Modernized Solar Radio Spectrograph in the L Band Based on Software Deﬁned Radio

electronics

Article Modernized Solar Radio Spectrograph in the L Band Based on Software Deﬁned Radio

Pavel Puricer 1,* , Pavel Kovar 1 and Miroslav Barta 2,*

1 Department of Radioelectronics, Faculty of Electrical Engineering, Czech Technical University, Technicka 2, 166 27 Prague, Czech Republic 2 Astronomical Institute, Czech Academy of Sciences, 25165 Ondrejov, Czech Republic * Correspondence: [email protected] (P.P.); [email protected] (M.B.); Tel.: +420-22-435-5969 (P.P.)

Received: 28 June 2019; Accepted: 1 August 2019; Published: 3 August 2019

Abstract: The paper presents the concept, implementation, and test operation of a modernized solar radio spectrograph for an investigation of the solar emission and solar bursts in radio frequency bands. Besides having a strong diagnostic significance for studying the flare energy release, the solar radio bursts can also cause strong interference for radio communication and navigation systems. The current spectrograph for the Ondrejov observatory (Astronomical Institute of Czech Academy of Sciences) was modernized by using a direct-conversion receiver connected to a field-programmable gate array (FPGA) for the fast Fourier transform (FFT) spectrum estimation and put into the test operation. The higher time and frequency resolution and lower noise in comparison with the existing analog instrument were reached by the implementation of the latest optimal signal processing methods. To reduce the costs for such modernization, the operating frequency range was divided into four sub-bands of bandwidth 250 MHz, which brings another benefit of greater scalability. The first observations obtained by the new spectrograph and their comparison with the analog device are presented in the paper with future steps to put the spectrograph into the regular operation.

Keywords: spectrograph; FPGA; direct conversion receiver; software deﬁned radio

1. Introduction

1.1. Radio Diagnostics in Solar Flare Research Solar eruptions and flares represent the most powerful energy release processes in the entire solar system. Disturbances connected with these phenomena (magnetic clouds, beams of accelerated particles, enhanced radiation—namely in X-ray, UV, and radio bands) propagating from the Sun through the interplanetary space, are now known as effects of the space weather. In the case of reaching the Earth and its vicinity, they represent a serious risk for technologies (e.g., enhanced geo-currents—a risk for power distribution grids, ionospheric/TEC disturbances—an impact to the navigation systems and communications [1]) and also for humans (astronauts in the open space, aircraft crews and passengers for near-pole flights, etc.). In order to be able to give deterministic predictions of the space weather, we need to understand its drivers first. As the eruptive flares belong to those most effective, a research in the solar-flare physics represents a key to the future space weather forecasts. Among all the radiation produced by solar flares the radio emission in decimetric range is the most closely related to the plasma processes of the flare energy release. Indeed, a radio emission connected with the beams of particles accelerated during the flare via a magnetic reconnection represents the first “messenger” of the solar flare ever [2]. Radio spectrographs, which are capable to record the dynamics of spectra on very short time scales, thus represent a key tool for remote radio diagnostics of the processes that started the flare. Modern approaches to the flare energy release via a magnetic

Electronics 2019, 8, 861; doi:10.3390/electronics8080861 www.mdpi.com/journal/electronics Electronics 2019, 8, 861 2 of 16 reconnection [3] involve the energy cascading and fragmentation of the magnetic ﬁeld structures in the solar atmosphere towards small scales. The small-scale structures, which are expected (as indicated by our numerical simulations) to be the actual locations of an energy release in ﬂares, have also short dynamic timescales (millisecond and shorter). In order to be able to observe their radio signatures, the radio spectrograph should have a comparable temporal resolution. This represents a very strong driver for the spectrograph upgrade from the point of view of the astrophysical research.

1.2. Direct Influence of the Solar Radio Bursts on the Radio Systems In addition to its important diagnostic role in the solar flare research, the solar radio emission (especially high-energy solar radio bursts) has a big impact on the navigation, communication, mobile, and Internet of things (IoT) systems and in a wider consequence to current and future information and transportation systems as well [4–9]. The influence of solar emissions can be observed as an increase of the effective noise temperature (or relevant noise factor) of the receiver as described in [10].The statistics of the solar radio bursts was published for example in [11] using a Poisson distribution for a description of the solar radio burst events together with an estimation of the distribution parameters from the long-term observation data. It can serve as a source for an estimation of the probability of high intensity events and their possible threats to the radio systems. The influence of a solar radio burst to the popular satellite navigation system GPS (global positioning system) was documented in [4,6,7,12]. The unique radio burst from December 2006 with the peak level of one million SFU (solar flux unit) caused a significant reduction of the signal to noise ratio of the most GPS receivers, some receivers had lost tracking of the satellites and the acquisition of the new satellites was not possible due to the higher noise background. However, the performance degradation can occur for lower levels of radio bursts as well, for example a solar radio burst with the intensity 100,000 SFU causes in a common GPS receiver with a typical noise temperature 150 K and antenna gain 3 dB a drop of signal to noise ratio (SNR) by 15 dB that can prevent the receiver to acquire new signals [13]. The performance degradation can be also expected for the satellite-based augmentation systems (SBAS) [6]. These systems were developed as an improvement of the precision and safety of the current satellite navigation systems [14]. The precision improvement is realized by the dissemination of wide range differential corrections while the safety is ensured by the provision of integrity information. The information about the navigation system failure must be detected within 6s; therefore, the drop of the message reception probability is critical. The probability of a reception of the message depends, inter alia, on a signal to noise ratio and a bit rate, which is 10 times higher than the GPS message bit rate. Thus, the SBAS receiver requires a much higher signal to noise ratio for proper reception of the navigation messages than the GPS [14], so the aforementioned SNR drop can surely endanger the integrity of the satellite navigation system. Even though the observations of a degradation of the SNR of GPS and SBAS caused by the solar bursts were documented in [6], the detailed information about the impact of observed phenomena onto the system performance is however missing and should be an object of a future research.

1.3. The Motivation for Modernization of Solar Radio Spectrography Instruments The solar radio emission in L band is in the research focus of the Astronomical Institute of the Czech Academy of Science in Ondrejov [15] for many years. The current scientific instrumentation RT5 (Figure1) is more than 25 years old. Its performance and architecture reflect the time of development. The spectrograph based on an analog technology is in the operation since 1993 without significant design changes. Its performance like a resolution in time and frequency as well as its dynamic range and linearity is currently below the current requirements demanded by both the advance in technology and the increasing level of knowledge in the solar plasma physics. Electronics 2019, 8, 861 3 of 16 Electronics 2019, 8, 861 3 of 16

Figure 1. Antenna of Ondrejov RT5 solar radio spectrographspectrograph telescope with 9.5 m antenna diameter and operating frequency range 0.8–2.0 GHz.

The aim of the modernization project is to developdevelop a modern but cost-ecost-effectiveffective instrument comparable or superior to the equipment of other observatories. The modern telescopes are usually designed as wideband radio receivers with a digita digitall signal processing [16,17] [16,17] and their technology enables us toto estimateestimate thethe frequency frequency spectrum spectrum in in parallel parallel for for all all frequency frequency bins bins in comparisonin comparison with with the theanalog analog technology technology that that is based is based on aon scanning a scanning principle. principle. The analoganalog spectrumspectrum estimation estimation is usuallyis usually based based on the on application the application of a super of a heterodynesuper heterodyne receiver receiverthat sequentially that sequentially scans the scans frequency the frequency spectrum sp channel-by-channelectrum channel-by-channel and measures and the measures signal power the signal [18]. powerThe modern [18]. digitalThe modern methods digital are based methods on an are application based on of an the application discrete Fourier of the transform discrete (DFT)Fourier or transformcomputationally (DFT) optimizedor computationa fast Fourierlly optimized transform fast (FFT) Fourier [19–21 ].tran Thesform common (FFT) method [19–21]. for The a consequent common methodspectrum for estimation a consequent based spectrum on the Fourier estimation transform based ison a the short-time Fourier Fouriertransform transform is a short-time (STFT) Fourier [22,23]. transformThere (STFT) are several [22,23]. possible ways how to implement such spectrum computation algorithms. Therefore,There theare critical several design possible analysis ways has how to beto madeimplement to choose such an spectrum optimal design computation not only algorithms. to fulfill the Therefore,system requirements the critical butdesign also analysis to respect has implementation to be made to issueschoose and an optimal realization design costs. not only to fulfill the systemThe observations requirements and but records also to from respect the modernizedimplementation spectrograph issues and will realization be used bothcosts. for a study of solarThe processes observations and for and an investigationrecords from ofthe the modernized impact of aspectrograph solar radio emission will be used onto both the currentfor a study and offuture solar communication processes and andfor an radio investigation navigation of systems the impact in the of frame a solar of radio several emission research onto projects. the current and future communication and radio navigation systems in the frame of several research projects. 2. Design Concepts Analysis 2. Design Concepts Analysis 2.1. Technical Requirements 2.1. TechnicalThe technical Requirements requirements on the solar radio spectrograph were defined on the base of the long-termThe technical experiments requirements with the currenton the solar spectrograph radio spectrograph with respect were to the defined parameters on the of base the similarof the long-terminstruments experiments [17,18,24,25 with]. the current spectrograph with respect to the parameters of the similar instrumentsThe main [17,18,24,25]. attention during the conceptual design was paid to a high sensitivity and a wide dynamicThe range.main attention The secondary during design the conceptual parameters design superseding was paid the to parameters a high sensitivity of the current and a analog wide dynamicinstrument range. [15] were:The secondary design parameters superseding the parameters of the current analog instrument [15] were: Processing frequency range min. 1/2 GHz. • • FrequencyProcessing binsfrequency spacing range<1 MHz. min. 1/2 GHz. • • TimeFrequency resolution bins spacing1 ms. <1 MHz. • Time resolution ≤≤1 ms. Electronics 2019, 8, 861 4 of 16

The high sensitivity requirement means that the noise floor of the telescope is below the quiet Sun radiation (QSR) level to be able to recognize its variation. The spectral flux density of the quiet Sun varies from 50 to 300 SFU while the maximal observed solar burst flux is up to 1,000,000 SFU [11]. The classical approach of the radio spectrograph design is based on a super heterodyne receiver that sequentially scans individual frequency bins. The number N of the investigated bins can be expressed as: fmax fmin N = − , (1) ∆ f where fmax f is a frequency range and ∆ f is a frequency spectra bins spacing. The processing time − min tp of an individual bin is: tr tp = tt, (2) N − where tr is a time resolution and tt is a time for returning to the next frequency bin. Let us note, that tt consists of a time for local oscillator retuning and a time for the removing of the transmission of the receiver filters. The time of measurement (i.e., the averaging in each frequency bin) is very short and thus the sensitivity (noise suppression) is poor. A modern approach oriented on the digital processing is based on the digitalization of the whole signal in early stages of the reception chain and frequency spectrum is then estimated by FFT. The results are further smoothed (a Bartlett’s approach [26]) with the aim of reduction of the power spectra estimation variance. The averaged samples of the power spectral density Ck0 are expressed by:

M 1 1 X− C = C , (3) k0 M k,m m=0 where Ck,m is a discrete power spectral density calculated from the m-th vector of input samples and M is a number of averaged consecutive vectors of the signal samples. The variance of the frequency spectra estimation is reduced M times.

h i 1 h i var C = var C . (4) k0 M k,m Since M N and number of frequency bins is several hundreds or thousands, the typical ≥ improvement of SNR in the modern “parallel” approach is about 20 dB or more in comparison to the “serial scanning” approach in the past.

2.2. Receiver Hardware Concept There are several existing projects of analog scanning and digital FFT based solar spectrographs [18, 24,25,27,28], where usually prevails an approach of the reception, detection, and sensing of signal in the analog domain with a final conversion of the detector output to digital data for the storage and viewing. On the other hand, a modern digital spectrograph is de facto a wideband receiver equipped with the FFT processor for the signal spectrum calculation from samples produced by an analog-to-digital converter (ADC) working in baseband or intermediate-frequency band output of receiver. As the spectrograph is a unique device, there are no usable application specific integrated circuits (ASICs) for its realization (apart from the extremely high costs for their development), so the digital processor is usually programmed to a field-programmable gate array (FPGA) or a digital signal processor (DSP) [24]. This architecture is often called a software radio or a software defined radio (SDR). The preliminary design study identifies several critical problems:

1. Need of the high-speed analog-to-digital converters; 2. Cost of the high-performance FPGA; Electronics 2019, 8, 861 5 of 16

Electronics 2019, 8, 861 5 of 16 3. Software IP cores for FPGA for interfacing the ADC; 4. Challenging design of high selectivity wideband radio frequency (RF) filtersfilters oror intermediateintermediate frequencyfrequency (IF) filters. filters. The first design concept assumed to use a high-speed ADC with the several gigahertz sampling The first design concept assumed to use a high-speed ADC with the several gigahertz sampling rate to be able to process a full 1 GHz bandwidth in a single channel (Figure 2a). Such a choice would rate to be able to process a full 1 GHz bandwidth in a single channel (Figure2a). Such a choice however lead to a very high cost of such ADC and a necessity to use a commercial proprietary IP would however lead to a very high cost of such ADC and a necessity to use a commercial proprietary (intellectual property) core for the interface between the ADC and the FPGA through JESD204B IP (intellectual property) core for the interface between the ADC and the FPGA through JESD204B standard [29] that was also very expensive. The further cost analyses had shown that if the standard [29] that was also very expensive. The further cost analyses had shown that if the processing processing bandwidth would be divided into several sub-bands (a multi-channel approach, Figure 2b), bandwidth would be divided into several sub-bands (a multi-channel approach, Figure2b), the much the much cheaper ADCs with LVDS (low-voltage differential signaling) interface that does not need cheaper ADCs with LVDS (low-voltage differential signaling) interface that does not need an expensive an expensive IP core can be used and thus the final cost of the instrument would be then IP core can be used and thus the final cost of the instrument would be then significantly reduced. Let significantly reduced. Let us note, that there is no need to process the working bandwidth in a single us note, that there is no need to process the working bandwidth in a single channel neither from signal channel neither from signal processing nor scientific point of view. The cost saving brought by the processing nor scientific point of view. The cost saving brought by the multi-channel solution is so multi-channel solution is so high that the total hardware cost of four channels is lower than the price high that the total hardware cost of four channels is lower than the price of JESD204B IP core for the of JESD204B IP core for the FPGA. Therefore, we decided to use the multi-channel concept in our FPGA. Therefore, we decided to use the multi-channel concept in our modernization project. modernization project.

(a)

(b)

FigureFigure 2.2. Spectrograph basic concept:concept: (a) Single channel and (b) multi-channel.

The problemproblem withwith a a design design and and realization realization of highof high selectivity selectivity RF orRF IF or filters, IF filters, required required in the in front the endfront for end a suppression for a suppression of the overlapping of the overlapping spectral components spectral components in case of the in heterodyne case of the concept, heterodyne can be solvedconcept, by can a selection be solved of theby a direct selection conversion of the receiverdirect conversion (Figure3) thatreceiver requires (Figure high 3) selectivity that requires low-pass high filtersselectivity instead low-pass of the band-passfilters instead ones. of Letthe usband-pass note, that ones. the low-passLet us note, filter that with the a bandwidthlow-pass filter of several with a hundredbandwidth megahertz of several can hundred be realized megahe as artz lumped can be elements realized filter as a fromlumped standard elements SMD filter components from standard while theSMD band-pass components IF filter while of a the super band-pass heterodyne IF filter receiver of a or super the RF heterodyne filter of a tuned receiver radio or receiverthe RF filter front endof a musttuned be radio realized receiver as a customizedfront end must product be realized based on as the a expensivecustomized ceramic product resonator based on filter the technology expensive orceramic on other resonator special filterfiltertechnology. technology The or on selectivity other special of the RFfilter bandpass technology. filter atThe the selectivity input of front of the end RF is notbandpass critical filter because at the it shallinput onlyof front additionally end is not suppress critical because the products it shall outside only additionally the processed suppress band 1–2the GHzproducts and itsoutside realization the processed is common band for 1–2 all GHz channels. and its realization is common for all channels. Electronics 2019, 8, 861 6 of 16 Electronics 2019, 8, 861 6 of 16

FigureFigure 3.3. FrontFront endend realized realized by by a directa direct conversion conversion receiver receiver with with an optionalan optional radio radio frequency frequency (RF) ﬁlter(RF) andfilter LNA and (lowLNA noise(low noise ampliﬁer) amplifier) at the at input. the input.

OnOn thethe otherother hand,hand, aa directdirect conversionconversion receiverreceiver [[30–33]30–33] susuffersﬀers fromfrom thethe followingfollowing problems: problems:

1.1. Problem with the semiconductor noise 11/f./f. 2.2. DC ooffsets,ffsets, and and an an interference interference of theof localthe oscillatorlocal oscillator that oscillates that oscillates in the middle in the of middle the processing of the processingfrequency band.frequency band. 3.3. Amplitude and phase unbalance that causes a crosstalk betweenbetween upper and lower sidebands. Fortunately, the first problem can be solved by the careful design of the level plan. The signal Fortunately, the first problem can be solved by the careful design of the level plan. The signal must be sufficiently amplified before the conversion to the baseband to avoid an influence of the 1/f must be sufficiently amplified before the conversion to the baseband to avoid an influence of the 1/f noise. In our case, the power consumption of the receiver front end is not critical so using a wide noise. In our case, the power consumption of the receiver front end is not critical so using a wide dynamic range quadrature mixer with higher power consumption enables successful mixing of the dynamic range quadrature mixer with higher power consumption enables successful mixing of the high-level signal from the amplified input stage. high-level signal from the amplified input stage. The problem with a DC offset can be solved by compensation feedback circuits, however paid The problem with a DC offset can be solved by compensation feedback circuits, however paid by by the distortion of the central frequency on which oscillates a local oscillator. The most radio the distortion of the central frequency on which oscillates a local oscillator. The most radio systems systems count with this fact and use modulation schemas that are resistive to this distortion. count with this fact and use modulation schemas that are resistive to this distortion. The solution to this problem in our design is to use a higher frequency resolution of the signal. The solution to this problem in our design is to use a higher frequency resolution of the signal. The distorted frequency bin is ignored and replaced by an interpolated value. The distorted frequency bin is ignored and replaced by an interpolated value. The crosstalk between upper and lower sidebands caused by the receiver imperfection or The crosstalk between upper and lower sidebands caused by the receiver imperfection or insufficient suppression is a problem that can completely spoil the measurement or can bring insufficient suppression is a problem that can completely spoil the measurement or can bring unwanted unwanted effects in the spectrum. The crosstalk suppression is expressed as: effects in the spectrum. The crosstalk suppression is expressed as: 1𝐴 2𝐴𝑐𝑜𝑠𝜃 𝐵𝑆𝑑𝐵𝑐 10𝑙𝑜𝑔 2 , (5)  11+ A𝐴 22A𝐴ucos𝑐𝑜𝑠𝜃θU  BS(dBc) = 10log U− , (5)  2  − 1 + A + 2AucosθU where 𝐴 and 𝜃 are the amplitude and phase unbalance,U respectively. A typical amplitude and phase unbalance of an uncompensated quadrature direct conversion mixer is about 0.5 dB and 1°. where AU and θU are the amplitude and phase unbalance, respectively. A typical amplitude and The crosstalk suppression is then about 30 dB. phase unbalance of an uncompensated quadrature direct conversion mixer is about 0.5 dB and 1◦. The The higher suppression can be achieved by the mixer calibration typically with the help of the crosstalk suppression is then about 30 dB. feedback circuits. The receiver front end must be equipped with a possibility to control the DC offset The higher suppression can be achieved by the mixer calibration typically with the help of the and gain of the baseband I and Q branches and adjust the phase shift of the local oscillator signal. feedback circuits. The receiver front end must be equipped with a possibility to control the DC offset The properly designed compensation can increase the cross talk suppression up to 60 dB that is and gain of the baseband I and Q branches and adjust the phase shift of the local oscillator signal. The enough for the most applications. properly designed compensation can increase the cross talk suppression up to 60 dB that is enough for The outputs of DC offset, amplitude, and phase imbalance detectors can be expressed: the most applications. The outputs of DC offset, amplitude, and phase imbalance detectors can be expressed: 𝑑, 𝐼 ,𝑑, 𝑄, (6) X X dDC,I = In, dDC,Q = Qn, (6) 𝑑n𝐼 𝑄n, (7) X X 2 2 damp = I∑n Qn, (7) 𝑑 − . (8) n∑ ∑n Electronics 2019, 8, 861 7 of 16

Electronics 2019, 8, 861 P 7 of 16 n InQn dp = . (8) P 2 + P 2 The values, produced by these detectors, aren In thenn usedQn for a compensation of negative effects describedThe values, above by produced controlling by these the oscillator detectors, block are thenand variable used for gain a compensation amplifiers (VGA) of negative in both e ﬀI ectsand describedQ branches. above by controlling the oscillator block and variable gain ampliﬁers (VGA) in both I and Q branches.The outputs of I and Q branches of the front-end receiver are then passed to dual-channel ADC and Thethe outputssamples ofare I and processed Q branches in a of FPGA the front-end realizing receiver the FFT are thencomputation passed to dual-channelof the full-bandwidth ADC and thespectrum. samples The are computed processed insamples a FPGA of realizing the spectrum the FFT are computation passed through of the a full-bandwidth standard interface spectrum. to a Theconnected computed computer samples or server of the for spectrum further areprocessi passedng, throughviewing, aand standard storage. interface The FPGA to a can connected be also computerprogrammed or server to realize for further a detection processing, of the viewing, receiver and imperfections storage. The described FPGA can above be also and programmed provide the to realizeresults ato detection the control of theand receiver setup processor imperfections for the described compensation above purposes. and provide the results to the control and setupTo be processor able to for simply the compensation combine measuremen purposes. ts of several spectrograph channels, the measurementsTo be able tomust simply be synchronized combine measurements with 1 PPS of several(pulse spectrographper second) pulses. channels, The the source measurements of such mustsynchronization be synchronized signal with can 1be PPS for (pulseexample per a second) 1 PPS output pulses. provided The source by ofa GPS such receiver synchronization or an arbitrary signal canfrequency be for examplestandard. a If 1 PPSthe pulses output are provided not present, by a GPSthe spectrograph receiver or an channels arbitrary are frequency switched standard. to the free If therunning pulses mode. are not The present, measurement the spectrograph cycle is prolonged channels on are to switched1.1 s. The to complete the free block running scheme mode. of Theone measurementchannel is at Figure cycle is4. prolonged on to 1.1 s. The complete block scheme of one channel is at Figure4.

Figure 4. Spectrograph channel block diagram.

The communicationcommunication between between blocks blocks and and the outputthe ou interfacetput interface can be basedcan be on based common on standardscommon (SPI,standards I2C, USB)(SPI, accordingI2C, USB) toaccording parameters to paramete of chosenrs hardware of chosen components. hardware components. The general The control general and setupcontrol are and then setup usually are then provided usually by provided some kind by of some microcontroller kind of microcontroller unit (MCU). unit (MCU).

3. Implementation and Results 3. Implementation and Results 3.1. Spectrograph Hardware 3.1. Spectrograph Hardware The described hardware concept was applied for a four-channel spectrograph in a frequency The described hardware concept was applied for a four-channel spectrograph in a frequency range of 1–2 GHz (Figure5) using the same RF band-pass filters and flat gain amplifiers both for the range of 1–2 GHz (Figure 5) using the same RF band-pass filters and flat gain amplifiers both for the common RF input and for the front-end part of the channels. The RF filter is an of-the-shelf LTCC common RF input and for the front-end part of the channels. The RF filter is an of-the-shelf LTCC (low temperature cofired ceramics) filter (BFCG-162W+) with the pass band 950–2200 MHz, pass band (low temperature cofired ceramics) filter (BFCG-162W+) with the pass band 950–2200 MHz, pass insertion loss 1.8 dB, and the stop band insertion loss better than 25 dB. The flat gain amplifier is band insertion loss 1.8 dB, and the stop band insertion loss better than 25 dB. The flat gain amplifier based on an off-the-shelf 40–4000 MHz RF gain block (TRF37C75) with 18 dB gain. These common is based on an off-the-shelf 40–4000 MHz RF gain block (TRF37C75) with 18 dB gain. These common blocks are placed together with an RF channel splitter, power supply modules, and a GPS module (for blocks are placed together with an RF channel splitter, power supply modules, and a GPS module 1 PPS signal provision) to separate the board. Each spectrograph channel is then constructed as an (for 1 PPS signal provision) to separate the board. Each spectrograph channel is then constructed as independent block in its own case (Figure6). The hardware parameters are summarized in Table1. an independent block in its own case (Figure 6). The hardware parameters are summarized in Table 1. Electronics 2019, 8, 861 8 of 16 ElectronicsElectronics 20192019,, 88,, 861861 88 ofof 1616

FigureFigure 5. 5. RealizedRealized spectrographspectrograph block block sche schemeschememe (only (only one one channel channel depicted). depicted).

((aa)) ((bb))

FigureFigure 6.6. SpectrographSpectrograph (single(single channel):channel): ((a (aa)) ) populatedpopulated populated printableprintable printable circuitcircuit circuit boardboard board (( (bb))) housedhoused to to the the aluminumaluminum case. case. Table 1. Spectrograph hardware parameters (values for one channel). TableTable 1.1. SpectrographSpectrograph hardwarehardware parameteparametersrs (values(values forfor oneone channel).channel). Parameter Value Note ParameterParameter ValueValue Note Note Usable bandwidth 250 MHz UsableUsable bandwidthbandwidth 250 250 MHzMHz Operating range 400 MHz/6 GHz (Center frequency) Operating range 400 MHz/6 GHz (Center frequency) OperatingFull Scale range 400 MHz/625/+5 GHz dBm Programmable, (Center frequency) step 1dB − FullFullSampling ScaleScale rate −−25/+525/+5 310 dBmdBm MHz Programmable, Programmable, stepstep 1dB1dB SamplingSamplingADC resolution raterate 310 310 2 MHzMHz14 bits IQ sampling × ADCADCBaseband resolutionresolution ﬁlters 6th order 2× 2× Cauer,1414 bitsbits symmetrical IQ IQ samplingsampling Filter pass-band 0/125 MHz, 0.5 dB ripple BasebandBaseband filtersfilters 6th 6th orderorder Cauer,Cauer, symmetricalsymmetrical Stop-band 155 MHz, Attenuation 50 dB ≥ ≥ FilterFilterData pass-bandpass-band interface 0/125 0/125 MHz,MHz, USB 0.50.5 2.0 dB devicedB rippleripple SynchronizationStop-bandStop-band ≥≥155155 MHz,MHz, AttenuationAttenuation 1 PPS ≥≥5050 dBdB Free running mode 1.1s if 1 PPS signal is not present DataData interfaceinterface USB USB 2.02.0 devicedevice 3.2.SynchronizationSynchronization FPGA Signal Processor Content 1 1 PPSPPS Free Free runningrunning modemode 1.1s1.1s ifif 11 PPSPPS signalsignal isis notnot presentpresent

3.2.3.2. FPGAFPGAThe receiver SignalSignal ProcessorProcessor is equipped ContentContent with a medium rate FPGA Artix7 by Xilinx (San Jose, CA, USA). The block diagram of the FPGA conﬁguration in Figure7 demonstrates a sequence of processing operations. TheThe receiverreceiver isis equippedequipped withwith aa mediummedium raterate FPGAFPGA Artix7Artix7 byby XilinxXilinx (San(San Jose,Jose, CA,CA, USA).USA). TheThe blockblock diagramdiagram ofof thethe FPGAFPGA configurationconfiguration inin FiFiguregure 77 demonstratesdemonstrates aa sequencesequence ofof processingprocessing Electronics 2019, 8, 861 9 of 16 Electronics 2019, 8, 861 9 of 16

Inoperations. order to meet In order the timing to meet constraints, the timing one constraints, second of aone sampled second signal of a sampled data stream signal is separated data stream in the is splitterseparated block in the to two splitter consecutive block to half two sized consecutive sub-blocks half that sized are sub-blocks put into two that parallel are put branches. into twoThe parallel FFT operationbranches. withThe FFT length operation 1024 is with therefore length calculated 1024 is th onlyerefore with calculated a half of processedonly with dataa half block of processed and can workdata block on a halfand ofcan the work sampling on a half frequency, of the sampling i.e., 155 MHz frequency, (details i.e., in Table155 MHz2). (details in Table 2).

FigureFigure 7.7. Field-programmable gate arrayarray (FPGA)(FPGA) blockblock diagram.diagram.

TableTable 2.2. SignalSignal processorprocessor settings.settings. Parameter Value Parameter Value FFT length 1024 FrequencyFFT bin length size 302,704.375 1024 Hz NumberFrequency of averaging bin (M) size 302,704.375 300 Hz Number of averaging (M) 300 The next block executes a signal spectrum averaging of 300 FFT measurements with the averaging The next block executes a signal spectrum averaging of 300 FFT measurements with the time of 1 ms. This block is synchronized to the 1 PPS signal to be able to synchronize the spectrograph averaging time of 1 ms. This block is synchronized to the 1 PPS signal to be able to synchronize the measurement onto an external time signal. spectrograph measurement onto an external time signal. The measurement is then sent via the USB 2.0 interface to the server for storage and further The measurement is then sent via the USB 2.0 interface to the server for storage and further processing. The data management software uses a standard FITS (flexible image transport system) processing. The data management software uses a standard FITS (flexible image transport system) format for data files [34]. format for data files [34]. The FPGA also realizes the detectors for a determination of the DC offset, amplitude imbalance, The FPGA also realizes the detectors for a determination of the DC offset, amplitude imbalance, and phase imbalance. The compensation loops are closed through an adjacent ARM processor that and phase imbalance. The compensation loops are closed through an adjacent ARM processor that controls through a SPI interface DC offsets, amplitude, and phase of the quadrature mixer. The other controls through a SPI interface DC offsets, amplitude, and phase of the quadrature mixer. The other function of the ARM processor is a receiver general control and a setup of the front end and ADC. function of the ARM processor is a receiver general control and a setup of the front end and ADC. 3.3. Testing 3.3. Testing During the spectrograph development and realization, the device was tested to verify the design parametersDuring fulfillment.the spectrograph The test development operation hadand startedrealization, in the the summer device ofwas 2018 tested at theto verify Ondrejov the astronomicaldesign parameters observatory fulfillment. (Figure The8) withtest operation the aim of had verification started ofin the spectrographsummer of 2018 performance at the Ondrejov and a comparisonastronomical of observatory its measurements (Figure with 8) with the currentthe aim analogof verification device. Theof the developed spectrograph spectrograph performance was testedand a withcomparison the backup of its 8 mmeasurements dish antenna originatedwith the cu fromrrent the analog Second device. World The War developed German radar spectrograph while the currentwas tested analog with spectrograph the backup 8 is m installed dish antenna at the modernoriginated 9.5 from m dish the antenna Second that World profits War from German the higher radar gain.while The the reasoncurrent for analog such spectrograph installation was is installed to be able at tothe test modern the new 9.5 design m dish parameters antenna that and profits operation from setupthe higher without gain. interruption The reason of for the such current installation astronomical was to experiments. be able to test Due the tonew a preliminary design parameters character and of theoperation tests, only setup one without channel interruption tuned to the of center the current frequency astronomical 1525 MHz experiments. was put in the Due operation. to a preliminary The test frequencycharacter bandof the was tests, chosen only to one cover channel the common tuned satelliteto the center navigation frequency systems’ 1525 frequencies MHz was to put be ablein the to investigateoperation. The the solartest frequency burst impacts band on was these chosen systems to ascover stated the in common the previous satellite part navigation of the paper. systems’ frequencies to be able to investigate the solar burst impacts on these systems as stated in the previous part of the paper. Electronics 2019, 8, 861 10 of 16

Electronics 2019, 8, 861 10 of 16 Electronics 2019, 8, 861 10 of 16

Figure 8. Spectrograph installation for tests at the Ondrejov observatory (one channel block and a common board with RF splitter, global positioning system (GPS) module, and power supply

modules). Figure 8. Spectrograph installation for for tests at the On Ondrejovdrejov observatory (one channel block and a 3.4. Testcommon Results boardboard with with RF RF splitter, splitter, global global positioning positioning system system (GPS) module,(GPS) module, and power and supply power modules). supply modules). 3.4. TestDuring Results the early one-year test period only one observable event at 20 March 2019 was registered. The reason is that the Sun activity is at its long-term minimum. The spectrum registered 3.4. TestDuring Results the early one-year test period only one observable event at 20 March 2019 was registered. by the current spectrograph is on Figure 9, while the measurements of the new digital spectrograph The reasonDuring isthe that early the Sunone-year activity test is period at its long-term only one minimum. observable The event spectrum at 20 March registered 2019 by was the are on Figures 10–13. All spectrograms were depicted relative to reference time (tr = 0 s) 11:11 UTC. registered.current spectrograph The reason is is on that Figure the Sun9, while activity the measurementsis at its long-term of theminimum. new digital The spectrographspectrum registered are on Figures 10 and 11 represent the same spectrum part as a spectrogram to respect the style of Figure 9 byFigures the current 10–13. Allspectrograph spectrograms is on were Figure depicted 9, whil relativee the measurements to reference time of (thetr = new0 s) 11:11digital UTC. spectrograph Figures 10 and as a 3D spectrum waterfall plot. There was chosen an interval of 40 s that symmetrically covers areand on 11 Figuresrepresent 10–13. the same All spectrograms spectrum part were as a spectrogramdepicted relative to respect to reference the style time of Figure(tr = 0 9s) and 11:11 as UTC. a 3D the reference time for a demonstration of a modernized spectrograph data in full resolution. The Figuresspectrum 10 waterfall and 11 represent plot. There the was same chosen spectrum an interval part as of a 40spectrogram s that symmetrically to respect the covers style the of reference Figure 9 datafile of the related interval represents a file of size 200 MB, therefore the long-time records are andtime as for a a3D demonstration spectrum waterfall of a modernized plot. There was spectrograph chosen an data interval in full of resolution.40 s that symmetrically The datafile covers of the stored as one-second averages (Figure 13). They are also used for a coarse detection of solar events therelated reference interval time represents for a demonstr a file of sizeation 200 of MB, a modernized therefore the spectrograph long-time records data are in storedfull resolution. as one-second The by an expert observer. The reference value of quiet Sun for all graphs is 100% (shown as 20 dB of datafileaverages of (Figure the related 13). Theyinterval are alsorepresents used for a file a coarse of size detection 200 MB, of therefore solar events the bylong-time an expert records observer. are relative power Pr for new spectrograph images and as a ratio 1 for the old analog spectrograph). The Thestored reference as one-second value of averages quiet Sun (Figure for all 13). graphs They is 100%are also (shown used asfor 20 a coarse dB of relative detection power of solar Pr for events new drops of the spectrum on the sides of the evaluated band were caused by channel low-pass filter byspectrograph an expert observer. images and The as reference a ratio 1 value for the of old quie analogt Sun spectrograph).for all graphs is The 100% drops (shown of the as spectrum 20 dB of characteristics. This drop would be compensated by software processing during combination of relativeon the sides power of P ther for evaluated new spectrograph band were images caused and by channelas a ratio low-pass 1 for thefilter old analog characteristics. spectrograph). This drop The overlapping adjacent channels. woulddrops of be the compensated spectrum byon softwarethe sides processing of the evaluated during band combination were caused of overlapping by channel adjacent low-pass channels. filter characteristics. This drop would be compensated by software processing during combination of overlapping adjacent channels.

Figure 9. SpectrogramSpectrogram of of solar solar radio radio burst, burst, 20 20 March March 2019, 2019, registered registered by by current current analog analog spectrograph. spectrograph. The rectanglesrectangles show show related related portions portions of the of spectrogram the spectrogram for a comparison for a comparison with the new with spectrograph, the new

spectrograph,Figure 10—gray, Figure Figure 10—gray, 13 (ﬁrst Figure 5 min)—orange). 13 (first 5 min) The—orange). colorbar values The colorbar are expressed values asare relative expressed values as relativeFigureof the quiet 9. values Spectrogram Sun of radiation the quiet of solar (QSR) Sun radio radiation level burst, (QSR (QSR) 20= March1). level 2019,(QSR registered = 1). by current analog spectrograph. The rectangles show related portions of the spectrogram for a comparison with the new spectrograph, Figure 10—gray, Figure 13 (first 5 min)—orange). The colorbar values are expressed as relative values of the quiet Sun radiation (QSR) level (QSR = 1). Electronics 2019, 8, 861 11 of 16

ElectronicsElectronics2019 2019, ,8 8,, 861 861 11 ofof 1616

Figure 10. Spectrogram of solar radio burst, 20 March 2019, registered by the new spectrograph. The colorFigure scale 10. isSpectrogram shown in dB of relative solar radio to QSR burst, = 20 20 dB. March 2019, registered by the new spectrograph. The Figure 10. Spectrogram of solar radio burst, 20 March 2019, registered by the new spectrograph. The color scale is shown in dB relative to QSR = 20 dB. color scale is shown in dB relative to QSR = 20 dB.

Figure 11. Solar radio burst (detailed 3D spectrum waterfall plot), March 20, 2019 registered by the Figure 11. Solar radio burst (detailed 3D spectrum waterfall plot), March 20, 2019 registered by the new spectrograph (the time scale orientation is switched for the sake of better readability). new spectrograph (the time scale orientation is switched for the sake of better readability). Figure 11. Solar radio burst (detailed 3D spectrum waterfall plot), March 20, 2019 registered by the new spectrograph (the time scale orientation is switched for the sake of better readability). Electronics 2019, 8, 861 12 of 16

ElectronicsElectronics2019 2019,,8 8,, 861 861 1212 of 1616

Figure 12. Solar radio burst (time record for selected frequencies), 20 March 2019 registered by the

Figurenew spectrograph. 12. Solar radio burst (time record for selected frequencies), 20 March 2019 registered by the Figure 12. Solar radio burst (time record for selected frequencies), 20 March 2019 registered by the new spectrograph. new spectrograph.

Figure 13. One-second averages (15 min total) of the 3D spectrum waterfall plot, 20 March 2019, reference time 11:11 UTC, registered by the new spectrograph. Figure 13. One-second averages (15 min total) of the 3D spectrum waterfall plot, 20 March 2019,

reference time 11:11 UTC, registered by the new spectrograph. ToFigure consider 13. One-second the system averages initial parameters,(15 min total) the of the measurements 3D spectrum forwaterfall a quiet plot, Sun 20 were March executed 2019, as well (Figure 14) and compared to the values obtained by a system with the antenna pointed to the clear Toreference consider time the 11:11 system UTC, registeredinitial parameters, by the new the spectrograph. measurements for a quiet Sun were executed as sky and during the night to determine a QSR-to-noise system ratio. well (Figure 14) and compared to the values obtained by a system with the antenna pointed to the To consider the system initial parameters, the measurements for a quiet Sun were executed as clear sky and during the night to determine a QSR-to-noise system ratio. well (Figure 14) and compared to the values obtained by a system with the antenna pointed to the clear sky and during the night to determine a QSR-to-noise system ratio. Electronics 2019, 8, 861 1313 of of 16

Figure 14. Quiet Sun case 3D spectrum waterfall plot obtained by the modernized spectrograph (a 10 Figuremin sample 14. Quiet for relative Sun case time 3D 11:40spectrum UTC). waterfall plot obtained by the modernized spectrograph (a 10 min sample for relative time 11:40 UTC). The observed difference of the signal level of about 3 dB for quiet Sun vs. clear sky does not soundThe very observed optimistic, difference but we of have the to signal take intolevel account of about the 3 non-optimal dB for quiet state Sun ofvs. the clear used sky old does antenna not soundin comparison very optimistic, to the final but installation. we have to Thetake most into significantaccount the improvement non-optimal can state be of achieved the used by old the antennaplacement in ofcomparison a low noise to amplifierthe final installation. directly to theThe antenna most significant feed output improvement as is in the can case be of achieved the RT5 byantenna. the placement Nevertheless, of a low we noise were amplifier still be able directly to observe to the quantitative antenna feed changes output of as the is spectrumin the case level of the in RT5comparison antenna. with Nevertheless, the quiet Sun, we thereforewere still we be can able expect to observe even better quantitative results forchanges the full of operation the spectrum state. levelThe expected in comparison values inwith the futurethe quiet are aboutSun, therefor 10% of thee we quiet can Sun expect level, even closer better to the results pure QSRfor the to noise full operationratio obtained state. from The theexpected comparison values ofin athe QSR future measurement are about with10% of a nightthe quiet operation, Sun level, where closer the to ratios the purewere 10QSR dB to for noise single ratio bin spikes obtained of the from spectrum the comparison and 15 dB for of the a averageQSR measurement value of noise. with a night operation,The frequency where the distribution ratios were of the 10 noisedB for level single in the bin realized spikes one of channelthe spectrum band is and mainly 15 determineddB for the averageby the used value low-pass of noise. filter characteristics as can be seen in 3D plots. Besides this impact, there were observedThe frequency two spurs ondistribution frequency of bins the 1.436 noise GHz level and in 1.615the realized GHz caused one bychannel internal band circuits is mainly of the determinedreceiver. These by the single-bin used low-pass spurs will filter be characteristics excluded from as the can final be dataseen duringin 3D plots. the signal Besides processing this impact, and theresubstituted were byobserved an interpolated two spurs value on frequency from adjacent bins bins. 1.436 GHz and 1.615 GHz caused by internal circuitsThe of testing the receiver. measurements These single-bin were also spurs used will to be identify excluded main from sources the final of radio data interference.during the signal The processingantenna placement and substituted is on the by south-western an interpolated slope value of the from hill adjacent in an elevation bins. of about 510 m. The nearby OndrejovThe testing village measurements is about 1 km fromwere thealso observatory used to identify and withmain an sources elevation of radio of about interference. 450 m, which The antennahelps to slightlyplacement reduce is on the the impact south-western of a possible slope close ofindustrial the hill in interference. an elevation Main of about distant 510 sources m. The of nearbythe radio Ondrejov interference village (considering is about 1 km the from final fullthe observatory frequency range and with 1–2 GHz) an elevation can be aof cellular about 450 phone m, whichnetwork helps transmitter to slightly or areduce nearby the mobile impact phone of a (850 possible MHz, close 950 MHz, industrial and 1800interference. MHz band Main and distant higher sourcesharmonics of ofthe 850 radio MHz interference band) and a (considering digital terrestrial the TVfinal transmitter full frequency (higher ra harmonicsnge 1–2 GHz) of 562–706 can be and a cellular714–858 phone MHz bands).network The transmitter impact of or these a nearby interference mobilesources phone (850 can beMHz, suppressed 950 MHz, by and preventing 1800 MHz the bandantenna and pointing higher directlyharmonics to the of transmitter850 MHz locations.band) and The a digital other sourceterrestrial of the TV interference transmitter signal (higher can harmonicsbe an ADS-B of (automatic562–706 and dependent 714–858 surveillance—broadcast)MHz bands). The impact signal of these at the interference frequency 1090sources MHz. can The be suppressedsignal is a product by preventing of an aircraft the surveillanceantenna pointing system directly transmitted to the in regulartransmitter intervals locations. during The the other flight sourceand it canof the pose interference the notable signal source can of interference be an ADS-B because (automatic the location dependent of a transmitter surveillance—broadcast) can be directly signalin the antennaat the frequency beam due to1090 the aircraftMHz. The movement signal acrossis a product the sky. Fortunately,of an aircraft such surveillance interference system will be transmittedactive only forin aregular short timeintervals interval. during the flight and it can pose the notable source of interference becauseThe the notable location interference of a transmitter was during can the be test dire operationctly in observedthe antenna at frequencies beam due 1.458to the GHz aircraft with movementrelative power across 36 dBthe and sky. 1.467 Fortunately, GHz with such relative interferen powerce 27 will dB. be Although active only the sourcefor a short of the time interference interval. was notThe identified, notable interference the observed was levels during are withinthe test designed operation dynamic observed range at frequencies of the system 1.458 and GHz therefore with relativedo not cause power a saturation36 dB and of ADC.1.467 TheyGHz willwith be rela digitallytive power filtered 27 in dB. the Although processed signalthe source data. of Let the us interferencenote that the was RT5 not spectrograph identified, isthe more observed covered levels by the are hill within slope designed from interference dynamic range sources of andthe system we can and therefore do not cause a saturation of ADC. They will be digitally filtered in the processed signal Electronics 2019, 8, 861 14 of 16 expect even better results in the future. Of course, the next interference analysis has to be made for the final installation to the location of current analog system.

4. Discussion and Conclusions Even though the developed spectrograph used a lower gain old antenna during the tests, the obtained measurements carry out more details and a comparable background noise with respect to the analog spectrograph. The comparison of the parameters of both spectrographs is presented in the Table3.

Table 3. Comparison of the parameters for both spectrographs.

Parameter RT5 Analog Spectrograph Designed Digital Spectrograph Time resolution 10 ms shared by 256 channels 1 ms Frequency resolution 4 MHz 303 kHz ADC resolution 12 bit 14 bit Dynamic range 36 dB 50 dB QSR/noise ratio 3.7 dB 3–10 dB (impact of antenna)

The measurement of the current spectrograph is saved in a proprietary format while the measurement of the developed spectrograph is archived in the FITS format that allows further data analyses and easy sharing and manipulation. The indisputable advantage of the new spectrograph is a precise amplitude scale in contrast to the current one, which has its amplitude scale distorted because of the imperfect analog processing. Moreover, the higher time and frequency resolution of the new device enables a better identification and analysis of possible interference sources, such as microwave data links, mobile phone systems signals, air traffic radio navigation signals (e.g., ADS-B), terrestrial broadcast of digital television signals, etc. The test of the L band solar spectrograph based on the modern digital signal processing at Ondrejov astronomical observatory confirmed the expected higher time and frequency resolution, and a high linearity of the amplitude measurement. The low processing noise that can be achieved by the massive averaging allowed by a parallel design (in contrast to the spectrum scanning approach of the current analog device) was however affected by the lower performance of the used antenna system and therefore the observed noise parameters persevered to stay at the original values. This problem is expected to be suppressed using the final installation antenna with better parameters. The crucial point was a proof of a validity of such design, which was accomplished according to the expectations. As the test operation was evaluated as successful, the next step, expected in the end of 2019, will be an installation of the complete multi-channel system to the modern 10 m dish antenna. The reasonable price of the spectrograph (approximately 2 kEuro per 250 MHz channel) was achieved by splitting the operating frequency range to several synchronized 250 MHz channels. This approach allows the use of moderately priced components and manufacture technologies without any negative impact on the developed instrument performance.

Author Contributions: Initiation of the spectrograph development and preliminary parameter specification, M.B.; development of the technical concept, P.K.; application of the spectrograph for investigation of the solar radio burst effect on the navigation and communication systems, P.K.; hardware development, FPGA programming and laboratory testing, P.K.; software for signal processing and the measurement format conversions and storage, P.P.; spectrograph installation P.P., P.K., M.B.; spectrograph operation, M.B.; preliminary measurement analyses, P.P.; writing—original draft, P.K.; writing—review and editing, P.P.; project administration P.K. Funding: This research was founded by OP RDE, MEYS, Czech Republic under the project CRREAT, CZ.02.1.01/0.0/0.0/15_003/0000481 and by CTU project SGS18/142/OHK3/2T/13. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Electronics 2019, 8, 861 15 of 16

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