signal, where its non-linear processing produces 1/4 in Figure 8, there is the 822 Hz disturbing signal, where its non-linear processing produces and 1/5 resonates, which directly act onto the 150 Hz navigation tone. Such an influence 1/4 and 1/5 resonates, which directly act onto the 150 Hz navigation tone. Such an influ- on the navigation tone is described by ence on the navigation tone is described by " × # m E2n FP(ϑ) U150HzDIST = m1 − EF2nP ( ) + UDIST (9) UU150HzDIST=2 1 −E1n × FL(ϑ) + DIST (9) 2 EF1nL ( ) where UDIST is the effective value of the amplitude of the disturbing signal. Consequently, thewhere resulting UDIST is value the effective of DDM valueis described of the amplitude by the equation of the disturbing signal. Consequently, the resulting value of DDM is described by the equation U − U − U DDM = 90Hz 150Hz DIST (10) U90Hz + U150Hz + UDIST The consequence of such disturbance is the error indication on the CPI equipment, where the device shows the command “fly right” in the case of the LOC, or “fly down” in the case of the GP disturbance in the UHF bandwidth. The difference arises in the normal navigation signal modulated with the 150 Hz navigation tone, which is substituted with a disturbance, as it is shown in Figure 11. It is shown that the levels and other parameters of the reception of the ILS/GP signals are the same as in the previous example in Figure 10; however, the ILS and MKR receivers face the intrusion of the EM signal generated by the power electronics. The first case (a) in Figure 11 shows only the disruption of the marker receiver that currently indicates the fly over the IM (inner marker), which uses the modulation frequency of 3000 Hz. It is evident that the intrusion is through the LF components of the MKR receiver. The optical indication can be potentially dangerous, as the integrity of the marker receiver is not monitored. Hence, there is only the acoustic check, when the pilot hears “Morse dots”. The second case (b) shows more serious effects of power electronics disruption. Although the received ILS/GP signal is strong enough (according to [5,15–17]), the ILS/GP receiver signal is completely eliminated. It is prohibited for the pilot to use the ILS information as from the presented flags. Nevertheless, the ILS Electronics 2021, 10, x FOR PEER REVIEW 13 of 27

UUU90Hz−− 150 Hz DIST DDM = (10) UUU90Hz++ 150 Hz DIST The consequence of such disturbance is the error indication on the CPI equipment, where the device shows the command “fly right” in the case of the LOC, or “fly down” in the case of the GP disturbance in the UHF bandwidth. The difference arises in the normal navigation signal modulated with the 150 Hz navigation tone, which is substituted with a disturbance, as it is shown in Figure 11. It is shown that the levels and other parameters of the reception of the ILS/GP signals are the same as in the previous example in Figure 10; however, the ILS and MKR receivers face the intrusion of the EM signal generated by the power electronics. The first case (a) in Figure 11 shows only the disruption of the marker receiver that currently indicates the fly over the IM (inner marker), which uses the modulation frequency of 3000 Hz. It is evident that the intrusion is through the LF components of the MKR receiver. The optical indication can be potentially dangerous, as the integrity of the marker receiver is not monitored. Hence, there is only the acoustic check, when the pilot hears “Morse dots”. The second case (b) shows more serious effects of Electronics 2021, 10, 777 13 of 26 power electronics disruption. Although the received ILS/GP signal is strong enough (according to [5,15–17]), the ILS/GP receiver signal is completely eliminated. It is prohibited for the pilot to use the ILS information as from the presented flags. Nevertheless, the ILS signalssignals can be can normally be normally presented. presented. In this In case, this case, the sum the ofsum depth of d ofepth modulation of modulation (SDM) (SDM) circuits,circuits, which which evaluate evaluate the usability the usability of the of ILS/GPthe ILS/GP channel, channel, are are currently currently intruded intruded by by the the disturbingdisturbing signal. signal. Figure Figure 11 11 shows shows the the effect effect of of the the 822 822 Hz Hz signal signal interference interference from from the the powerpower electronics electronics circuits. circuits. Figure 1111 also also shows shows the the situation situation when when the the MARKER MARKER function functionhas hasbeen been affected affected (see (see Figure Figure 11a )11. Ina). this In thiscase, case, the pilot the pilot has hasno information no information that thatthe inter- the interferenceference has has occurred occurred and and potentially potentially may may mistakenly mistakenly assume assume that that he has he hasflown ﬂown over the over theMARKER MARKER station, station, which which in ina anormal normal ILS ILS approach approach indicates thethe pilot’spilot’s distance distance to to the the thresholdthreshold of of the the runway runway (RWY) (RWY) [5 ,[105,10,17],17]. .

(a)

(b)

Figure 11. Disturbed Figuremarker 11.(MKRDisturbed) and GP marker channel (MKR) of ILS and receiver GP channel, (a) only of ILS inner receiver, marker (a) onlyinterference, inner marker (b) disrupted interference, GP Electronics 2021receiver., 10, x FOR PEER REVIEW 14 of 27 (b) disrupted GP receiver. AnotherAnother example example for navigation for navigation is from is from the the under-voltage under-voltage in in an an airborne airborne power power net- network.work. The The nominal nominal voltage voltage of anof airbornean airborne DC DC network network is 28 is volts 28 v (orolts 27 (or volts 27 v inolts special in special cases); however, producers of avionics often state the usability of instruments from 18 cases); however, producers of avionics often state the usability of instruments from 18 volts volts as it is shown in Figure 11. The ILS receiver works in the same environment it was as it is shown in Figure 11. The ILS receiver works in the same environment it was presented presented in; nevertheless, the CPI measures an additional indication error, which corre- in; nevertheless, the CPI measures an additional indication error, which corresponds to sponds to 0.0011 DDM (DDM for the whole glide path sector is 0.1750), implying circa 9 0.0011 DDM (DDM for the whole glide path sector is 0.1750), implying circa 9 µA of µA of current in the GP channel in the CPI (see Figure 12). Despite the defective navigation current in the GP channel in the CPI (see Figure 12). Despite the defective navigation data, the CPI indicates the inapplicability of the ILS receiver by flags, as monitored by the data, the CPI indicates the inapplicability of the ILS receiver by flags, as monitored by SDM which is out of the expected value. The effect of the power supply is evident in Fig- the SDM which is out of the expected value. The effect of the power supply is evident in ure 11b, where the GP channel is affected due to the interference from the power electron- Figure 11b, where the GP channel is affected due to the interference from the power ics. The examples above document the influence of circuits on the normal function of air- electronics. The examples above document the influence of circuits on the normal function craftof aircraft instruments instruments—avionics—and—avionics—and potentially potentially might might affect affect the reliability the reliability and andintegrity integrity of airof traffic, air traffic, and andpartially partially also also the thesafety safety [5,14,15,19] [5,14,15., 19].

Figure 12. Indication error caused by ILS/GP disturbance. Figure 12. Indication error caused by ILS/GP disturbance.

3. Backup Aircraft Superheterodyne Receiver into Disturbing Environment Another example of impacts of power electronics are EMI measurement results chal- lenging the certified airborne radio communication system LUN-3520 (3524), which complies with Radio Technical Commission for Aeronautics (RTCA) and International Civil Aviation Organization (ICAO) standards. LUN is a VHF (118–136.975 MHz) radio communication system using AM modulation and frequency channel separation of 25 kHz. LUN-3520 (3524) complies with the following: • RTCA DO-160C—“Environmental Conditions and Test Procedures for Airborne Equipment” (currently in version DO-160G, when after December 2014, parts “Radio Frequency Susceptibility, Radiated and Conducted” and “Emission of Radio Fre- quency Energy” were removed and placed into the new DO-357); • RTCA DO-186A—“Minimum Operational Performance Standards (MOPS) for Air- borne Radio Communications Equipment Operating within the Radio Frequency Range 117.975–137.000 MHz” (since 2005 in version DO-186B). Since the equipment is used in a variety of aircraft, LUN also complies with other standards [4,5,20–23]. The experimental radio receiver tested in detail is the RTCA/DO-186A class C and E certified and equipped with a squelch function. The declared sensitivity of the receiver is better than 5 µV. The level of suppression of unwanted (spurious and image) responses is at least 60 dB, in accordance with [20–24] and RTCA/DO-180A, part 2.2.8. The detailed synthesis of the VHF radio system immunity and effects of the modern power electronics show the impact of the interferences. The analyses demonstrate significant threats and risks resulting from interferences between VHF radio systems and power electronics switching. As mentioned above, most airborne VHF/UHF receivers are based on the superheterodyne or double-superheterodyne concept. A simplified block diagram is shown in Fig- ure 13. The superheterodyne receiver uses frequency mixing of a received signal to fix an

intermediate frequency IF. The idea is to reduce the incoming frequency to a suitable frequency to be amplified efficiently. The efficiency of operation in view of resistivity versus interfering signals of the superheterodyne receiver (SHR) depends on the frequency mixing, which is shown in Figure 13.

Electronics 2021, 10, 777 14 of 26

3. Backup Aircraft Superheterodyne Receiver into Disturbing Environment Another example of impacts of power electronics are EMI measurement results chal- lenging the certified airborne radio communication system LUN-3520 (3524), which complies with Radio Technical Commission for Aeronautics (RTCA) and International Civil Aviation Organization (ICAO) standards. LUN is a VHF (118–136.975 MHz) radio communication system using AM modulation and frequency channel separation of 25 kHz. LUN-3520 (3524) complies with the following: • RTCA DO-160C—“Environmental Conditions and Test Procedures for Airborne Equip- ment” (currently in version DO-160G, when after December 2014, parts “Radio Fre- quency Susceptibility, Radiated and Conducted” and “Emission of Radio Frequency Energy” were removed and placed into the new DO-357); • RTCA DO-186A—“Minimum Operational Performance Standards (MOPS) for Air- borne Radio Communications Equipment Operating within the Radio Frequency Range 117.975–137.000 MHz” (since 2005 in version DO-186B). Since the equipment is used in a variety of aircraft, LUN also complies with other standards [4,5,20–23]. The experimental radio receiver tested in detail is the RTCA/DO-186A class C and E certified and equipped with a squelch function. The declared sensitivity of the receiver is better than 5 µV. The level of suppression of unwanted (spurious and image) responses is at least 60 dB, in accordance with [20–24] and RTCA/DO-180A, part 2.2.8. The detailed synthesis of the VHF radio system immunity and effects of the modern power electronics show the impact of the interferences. The analyses demonstrate significant threats and risks resulting from interferences between VHF radio systems and power electronics switching. As mentioned above, most airborne VHF/UHF receivers are based on the superheterodyne or double-superheterodyne concept. A simplified block diagram is shown in Figure 13. The superheterodyne receiver uses frequency mixing of a received signal to fix an intermediate frequency IF. The idea is to reduce the incoming frequency to a suitable frequency to be amplified efficiently. The efficiency of operation in view of resistivity Electronics 2021, 10, x FOR PEER REVIEW 15 of 27 versus interfering signals of the superheterodyne receiver (SHR) depends on the frequency mixing, which is shown in Figure 13.

Figure 13. SuperheterodyneFigure 13. Superheterodyne receiver—simpliﬁed receiver block—simplified diagram block [5,14 diagram,20]. [5,14,20].

Selection and amplification of the desired signal at the tuned frequency ωD is the Selection and amplification of the desired signal at the tuned frequency ωD is the first first part of the SHR at the RF front-end. The first block is a high-quality band-pass filter part of the SHR at the RF front-end. The first block is a high-quality band-pass filter (BPF) (BPF) with appropriate sensitivity and selectivity. The BPF circuit is responsible for the with appropriate sensitivity and selectivity. The BPF circuit is responsible for the overall overall receiver’s selectivity and its ability to reject signals in close proximity to the desired frequency band.receiver In most’s selectivity cases, the SHRand its embeds ability the to RFreject tunable signals band-pass in close proximity filter and RFto the desired fre- amplifier. The signalquency at band. the output In most of cases, the RF the front-end SHR embeds is preprocessed the RF tunable to be convertedband-pass infilter and RF am- the frequency. Theplifier. frequency The signal converter at the output is translated of the from RF front the RF-end frequency is preprocessed band to to the be IF converted in the frequency. The frequency converter is translated from the RF frequency band to the IF band by using the mixer and ωLO signal of the local oscillator—synthesizer. The result is the combined signals,band by which using containthe mixer a largeand ω varietyLO signal of signalof the local characteristics oscillator— suchsynthesizer. as the The result is series of harmonics,the combined according signals, to the equation which contain a large variety of signal characteristics such as the series of harmonics, according to the equation mωLO ± nωD (11) 푚휔퐿푂 ± 푛휔퐷 (11)

where m, n are integers {0,1,2,3, …} expressing multiples of frequencies; ωD is the fre-

quency of the desired input signal; and ωLO is the frequency of a signal produced by the

local oscillator. The output signal ωIF of the mixing process is constant and does not change

for any possibly tuned ωD. To achieve the desired effect, the ωLO has to be changed in

parallel with ωD to keep a constant difference [1,25]. In view of power electronics effects, the input signal from the antenna is filtered to reject the image frequencies. In the superheterodyne receiver, the oscillator produces a

signal for mixing to shift the incoming signal to a specific intermediate frequency ωIF. The

ωIF signal is filtered and amplified. The demodulator demodulates the ωIF signal to obtain the original modulation, generally supporting the aircraft LF block. Despite all the advantages of the heterodyning in a receiver, there are many potentially critical channels—frequencies—which open “back doors” that make the receiver susceptible to interferences produced by power electronics processing. Practically, it is not always an easy task to identify what mechanism is the cause of the one particular interference.

One of the critical frequencies is the image frequency ωIMG. The signal, if present, at

the ωIMG can penetrate through the input RF band-pass filter in case of an insufficiently low quality of this filter. Generally, the input RF filter does not have too steep characteristics, and it is solved by the superheterodyne reception itself. Nevertheless, such way of reception cannot effectively confirm all the interfering signals are emitted by the power electronics.

The difference between the desirable signal at ωD and a critical signal at ωIMG is 2x

ωIMG.

휔퐼푀퐺 = 휔퐷 ± 2휔퐼퐹. (12)

Once the signal at the image frequency ωIMG penetrates into the receiver, it generates an unwanted response at the IF band after the mixer (MIX) which will be amplified by the IF amplifier and cannot be suppressed anymore. The only way to prevent the signal at the image frequency from interfering with the receiver and to increase immunity against the image response is to increase the selectivity (quality) of the RF circuits.

Electronics 2021, 10, 777 15 of 26

where m, n are integers {0,1,2,3, ... } expressing multiples of frequencies; ωD is the frequency of the desired input signal; and ωLO is the frequency of a signal produced by the local oscillator. The output signal ωIF of the mixing process is constant and does not change for any possibly tuned ωD. To achieve the desired effect, the ωLO has to be changed in parallel with ωD to keep a constant difference [1,25]. In view of power electronics effects, the input signal from the antenna is filtered to reject the image frequencies. In the superheterodyne receiver, the oscillator produces a signal for mixing to shift the incoming signal to a specific intermediate frequency ωIF. The ωIF signal is filtered and amplified. The demodulator demodulates the ωIF signal to obtain the original modulation, generally supporting the aircraft LF block. Despite all the advantages of the heterodyning in a receiver, there are many potentially critical channels—frequencies—which open “back doors” that make the receiver susceptible to interferences produced by power electronics processing. Practically, it is not always an easy task to identify what mechanism is the cause of the one particular interference. One of the critical frequencies is the image frequency ωIMG. The signal, if present, at the ωIMG can penetrate through the input RF band-pass filter in case of an insufficiently low quality of this filter. Generally, the input RF filter does not have too steep characteristics, and it is solved by the superheterodyne reception itself. Nevertheless, such way of reception cannot effectively confirm all the interfering signals are emitted by the power electronics. The difference between the desirable signal at ωD and a critical signal at ωIMG is 2x ωIMG. ωIMG = ωD ± 2ωIF. (12)

Once the signal at the image frequency ωIMG penetrates into the receiver, it generates an unwanted response at the IF band after the mixer (MIX) which will be ampliﬁed by the IF ampliﬁer and cannot be suppressed anymore. The only way to prevent the signal at the image frequency from interfering with the receiver and to increase immunity against the image response is to increase the selectivity (quality) of the RF circuits. Another hazardous channel into the receiver is through the frequency equal exactly to ωIF. If the signal at the input of the receiver is at the ωIF and penetrates through the RF front-end, then it appears at the input of the MIX. Such an MIX interference is independent of the current frequency tuning of the receiver and the desirable ωD.A hazardous frequency is therefore the difference between ωD and ωLO:

ωIF = ωLO − ωD or ωIF = ωD + ωLO. (13)

The MIX interference occurs when a strong source generates a signal at ωIF and the input band-pass RF ﬁlter is not able to suppress it under the acceptable level. In the EMI case, a hazardous signal penetrates into the IF stage and interferes with the desirable signal at ωD. In addition, the imperfect local oscillator produces a signal, which is not ideally harmonic. It means the frequency spectrum of such a signal contains not only the basic ωLO but also a series of higher-order harmonics nωLO, where n = {2, 3, 4, ... }. The entire series of harmonics from the LO is introduced to the mixer MIX. The MIX then produces a combinatorial signal for every speciﬁc harmonic, creating a much more complex scenario with many more various possible combinations, resulting in a critical ωIF frequency [13–15]. For instance, the presented example of the second harmonic of the LO, 2ωLO, is

2 ωIMG − 2ωLO = (2 f ωLO + ωIF) − 2 fLO. (14)

From Equation (14), if any signal at the frequency (2ωIMG—2ωLO) or (2ωLO—2ωIMG) can successfully enter the receiver, then its output of the MIX is translated as the ωIF and its rejection is not possible any more. As such, the signal then interferes with the desired signal at ωIMD. As shown above, one of the critical frequencies is the image frequency f IMG [4,5,20–23], deﬁned as: the mixer uses non-linear components and processes, and the output of the Electronics 2021, 10, 777 16 of 26

mixer may include the original RF signal at f D, the local oscillator signal f LO, and two new frequencies f D + f LO and f D + f LO. The mixer may also produce additional signals of higher-order inter-modulation products. The undesired signals are removed by the IF band-pass ﬁlter to obtain only the IF signal that includes the original modulation. The quality of the IF modulation is important to provide to the receiver to mitigate the effects of the power electronics interfering signals. One major disadvantage is the problem of determining ωIF so the receiver can reject interfering signals at the image frequency [25,26]. Likewise, additional dangerous frequencies—hazardous channels—exist due to the non-linear input/output characteristics of the active circuits (ampliﬁers and mixer) of the receiver. When the signal is processed by a non-linear circuit, the circuit not only excites the output of the spectrum with the same harmonics content but also generates completely new harmonic members— called non-linear distortion. For a power electronics example, let the input of a non-linear circuit be two signals with angular frequencies ω1 and ω2 as described: x(t) = A1 cos ω1t + A2 cos ω2t, (15)

where A1 and A2 are the amplitudes of the signal. The output of the circuit is the signal which contains the series of harmonics:

mω1 ± nω2 ± pω3 ± . . . , (16)

where m,n = {0,1,2, 3, 4, ... }. The interferences and disturbances which occur in this case and in one of the most intrusive cases are called inter-modulation. The inter-modulation disturbance appears when two or more strong signals with different frequencies are received simultaneously and both proliferate at the input of the IF amplifier. Practically, there is a desired signal at ωD and other more strong signals. The RTCA DO-186B, part 2.6.2.10 describes the procedure for inter-modulation immunity tests using a two-signal method. A dangerous situation arises when the disturbing signal is at the frequency close to ωD and the radio combines the signal with the frequency inside of the band pass of the IF filter [25,26]. Apparently, due to a non-linear character of the input–output characteristics of the receiver’s circuits, higher-order harmonics combinations between the desired and disturbing signals occur. Some of them, causing the receiver to become susceptible to the inter-modulation type of interference, appear in a frequency band passing through the IF filter and IF amplifier. Hence, two signals at frequencies ω1 and ω2 are received, resulting in ω1, ω2, 2ω1, 2ω2, 3ω1, 3ω2 . . . .., and higher frequencies being present at the input of the MIX. At the input of the IF filter, therefore, combinations of 2ω1 ± ω2, 3ω1 ± 2ω2, 2ω2 ± ω1, and others occur. Hence, the critical frequencies are

ωdisturbing = ωs − 2ωIF, (17)

and the interfering signal penetrates as an image signal, since

ωs − 2ωIF − ωLO = ωIF (18)

When the interfering signal is at

ωdisturbing = ωIF, (19)

it penetrates directly through the IF ﬁlter and ampliﬁer as an IF signal. Interferences at 2ωs are disturbing when

1 ω = ω , (20) disturbing 2 s since 1 2 ω = ω . (21) 2 s s Electronics 2021, 10, 777 17 of 26

Similarly, disturbances occur when

1 ω = ω = γ , (22) disturbing k s k where k = (1, 2, 3, . . . ). The inter-modulation disturbance of the third order

2ωs − ωdisturbing, (23)

When the interferences are at its frequency, is close to ωs. The inter-modulation disturbance of the fourth order according to

1 3 ω = ω − ω = ω − ω , (24) disturbing LO 2 IF LO 2 IF interferes since 3 2(ω − (ω − ω >)) = ω (25) LO D 2 IF IF

4. Measuring and Testing Workplace For testing purposes, an instrumentation workplace that enables interference testing of systems with the superheterodyne radio receiver concept was created. For such testing, the main part of the workplace is a jammer system based on a transistor PWM power switch, which allows generation of interference by switching the power to inductive load with a square waveform with an adjustable carrier frequency. The system enables targeted Electronics 2021, 10, x FOR PEER REVIEW 18 of 27 setting of interfering functions of the electronic radio receiver in the bandwidth of 500 Hz to 200 kHz. Figure 14 shows the jammer system according to [12].

FigureFigure 14.14.Testing Testing source source of of electromagnetic electromagnetic interference. interference.

TheThe EMIEMI testingtesting systemsystem consistsconsists ofof aa powerpower switchswitch andand aa PWMPWM modulator,modulator, whichwhich providesprovides a a power power electronic electronic interface interface between between an an electric electric source source and and an electric an electric load: load in our: in case,our case the power, the power amplifier amplifier and antenna. and antenna. Such conceptSuch concept allows allows not only not setting only setting the frequency the fre- butquency also simulatingbut also simulat the PWMing the control PWM of thecontrol used of frequency the used convertersfrequency onconverters board the on aircraft, board usingthe aircraft, the possibility using the of possibility duty cycle of changing duty cycle and changing modulation and control, modulation or unmanned control, aerialor un- vehiclemanned (UAV) aerial drives,vehicle which(UAV) can drives, be a which source can of interferencebe a source of of interference ILS ground of transmitters ILS ground whichtransmitters can fundamentally which can fundamentally influence aircraft influence in the approachaircraft in tothe landing approach phase. to landing Furthermore, phase. theFurthermore, interference the system interference is equipped system with is anequipped amplifier, with which an amplifier allows continuous, which allows adjustment contin- ofuous the adjustment output power of the level output with apower modular level antenna with a connection.modular antenna The implemented connection. systemThe im- enablesplemented the generationsystem enables of interference the generation and theof interference simulation ofand the the formed simulation interfering of the signals,formed whichinterfering can be signals, produced which on commutatorscan be produced of electric on commutators motors orelectric of electric DC motors drives ofor aircraft. electric ItDC also drives allows of theaircraft. simulation It also ofallows interference the simulation by power of electronicsinterference circuits by power that useelectronics PWM modulationcircuits that to use power PWM supply modulation of asynchronous to power and supply synchronous of asynchronous generators and or servosynchronous drives usedgenerators on board or servo the aircraft. drives used on board the aircraft. TheThe interfering interfering signal signal is is propagated propagated both both by by electromagnetic electromagnetic emission emission and and through through the aircraft’sthe aircraft distribution’s distribution buses. buses. In particular, In particular frequency, frequency converters converters are the are source the source of such of inter- such ferenceinterference on board on board the aircraft. the aircraft. The measuring The measuring workplace workplace enables enables testing testing where interferencewhere interference directly penetrates into electronic circuits through the air. As mentioned above, generated interference frequencies, especially in the order of 1 kHz, are potentially possible because such frequencies interfere with the signal processing circuits of the superheterodyne receivers. In the article, the cases with frequencies around 800 Hz are presented. These frequencies are potentially dangerous as similar frequencies are used in power systems because the choice of frequency is very closely related to the level of the processed power. The higher the power level processed by the power electronics converter, the lower the frequencies that can be used in the switching circuit, as the switching losses of the converter increase with the frequency. For these reasons, frequencies from 800 Hz to 2.5 kHz are used very often. In addition, these high-level frequencies of processed power operate with voltages of up to hundreds or thousands of volts, which can very easily penetrate into the circuits and produce interference of feedback control signals or interference of data signals on board the aircraft. Furthermore, these frequencies may produce interference on radio communications. On board the aircraft, interference can be propagated by galvanic, capacitive, inductive, or electromagnetic emission in direct or secondary coupling. In this case, only the transmission by electromagnetic wave emission of a distant electromagnetic field is the subject of the test. Transmission by electrical or magnetic coupling in the near field between the receiver object and the frequency converter, for example, is less probable. In particular, in case of radiation transmission, the intensities of the electric Ei or the magnetic field Hi are used. These intensities of the interfering fields Ei and Hi are converted by measuring antennas to the voltage Vi at the terminals of the system, producing simulated interference. There are obvious coefficients of antenna factors of used antennas for a given measuring frequency of interference or bandwidth of interference. The basic device for evaluation is a spectrum analyzer, operating in the frequency range of 10 Hz to 9 GHz.

Electronics 2021, 10, 777 18 of 26

directly penetrates into electronic circuits through the air. As mentioned above, generated interference frequencies, especially in the order of 1 kHz, are potentially possible because such frequencies interfere with the signal processing circuits of the superheterodyne receivers. In the article, the cases with frequencies around 800 Hz are presented. These frequencies are potentially dangerous as similar frequencies are used in power systems because the choice of frequency is very closely related to the level of the processed power. The higher the power level processed by the power electronics converter, the lower the frequencies that can be used in the switching circuit, as the switching losses of the converter increase with the frequency. For these reasons, frequencies from 800 Hz to 2.5 kHz are used very often. In addition, these high-level frequencies of processed power operate with voltages of up to hundreds or thousands of volts, which can very easily penetrate into the circuits and produce interference of feedback control signals or interference of data signals on board the aircraft. Furthermore, these frequencies may produce interference on radio communications. On board the aircraft, interference can be propagated by galvanic, capacitive, inductive, or electromagnetic emission in direct or secondary coupling. In this case, only the transmission by electromagnetic wave emission of a distant electromagnetic field is the subject of the test. Transmission by electrical or magnetic coupling in the near field between the receiver object and the frequency converter, for example, is less probable. In particular, in case of radiation transmission, the intensities of the electric Ei or the magnetic field Hi are used. These intensities of the interfering fields Ei and Hi are converted by Electronics 2021, 10, x FOR PEER REVIEWmeasuring antennas to the voltage Vi at the terminals of the system, producing simulated19 of 27 interference. There are obvious coefficients of antenna factors of used antennas for a given measuring frequency of interference or bandwidth of interference. The basic device for evaluation is a spectrum analyzer, operating in the frequency range of 10 Hz to 9 GHz. The The interfering signals that have been radiated into the area of the superheterodyne re- interfering signals that have been radiated into the area of the superheterodyne receiver ceiver are at frequencies from 500 Hz up to 1000 MHz. For testing, the loop antenna in the are at frequencies from 500 Hz up to 1000 MHz. For testing, the loop antenna in the rangerange up up to to 30 30 MHz MHz with with a a H-component H-component and and the the rod rod antenna antenna with with an an E-component E-component were were used.used. In Inthe thebandwidth bandwidth up up to to200 200kHz, kHz, interferenceinterference phenomena phenomena caused caused by by the the H-magnetic H-magnetic componentcomponent of of the the electromagnetic electromagnetic field field predominates. predominates. Loop Loop antennas antennas with with amplifiers amplifiers for for a givena given frequency frequency band band are are typically typically used used for measurement.for measurement. For higherFor higher frequencies, frequencies, both both the Hi-componentthe Hi-component and theand Ei-component the Ei-component were were tested. tested Interfering. Interfering signals signals in the in operating the operating area ofarea electronic of electronic circuits ci werercuits realized were realized by special by special electric electric and magnetic and magnetic field probes. field probes. The use The of theuse probes of the isprobes shown is inshown Figure in 15Figure. 15.

FigureFigure 15. 15.Realization Realization of of special special electric electric and and magnetic magnetic ﬁeld field probes. probes.

TheThe testtest designdesign (Figures(Figures 14 and 15)15) allows allows monitoring monitoring the the undesirable undesirable p phenomenahenomena of ofindividual individual interfering interfering components components simulating simulating power power electronics circuits circuits on onindividual individual blocksblocks of of the the superheterodyne superheterodyne receiverreceiver and,and,consequently, consequently, determining determiningthe thelimits limitsof ofelec- elec- tromagnetictromagnetic immunityimmunity andand weaknessesweaknesses ofof thethe receiverreceiver fromfrom thethe EMCEMC pointpoint of of view view of of testedtested devicesdevices onon board the aircraft. aircraft. In In addition, addition, the the designed designed system system allows allows obtaining obtaining the therelative relative information information about about the theintensity intensity of interference of interference in a in given a given operation operation area area or in or a ingiven a given circuit. circuit. Consequently Consequently,, it affords it affords identifying identifying the theplace placess of radiation of radiation or places or places of pen- of penetrationetration of interference of interference into into the the electronic electronic circuits circuits of ofthe the receiver. receiver. It should be emphasized that the effects of emitted measuring interference signals cause a measurement uncertainty. Although the overall inaccuracy of the whole measuring chain, i.e., interference source, amplifier, and antenna, is quite high, for testing purposes, the system is able to analyze receiver capabilities against unwanted reception channels. An example of the use of our interference testing systems is shown in Figure 16, where an 800 Hz interference signal is generated. The voltage from the PWM modulator is led to the control of the switching transistor.

Figure 16. Generated interference signals.

Electronics 2021, 10, x FOR PEER REVIEW 19 of 27

The interfering signals that have been radiated into the area of the superheterodyne receiver are at frequencies from 500 Hz up to 1000 MHz. For testing, the loop antenna in the range up to 30 MHz with a H-component and the rod antenna with an E-component were used. In the bandwidth up to 200 kHz, interference phenomena caused by the H-magnetic component of the electromagnetic field predominates. Loop antennas with amplifiers for a given frequency band are typically used for measurement. For higher frequencies, both the Hi-component and the Ei-component were tested. Interfering signals in the operating area of electronic circuits were realized by special electric and magnetic field probes. The use of the probes is shown in Figure 15.

Figure 15. Realization of special electric and magnetic field probes.

The test design (Figures 14 and 15) allows monitoring the undesirable phenomena of individual interfering components simulating power electronics circuits on individual blocks of the superheterodyne receiver and, consequently, determining the limits of electromagnetic immunity and weaknesses of the receiver from the EMC point of view of

Electronics 2021, 10, 777 tested devices on board the aircraft. In addition, the designed system allows obtaining19 of the 26 relative information about the intensity of interference in a given operation area or in a given circuit. Consequently, it affords identifying the places of radiation or places of pen- etration of interference into the electronic circuits of the receiver. ItIt shouldshould bebe emphasizedemphasized thatthat thethe effects effects of of emitted emitted measuring measuring interference interference signals signals causecause a a measurement measurement uncertainty. uncertainty. Although Although the the overall overall inaccuracy inaccuracy of theof the whole whole measuring measur- chain,ing chain, i.e., interferencei.e., interference source, source, ampliﬁer, amplifier and, antenna, and antenna is quite, is quite high, forhigh testing, for testing purposes, pur- theposes, system the system is able is to able analyze to analyze receiver receiver capabilities capabilities against against unwanted unwanted reception reception channels. chan- Annels. example An example of the useof the of ouruse interference of our interference testing systems testing issyste shownms i ins shown Figure 16in, Figure where an16, 800where Hz interferencean 800 Hz interference signal is generated. signal is Thegenerated. voltage The from voltage the PWM from modulator the PWM is modulator led to the controlis led to of the the control switching of the transistor. switching transistor.

Figure 16. Generated interference signals. Figure 16. Generated interference signals. The measurement system, described above, was also extended with the possibility of testing for EMI with narrowband or wideband signals. The switching slew rate significantly affects the generated interference for the radio receiver testing with narrowband or wideband signals [24,25]. The signals’ slew rate is set on the transistor by the change in the VGS (voltage from gate to source) of the gate voltage with a constant emitter–collector voltage. The dynamics of the switching edge is influenced by the series of capacitances of the transistor and the gate resistor Rg. These capacitances appear in the transition of the P-channel capacitance and gate in the N-MOSFET (N channel metal–oxide–semiconductor field-effect transistor ) switch. The width of the channel varies depending on the width of the spatial charge between the P layer and the epitaxial layer of the MOSFET and thus its total charge [27]. This charge describes the equation

dQ C = , (26) dV where dQ is the derivative of charge from the power source. Examples of the generated interference when changing the steepness of the rising edge are shown in Figure 17. An example of a higher level of interference is the case of Figure 17a, where the steeper edge produces higher peaks of voltage up to 700% and thus gets into the systems more easily, in contrast to the case in Figure 17b, where a slower slew rate of the rising edge is visible and thus a smaller post-transition overshoot and consequently less interference, which is produced by the test system. Electronics 2021, 10, x FOR PEER REVIEW 20 of 27

The measurement system, described above, was also extended with the possibility of testing for EMI with narrowband or wideband signals. The switching slew rate significantly affects the generated interference for the radio receiver testing with narrowband or wideband signals [24,25]. The signals’ slew rate is set on the transistor by the change in the VGS (voltage from gate to source) of the gate voltage with a constant emitter–collector voltage. The dynamics of the switching edge is influenced by the series of capacitances of the transistor and the gate resistor Rg. These capacitances appear in the transition of the P- channel capacitance and gate in the N-MOSFET (N channel metal–oxide–semiconductor field-effect transistor) switch. The width of the channel varies depending on the width of the spatial charge between the P layer and the epitaxial layer of the MOSFET and thus its total charge [27]. This charge describes the equation 푑푄 퐶 = , 푑푉 (26) where dQ is the derivative of charge from the power source. Examples of the generated interference when changing the steepness of the rising edge are shown in Figure 17. An example of a higher level of interference is the case of Figure 17a, where the steeper edge produces higher peaks of voltage up to 700% and thus Electronics 2021, 10, 777 gets into the systems more easily, in contrast to the case in Figure 17b, where 20a ofslower 26 slew rate of the rising edge is visible and thus a smaller post-transition overshoot and consequently less interference, which is produced by the test system.

(a)

(b)

Figure 17. Rising edge asFigure the sourc 17.eRising of electromagnetic edge as thesource interference. of electromagnetic (a) High level interference. of interference (a) caused High level by fast of switching; interference (b) interference with normalcaused switching. by fast switching; (b) interference with normal switching.

TheThe main main benefits benefits of of the the design design of of the the EMI EMI testtest sourcesource are described below. below. Narrow- Nar- rowbandband interference interference can can be be suppressed suppressed with with special special tuned tuned filters. filters. However, However, as as can can be be seen seen from the measurements, the interference of the power electronics circuits is rather wideband interference, i.e., such that the spectral width is greater than the bandwidth of the intermediate part of the superheterodyne receiver. These interfering signals are mainly produced with one-time or periodic pulses, which the designed system allows generating. The advantages of the concept can be seen especially in the area of measures recommended for airborne radio systems to avoid interferences. The problem of interference suppression in onboard aircraft systems is usually very problematic and presents a complicated solution. Such solutions aim to increase the immunity of the airborne superheterodyne receivers so that receivers are able to resist the interference in an electrically problematic environment. However, the backup receivers used on board the aircraft generally represent a simplified solution, which also represents a reduced ability to suppress external interference. The design and topology of the interference filters are determined by the size of the processed power and frequencies. In this case, it is obvious that the circuit concept based on mass relevant materials should be used for low-frequency interference suppression, which in aeronautics represents an additional load and increase in the weight of the filter and consequently the whole electronic device. The design issues of filter circuits that play an important role in reaching the goal of parameter optimization such as the weight and size can even lead to a reduction in the overall device resistance to EMI. These principles, as has already been mentioned, are mainly used in backup systems so that the backup system should not represent a weight load on board the aircraft. In addition, there is a problem where the use of interference filters can degrade the operating conditions of the used device, e.g., the receiver’s sensitivity. Electronics 2021, 10, 777 21 of 26

Interference is also related to the shielding thickness and shielding efficiency. The total shielding efficiency with the shielding thickness of 1 mm for frequencies up to 1000 Hz is around 60–80%. The 3 mm shielding efficiency is up to 90% [15,16,28]. It is evident from the examples that the methods of interference suppression by power electronics circuits are also significant from the design point of view, as the measures significantly increase the weight of the systems on board the aircraft [20,23]. The testing method (Figure 14) not only enables the measurement of interfering signals and the behavior of the radio system in the environment of the EMI but it also enables the solution of issues in the design of onboard equipment when choosing optimization processes between mass relevant filters and EMI shielding. Since it is not possible to achieve ideal EMC of superheterodyne receivers for the mentioned reasons, it is necessary to perform measurements and testing for verification of the correct functions of onboard devices.

5. Experimental Verifications On board an aircraft, electrical power-to-energy results from power converters with PWM switching, where SiC IGBTs (silicon carbide, insulated-gate bipolar transistors) are mainly used, which can produce disturbing signals and consequently reduce the quality of the radio reception. The problems of such interfering signals are analyzed in this section using the superheterodyne receiver concepts [12,26–29]. High-speed switching of power devices produces an EMI, which is potentially dangerous for aircraft radio jamming. Possible electronics interference that may arise at the aircraft electronics include power switching, where a short rise and fall of power contains significant harmonic energy levels. The interference power does not focus just on a certain part of the frequency spectrum but occurs in a wide band of frequencies from kHz to up to tens of MHz (see Figure3b), where wideband interfering signals are produced by power electronics processing. It is evident from Figure3b that higher speeds of switching produce higher peaks of voltage, producing serious EMI stresses, noise, and electromagnetic interferences. The HF switching in the PWM converter may produce harmonics and inter-harmonics in the range of wide-spread Hz and can have a serious impact on the VHF/UHF radio systems. Such an effect may influence the basic integrity and normal operation of the airborne equipment [28,30]. An example of EMI is discussed below and the results of experimental verification of the backup radio system are shown in Figure 18. The interfering signals that produce the interferences of VHF radios are marked by red colors. Experimental verifications of the backup radio certificated in accordance with RTCA/DO-186A class C and E are presented here. The sensitivity of the receiver is declared better than 5 µV. The level of suppression of unwanted (spurious and image) responses, in accordance with RTCA/DO-180A, part 2.2.8, is at least 60 dB. From the receiver EMI signal analyses above, we analyzed the signals that can be potentially dangerous and can cause the intrusion into the receiver circuits. The frequencies that are sensitive to the potential hazard of the VHF receiver based on the superheterodyne or double-superheterodyne concept in the undesired response can be categorized into several groups, as can be seen in Table1. The frequency fs is selected—the desired frequency of the input signal—and fIF is an intermediate frequency of the receiver. The results of the experimental verification of the influence of aircraft power electronics versus radio compatibility are shown in Figure 18. In the following experimental results, we will focus on the intrusion of the interference signal into the reception circuits and into the receiving channel according to Equation (22) (i.e., through f = 59 MHz), as shown in Figure 18, which aims to summarize measurements on radio stations on selected channels according to Table1. Electronics 2021, 10, x FOR PEER REVIEW 22 of 27

An example of EMI is discussed below and the results of experimental verification of the backup radio system are shown in Figure 18. The interfering signals that produce the interferences of VHF radios are marked by red colors. Experimental verifications of the backup radio certificated in accordance with RTCA/DO-186A class C and E are presented here. The sensitivity of the receiver is declared better than 5 µV. The level of suppression of unwanted (spurious and image) responses, in accordance with RTCA/DO-180A, part 2.2.8, is at least 60 dB. From the receiver EMI signal analyses above, we analyzed the signals that can be potentially dangerous and can cause the intrusion into the receiver circuits. The frequencies that are sensitive to the potential hazard of the VHF receiver based on the superheterodyne or double-superheterodyne concept in the undesired response can be categorized into several groups, as can be seen in Table 1. The frequency fs is selected—the desired frequency of the input signal—and fIF is an intermediate frequency of the receiver. The results of the experimental verification of the influence of aircraft power electronics versus radio compatibility are shown in Figure 18. In the following experimental results, we will focus on the intrusion of the interference signal into the reception circuits and into the receiving channel according to Equation (22) (i.e., through f = 59 MHz), as shown in Figure 18, which aims to summarize measurements on radio stations on selected channels according to Table 1.

Table 1. Intermediate frequency of the receiver.

Group A: A: fif Group B: B1: fs – 2 fif; B2: fs – 3/2 fif Group C: C2: 1/2 fs; C3: 1/3 fs; C4: 1/4 fs; C5: 1/5 fs; C6: 1/6 fs Electronics 2021, 10, 777 Group D: D: fs – ½ fif 22 of 26 Group E: E1: 3/2fs – ½ fif, E2: fs – ½ fif

Figure 18. Measurements of susceptibility of the RF receiver LUN related to undesired signals of Figurepower 18. electronics. Measurements of susceptibility of the RF receiver LUN related to undesired signals of power electronics. Table 1. Intermediate frequency of the receiver. The results of our experimental verifications in Figure 18 show the serious situation

whereGroup aircraft A: power electronicsA: fif affects airborne radio systems. The selectivity of the radio receiverGroup B:is a crucial parameterB1: fs – 2 when fif; B2: consideringfs – 3/2 fif the immunity of the receiver [5–7,22–24].

Group C: C2: 1/2 fs; C3: 1/3 fs; C4: 1/4 fs; C5: 1/5 fs; C6: 1/6 fs 1 Group D: D: fs – 2 fif 1 1 Group E: E1: 3/2fs – 2 fif, E2: fs – 2 fif

The results of our experimental verifications in Figure 18 show the serious situation where aircraft power electronics affects airborne radio systems. The selectivity of the radio receiver is a crucial parameter when considering the immunity of the receiver [5–7,22–24]. The ability of the receiver to suppress or reject unwanted and disturbing signals determines the amount of possible frequencies at which those disturbances can influence the receiver and trigger the response. Practical results indicate that not only strong but also weak disturbing signals can interfere within the receiver. Disturbances generated by power electronic circuits are mostly significantly stronger comparing to weak signals received by the antenna. The experiment design presented in Figure 14 was prepared to use the switching frequency over an interval of 800 Hz–20 kHz, which is currently normally used [7,8,29]. The presented experimental results are for fs = 118 MHz. The power electronics uses 822 Hz (see Figure7b), and 2 kHz of power switching. As can be seen in Figure 18, power electronics affects, for example, case B2, at the frequency of 59 MHz. Very likely, some of the higher-order harmonics products from the 2 kHz power electronic switching belong to the specific critical channel B2, and it can potentially reduce the immunity of the radio receiver. The problem is when the interfering signal physically intrudes into the radio station and consequently causes such interferences. The results of the previous analyses versus the radio RF receiver LUN were experimentally verified together with the power electronics system of the servomotors. Power electronics switches (probably based on the MOSFET technology) operate in high frequency and provide receiver disturbances. The consequence of the disturbance emitted by the power electronics was serious in terms of the reliability of the receiver communication system. Switching power electronics of the servo system caused an emitted electromagnetic disturbance that covers the large-frequency range, as can be seen in Figure 19. The Electronics 2021, 10, x FOR PEER REVIEW 23 of 27

The ability of the receiver to suppress or reject unwanted and disturbing signals determines the amount of possible frequencies at which those disturbances can influence the receiver and trigger the response. Practical results indicate that not only strong but also weak disturbing signals can interfere within the receiver. Disturbances generated by power electronic circuits are mostly significantly stronger comparing to weak signals received by the antenna. The experiment design presented in Figure 14 was prepared to use the switching frequency over an interval of 800 Hz–20 kHz, which is currently normally used [7,8,29]. The presented experimental results are for fs = 118 MHz. The power electronics uses 822 Hz (see Figure 7b), and 2 kHz of power switching. As can be seen in Figure 18, power electronics affects, for example, case B2, at the frequency of 59 MHz. Very likely, some of the higher-order harmonics products from the 2 kHz power electronic switching belong to the specific critical channel B2, and it can potentially reduce the immunity of the radio receiver. The problem is when the interfering signal physically intrudes into the radio station and consequently causes such interferences. The results of the previous analyses versus the radio RF receiver LUN were experimentally verified together with the power electronics system of the servomotors. Power electronics switches (probably based on the MOSFET technology) operate in high frequency and provide receiver disturbances. The consequence of the disturbance emitted

Electronics 2021, 10, 777 by the power electronics was serious in terms of the reliability of the receiver communi-23 of 26 cation system. Switching power electronics of the servo system caused an emitted electromagnetic disturbance that covers the large-frequency range, as can be seen in Figure 19. The frequencies ranging from 10 kHz to 10 MHz are extremely ranged spectral contents frequenciesthat are affected ranging by fromswitching 10 kHz harmonics to 10 MHz of arethe extremelyswitch mode ranged power spectral supply contents of the servo that aresystem. affected As by mention switchinged above, harmonics the of problem the switch at highmode frequencies power supply is ofthe the transient servo system. in the Asswitching mentioned of power above, devices. the problem We described at high frequencies the system isto thechange transient the edge in the of switchingthe switching of powerprocess devices. in our approach We described because the system the edge to steepness change the is edge a very of important the switching parameter process in in anal- our approachyses of an because EMI of thepower edge electronics. steepness isSwitching a very important in the transient parameter of devices in analyses used of in an the EMI servo of powersystem electronics. produced Switchinghigh-frequency in the transientresonances of devicesfrom 10 used MHz in to the 30 servo MHz. system In addition produced, the high-frequencyinterfering signals resonances of the control from 10 stage MHz produced to 30 MHz. an EMI In addition, as well. theThe interfering results of this signals exper- of theimental control verification stage produced of the anservo EMI system as well. show The that results power of this electronics experimental produce veriﬁcations very com- of theplex servo interfering system showsignals that to provide power electronics jamming produceseffects of verythe backup complex radio interfering system signalsLUN. De- to providetailed and jamming complex effects analyses of the of backup how the radio EMI system signals LUN. transmit Detailed into andthe receiver complex are analyses shown ofbelow how theand EMIin Figure signals 20. transmit into the receiver are shown below and in Figure 20.

Electronics 2021, 10, x FOR PEER REVIEW Figure 19. Measurements of power electronics effects as a function of the frequency range24 ofof 27 Figure 19. Measurements of power electronics effects as a function of the frequency range of powerpower electronics. electronics.

(a) (b)

1 FigureFigure 20. 20.ResultingResulting frequency frequency spectrum spectrum for case for of case ½ f ofs, (2a)f singles,(a) singleamplitude amplitude modulation modulation (AM) (AM) signal,signal, (b) (AMb) AM signal signal when when DC DC-DC-DC power power converter converter is active, is active, (c) (c)single single AM AM signal signal image image fre- frequency quencyrejection, rejection, (d) DC-DC (d) DC- converterDC converter image image frequency frequency rejection. rejection.

6. Discussions Besides others, the LUN-3520 (LUN-3524) radio communication system contains an AM receiver (specifically A3E with DSB) for the frequency band of 118–137 MHz. How- ever, the receiver is professionally built and certified, which may allow interfering signals generated by power electronics to penetrate and can decrease the basic quality of normal reception in backup radio systems. As mentioned in the Introduction, a backup VHF (UHF) communication or navigation receiver and its instrument parts are typically built as simple or double-heterodyne receivers in low-dimension blocks with a low mass [5,11,29]. The design does not allow using a high level of mechanical shielding and consequently allows the intrusion of specific disturbing signals. Figure 20 shows the effect of switching with detailed analysis of the situation inside the radio circuits. Figure 20 also shows the effect of how the interference signal intrudes the reception channel, according to Equation (22), when the interference penetrates the radio at the frequency of 59 MHz. The interfering signal is 118 MHz, the intermediate frequency is in red (22.114 MHz), the distortion is in black (1 kHz), green is the interfering signal (frequency 822 Hz) produced by the power electronics, and brown is the switching frequency (2 kHz) of the power electronics. To understand how the interference signal reaches the output of the radio station, it is necessary to describe the effect of the IF filter. The IF filter is the custom-designed unbalanced discrete crystal filter (DCF) with Chebyshev approximation [24,25,31]. The IF filter is designed to provide the required selectivity in accordance with current standards for VHF airborne radios. The IF frequency is 22.114 MHz which gives us the opportunity to use the fixed RF filter for the entire frequency band (118–137.000 MHz) with appropriate image frequency rejection. Measured results are presented in Figure 20c,d.

Electronics 2021, 10, 777 24 of 26

6. Discussions Besides others, the LUN-3520 (LUN-3524) radio communication system contains an AM receiver (specifically A3E with DSB) for the frequency band of 118–137 MHz. However, the receiver is professionally built and certified, which may allow interfering signals generated by power electronics to penetrate and can decrease the basic quality of normal reception in backup radio systems. As mentioned in the Introduction, a backup VHF (UHF) communication or navigation receiver and its instrument parts are typically built as simple or double-heterodyne receivers in low-dimension blocks with a low mass [5,11,29]. The design does not allow using a high level of mechanical shielding and consequently allows the intrusion of specific disturbing signals. Figure 20 shows the effect of switching with detailed analysis of the situation inside the radio circuits. Figure 20 also shows the effect of how the interference signal intrudes the reception channel, according to Equation (22), when the interference penetrates the radio at the frequency of 59 MHz. The interfering signal is 118 MHz, the intermediate frequency is in red (22.114 MHz), the distortion is in black (1 kHz), green is the interfering signal (frequency 822 Hz) produced by the power electronics, and brown is the switching frequency (2 kHz) of the power electronics. To understand how the interference signal reaches the output of the radio station, it is necessary to describe the effect of the IF filter. The IF filter is the custom-designed unbalanced discrete crystal filter (DCF) with Chebyshev approximation [24,25,31]. The IF filter is designed to provide the required selectivity in accordance with current standards for VHF airborne radios. The IF frequency is 22.114 MHz which gives us the opportunity to use the fixed RF filter for the entire frequency band (118–137.000 MHz) with appropriate image frequency rejection. Measured results are presented in Figure 20c,d.

7. Conclusions During standard as well as non-standard air trafﬁc situations, many air trafﬁc management (ATM) services are provided using standardized VHF/UHF radio channels. The same channel bandwidth is used for emergency services and, consequently, it is far more critical when emergency services are unavailable. A potential failure of VHF/UHF radio channels is due to disturbances by the electromagnetic interferences (EMI) emitted by the thousands of electronic circuits and power processing on board an aircraft and around radio systems. In aeronautical systems, the EMIs can be very dangerous for electromagnetic compatibility processes and can potentially reduce the safety level in the air transportation. On board the aircraft, there are many systems based on traditional technologies, especially in backup systems that belong, as their main counterpart, to the group of critical airborne systems. One of the backup instruments is the radio system, which is used for aeronautical communication—ATM services—as well as for navigation, where the most critical are VHF/UHF systems for approaching and landing. Although typical communication and navigation systems are normally based on SDR technologies, there are backup equipment that must operate in non-standard conditions and therefore they are based on simple analog technologies such as superheterodyne or double-superheterodyne VHF/UHF radios, ILS receivers, and/or analog indicators. Beside the use of traditional technologies, newer power electronics can cause unique types of disruptions, especially the use of SiC or GaN switches, from which their switching action can cause wideband interferences, including interferences in VHF and UHF bandwidths used for provision of communication and navigation services in aviation. The presented results show that the designed airborne receivers, based on traditional technologies, are not resistant to such new types of interferences from emerging power switching devices and there are many ways to get the disturbing signal into the receivers’ circuits that have inadequate shielding. Such interferences have various levels of danger; as the sensitivity decreases, the marker receiver indicates incorrect navigation positions. Such problems must be taken into consideration not only during the design of new aircraft systems but also through modernization and additional avionics installation, where such types of technologies can cooperate. Electronics 2021, 10, 777 25 of 26

Author Contributions: Conceptualization, J.L., R.B. and J.B.(Josef Bajer); methodology, J.L., R.B., J.B.(Josef Bajer); validation, E.B., J.B.(Jan Boril); formal analysis, J.L. and R.B.; investigation, J.L., E.B. and R.B.; writing—original draft preparation, J.L., R.B. and E.B.; writing—review and editing, J.L., R.B. and E.B; visualization, J.B.(Josef Bajer); supervision, J.L.; project administration, J.B.(Josef Bajer); All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The work presented in this paper has been supported by the Czech Republic Ministry of Defence—University of Defence development program “AIROPS”. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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