Magnetic-Free Nonreciprocal Multifunction Device Based on Switched Delay Lines

electronics

Article Magnetic-Free Nonreciprocal Multifunction Device Based on Switched Delay Lines

Fengchuan Wu, Yuejun Zheng and Yunqi Fu * National University of Defense Technology, Changsha 410073, China * Correspondence: [email protected]

Received: 10 July 2019; Accepted: 1 August 2019; Published: 3 August 2019

Abstract: A magnetic-free multifunction nonreciprocal device based on switched delay lines (SDLs) has been proposed in this paper. It is constructed with two double balanced gyrators (DBGs) and four baluns, each pair of differential ports of the balun connect the ports at the same orientation of the two DBGs, respectively. Due to the asymmetry of the clock control signals acting on the switches, the time reversal symmetry of the transmission line between the Gilbert quad-switch-sets (GQSS) can be broken to achieve non-reciprocity. It can be used as a circulator, gyrator, or isolator by setting different control signals. The device has infinite working bandwidth in theory based on the SDLs. Common mode interference can be better suppressed by using differential transmission structures. Moreover, power capacity can be improved compared to the previous work. Then, experiments have been done to verify the device as a circulator. Broadband property and the anti-interference property have been verified.

Keywords: anti-interference; circulator; gyrator; isolator; magnetic-free; multifunction; nonreciprocal; power capacity; switched delay lines; ultra-broadband

1. Introduction In the future, wireless cellular network communication and WIFI will be further improved, the integrated full-duplex system is expected to improve the channel capacity or communication speed of wireless cellular networks and WIFI. Circulator and isolator, as full-duplex or decoupling devices between transmitter and receiver, to avoid self-interference, play an irreplaceable role in duplex communication systems. The main feature of circulators and isolators are their nonreciprocity. Conventional nonreciprocal devices usually use magnetic materials such as ferrites [1,2] to achieve non-reciprocal property. However, the ferrite-based nonreciprocal devices are quite bulky and diﬃcult to be integrated, which limits its further miniaturization. Moreover, non-reciprocal is exceptionally strong in structures with near-zero parameters, which is expected to be used to design nonreciprocal devices [3,4]. However, it has not been fully carried out and explored. So, several magnetic-free devices have been proposed to realize integration of nonreciprocal devices [5–17]. For magnetic-free nonreciprocal devices, there are several ways to realize them. Reciprocity can be broken with active transistors, either in discrete circuits or integrated circuits [5,6]. However, the use of active transistors severely limits the linearity and noise performance. Reciprocity can also be broken using nonlinearities [7]. However, nonlinear devices typically exhibit non-reciprocity over a limited range of signal powers, and have limited bandwidth, precluding their application in scenarios where linearity to the desired signal is required. Parametrically modulated coupled-resonator loops have been proposed to construct the magnetic-free circulators with subwavelength dimensions [8,9]. However, they will produce intermodulation and harmonic products, and the working bandwidth is limited. At microwave frequencies, permittivity modulation is typically achieved using varactors that exhibit a limited maximum-to-minimum capacitance ratio ranging from 2 to 4, thus leading to large devices.

Electronics 2019, 8, 862; doi:10.3390/electronics8080862 www.mdpi.com/journal/electronics Electronics 20192019,, 88,, 862x FOR PEER REVIEW 22 of of 1213

time varying transmission lines (TVTL) to construct magnetic-free nonreciprocal devices [8], where Shihanthe distributed Qin et al.capacitive proposed mixers the time are used varying to the transmission TVTL. When lines the direction (TVTL) to of construct the transmission magnetic-free signal nonreciprocalis the same as devices the carrier’s, [8], where the thetransmission distributed si capacitivegnal will mixersmix with are the used ca torrier. the TVTL.However, When if the directiontransmission of the signal transmission and the carries signal are is the in opposite same asthe directions, carrier’s, they the will transmission not mix up. signal So, willthe TVTL mix with can thebe used carrier. as a However, frequency if conversion the transmission isolator signal or a andcirculator. the carries However, are in oppositea lengthy directions,circuit is needed they will to notrealize mix the up. spectrum So, the TVTL shift. canReiskarimian be used as et a al. frequency proposed conversion a staggered isolator commutation or a circulator. theory However,to realize amagnetic-free lengthy circuit circulators, is needed and to realizefabricated the an spectrum integrated shift. circulator Reiskarimian for the etfirst al. time proposed [11,12]. a staggeredHowever, commutationthis design has theory limited to power realize handling magnetic-free ability circulators, and is hard and to work fabricated at millimeterwave an integrated band. circulator Nagulu for theet al. first proposed time [11 a, 12new]. However, kind of TVTL this designwhich hasis called limited SDL, power to realize handling magnetic-free ability and circulator is hard to working work at millimeterwaveat millimeterwave band. band Nagulu [13–15]. et al. However, proposed ath newe working kind of TVTLband whichof the is circulator called SDL, is tolimited. realize magnetic-freeUltra-wideband circulator circulators working have atbeen millimeterwave proposed in band previous [13–15 work]. However, based on the the working balanced band gyrator of the circulator(BG) and double is limited. balanced Ultra-wideband gyrator (DBG), circulators respecti havevely been [15]. proposed Adjusting in the previous clock workcontrol based signal, on the balancedDBG can gyrator be used (BG) andas a double frequency balanced conversion gyrator (DBG), isolator respectively [15]. However, [15]. Adjusting the anti-interference the clock control signal,performance the DBG and can power be used capa ascity a frequency of the ultra-broadband conversion isolator circulat [15or]. based However, on the the DBG anti-interference are limited. performanceAlso, the bandwidth and power of the capacity frequency of the conversion ultra-broadband isolator circulator is limited. based on the DBG are limited. Also, the bandwidthIn this paper, of the a frequencymagnetic-free conversion nonreciprocal isolator ismultifunction limited. device, which can be used as a circulator,In this paper,gyrator, a magnetic-freeor isolator by nonreciprocal changing the multifunction clock control device, sign whichals, is can proposed. be used asBesides, a circulator, the gyrator,anti-interference or isolator performance by changing and the po clockwer capacity control signals, of the device is proposed. can be improved Besides, the at the anti-interference same time by performanceusing differential and powertransmission capacity line of thesegments. device canIt also be improvedhas infinite at working the same bandwidth time by using in theory. differential The transmissionproperties mentioned line segments. above It can also be has realized infinite at working the same bandwidth time in this in theory.work. This The paper properties is organized mentioned as abovefollows: can be realized at the same time in this work. This paper is organized as follows: Section2 2introduces introduces the the principle principle of the of magnetic-free the magnetic-free nonreciprocal nonreciprocal device asdevice a gyrator, as a circulator, gyrator, andcirculator, isolator. and Section isolator.3 presents Section 3 the presents simulation the simulation verification verification of the device of the as device a circulator as a circulator and isolator, and andisolator, theanti-interference and the anti-interference performance performance of the device of th ise also device verified. is also Section verified.4 presents Section the 4 presents experiment the ofexperiment the device of as the a circulator.device as a Finally, circulator. a conclusion Finally, a is conclusion made in Section is made5. in Section 5. 2. Principle of the Device 2. Principle of the Device The schematic of the proposed device is shown in Figure1, which consists of two DBGs and four The schematic of the proposed device is shown in Figure 1, which consists of two DBGs and baluns. Each pair of differential ports of the balun connect the ports at the same orientation of the two four baluns. Each pair of differential ports of the balun connect the ports at the same orientation of DBGs, respectively. The GQSS in the DBG are controlled by the clock signals. The switch keeps on (off) the two DBGs, respectively. The GQSS in the DBG are controlled by the clock signals. The switch when at a high (low) level of the control signal acting on the switches. keeps on (off) when at a high (low) level of the control signal acting on the switches.

S1(t) S3(t)

S2(t) Z0,Td=Ts/4 S4(t) Balun Balun 1 S2(t) Z0,Td=Ts/4 S4(t) 3 S1(t) S3(t) S1(t) S3(t) Z0,Td=Ts/4 S2(t) S4(t) Balun Balun 2 2 4 S (t) Z0,Td=Ts/4 S4(t)

S1(t) S3(t) S1(t) S2(t) S3(t) S4(t) Ts/4

Figure 1. Schematic of the proposed magnetic-free device. Figure 1. Schematic of the proposed magnetic-free device. 2.1. Gyrator 2.1. Gyrator For the gyrator, keeping the clock control signal is shown in Figure1. Regard port 1(2) and port 3(4) asFor port the 1 gyrator,+ (1 ) and keeping port 2 the+ (2 clock), respectively. control signal The is device shown can in Figure be regarded 1. Regard as a two-portport 1(2) and network. port − − 3(4) asWhen port the1+ (1 transmission−) and port 2+ signals (2−), arerespectively. input from The port device 1 and can port be 3, regarded the output as a signals two-port at port network. 2 and port 4When can be the written transmission as: signals are input from port 1 and port 3, the output signals at port 2 and port 4 can be written as:

Electronics 2019, 8, x FOR PEER REVIEW 3 of 13

±±TT T TT T = −ss −× + −× s + − ss −× + −× s V21()()()()()()()()()()() t V t St 1 St 32 St St 4 V 1 t St 1 St 42 St St 3 44 4 44 4

±±TT TT Electronics 2019=−×−×+−×−×Vt, 8,()()()()()() 862 ss St StVt s St s St 3 of 12 11312444 44 (1)

± T =−Vt(),s 1 4

h i h i Ts Ts Ts Ts Ts Ts V±(t) = V±(t ) S1(t ± ) S3(t) + S2(t ) S4(t) + V∓(t ) S1(t ) S4(t) + S2(t ) S3(t) 2 1 Vt− ±4() Vt−()4 × − 4 × 1 − 4 − 4 × − 4 × = V Where(t Ts ) 1S (t , Ts 2) S are(t) +theV (ttransmissionTs ) S (t Tsignals ) S (tat) port 1(2) and port 3(4), respectively. (1) 1± − 4 × 1 − 4 × 3 1± − 4 × 2 − 4 × 4 Sn(t)= (n=1,2,3,4)( Ts ) is the clock control signal. Ts is the period of the clock signals. The clock signals of V1± t 4 , the right− switches are delayed with respect to the left switches by a value which equals to the propagation delay( ) of( the) delay line Ts/4. Where V1± t , V2± t are the transmission signal at port 1(2) and port 3(4), respectively. Sn(t) (n = 1, 2, 3, 4)When is the the clock transmission control signal. signalsTs isare the input period from of port the clock2 and signals.port 4, the The output clock signals at of port the right1 switchesand port are 3 can delayed be written with respectas: to the left switches by a value which equals to the propagation delay of the delay±± line Ts/4. TT T TT T = −ss −× + −× s + − ss −× + −× s Vt12()()()()()()()()()()() Vt St 3 St 14 St St 22 Vt St 4 St 13 St St 2 When the transmission44 signals are input 4 from port 2 and 44 port 4, the output signals 4 at port 1 and port 3 can be writtenTT as: TT =Vt()()()()()() −×ss St −× StVt + −× ss St −× St 24123244 44 (2) h i h i Ts Ts Ts Ts Ts Ts ( ) = ( T)s ( ) ( ) + ( ) ( ) + ( ) ( ) ( ) + ( ) ( ) V1± t V=−2± t 4 S3 t 4 S1 t S4 t 4 S2 t V2∓ t 4 S4 t 4 S1 t S3 t 4 S2 t Vt2 −()− × − × − − × − × = ( Ts ) (4 Ts ) ( ) + ( Ts ) ( Ts ) ( ) V2∓ t 4 S4 t 4 S1 t V2∓ t 4 S3 t 4 S2 t (2) − T × − × − × − × = V (t s ) 2∓ − 4 The scattering parameter matrix of DBG can be written as: The scattering parameter matrix of DBG can be written as: T − jω s  T4  − jω s  0 e 4  =  0 e−  SS =  Ts − , (3) (3)  jωTs   e−−jω 4 0  4 e 0 From Equation (2), the nonreciprocal phase property of the gyrator is also frequency independent. TransmissionFrom Equation procession (2), of the the nonreciprocal signals in the devicephase property as a circulator of the or gyrator gyrator is shownalso frequency in Figure 2. Whenindependent. signals are Transmission input from procession port 1 (1+ of) andthe signals port 2 (1in the), theydevice will as bea circulator transmitted or gyrator to port is 3 shown (1+) and in Figure 2. When signals are input from port 1 (1+) and− port 2 (1−), they will be transmitted to port 3 port 4 (1 ); when signals are input from port 3 (2+) and port 4 (2 ), they will be transmitted to port 2 (1+) and− port 4 (1−); when signals are input from port 3 (2+) and port− 4 (2−), they will be transmitted (1 ) and port 1 (1+). to− port 2 (1−) and port 1 (1+).

Signal(1)+ Signal(4)+ Signal(1)+

Signal(4)+ 1 1 Balun Balun 3 Balun Balun 3 1+ Signal(2)+ 2+ 1+ 2+

Signal(3)+ Signal(2)+ Signal(3)+ Signal(1)- Signal(4)- Signal(1)-

2 Signal(4)- Balun Balun 4 2 Balun Balun 4 1- Signal(2)- 2- 1- 2-

Signal(3)- Signal(2)- Signal(3)- 0 Ts/4 time Figure 2. Transmission procession of the signals in the gyrator (circulator). Figure 2. Transmission procession of the signals in the gyrator (circulator). 2.2. Circulator 2.2. Circulator As for the circulator, the DBG proposed in previous work can be considered as an ultra-broadband As for the circulator, the DBG proposed in previous work can be considered as an single-ended four-port circulators, which has been veriﬁed by R. Lu et al. [16]. However, it has limited ultra-broadband single-ended four-port circulators, which has been verified by R. Lu et al. [16]. power capacity and anti-interference performance by using single-ended transmission structures. However, it has limited power capacity and anti-interference performance by using single-ended Assumingtransmission the DBGstructures. is a well-matched Assuming the four-port DBG is network a well-matched without considering four-port network the bandwidth without and dispersion of the transmission line, the scattering parameter matrix can be written as:

 jω Ts   0 0 0 e 4   −   jω Ts   e 4 0 0 0  S =  − , (4)  jω Ts   0 e− 4 0 0     jω Ts  0 0 e− 4 0 Electronics 2019, 8, x FOR PEER REVIEW 4 of 13

considering the bandwidth and dispersion of the transmission line, the scattering parameter matrix can be written as:

T − jω s 000e 4 T − jω s e 4 000 S = , T (4) − jω s 4 000e T − jω s 4 00e 0

Ts is the period of the clock signals. The clock signals of the right switches are delayed with respect to the left switches by a value which equals to the propagation delay of the transmission line ElectronicsTs/42019. ,And8, 862 ω represents the transmission frequency. For the differential transmission circulator 4 of 12 (DTC), combine the two DBGs with four frequency-independent baluns shown in Figure 1. The scattering parameter matrix can also be written as Equation (4). The nonreciprocal property is Ts isfrequency the period independent. of the clock signals. The clock signals of the right switches are delayed with respect to the left switches by a value which equals to the propagation delay of the transmission line Ts/4. And ω 2.3. Isolator represents the transmission frequency. For the differential transmission circulator (DTC), combine the For lower DBG structures in the DTC, setting S1(t), S2(t), S3(t), S4(t) as DC voltages. S1(t) and two DBGs with four frequency-independent baluns shown in Figure1. The scattering parameter S3(t) acting on the lower DBG are at the high level to keep the switches on; S2(t) and S4(t) acting on matrix canthe lower also beDBG written are at the as low Equation level to (4).keep The the switches nonreciprocal off. Thus, property the lower is DBG frequency can be regarded independent. as a pair of reciprocal transmission line segments. The clock control signals acting on the upper DBG 2.3. Isolatorkeep the same as shown in Figure 1. Thus, the device can be regarded as a magnetic-free isolator Forcalled lower a differential DBG structures transmission in the isolator DTC, (DTI), setting which S1(t), is similar S2(t), in S3(t), the previous S4(t) as work DC [17]. voltages. However, S1(t) and the Wilkinson power splitters (combiners) are replaced by the baluns in this paper, which can S3(t) acting on the lower DBG are at the high level to keep the switches on; S2(t) and S4(t) acting on suppress the common mode signals. So, the reverse transmission signals can be designed as the the lowercommon DBG signals are at theto be low suppressed level to by keep the baluns. the switches off. Thus, the lower DBG can be regarded as a pair of reciprocalThe transmission transmission process lineof the segments. signals is shown The in clock Figure control 3. When signals a pair of acting differential on the signals upper DBG keep theinput same from as port shown 1+ and in 1 Figure−, the baluns1. Thus, divide the the device two sets can of be signals regarded into four as asets magnetic-free of signals with isolator called aequivalent differential power. transmission Then, the signals isolator will (DTI), be combined which at is the similar baluns in with the antiphase previous at workport 2+ [ 17and]. 2 However,− the Wilkinsonrespectively. power Finally, splitters the signals (combiners) will be output are replaced from port by 2+ the and baluns 2−. in this paper, which can suppress When a pair of differential signals are input from port 2+ and 2−, respectively, the signals will the commonbe combined mode in signals. phase at So, the the baluns reverse at port transmission 1+ and 1-, respectively. signals can So, be the designed signals can as be the regarded common as signals to be suppressedcommon mode by thesignals. baluns. The common mode signals will be suppressed significantly by the baluns. TheAssume transmission that small process parts of ofsignals the signalsare reflected is shown back to in the Figure baluns3 .at When port 2+ aand pair 2− of, the di signalsfferential will signals input frombe combined port 1+ inand phase 1 at, thethe baluns twice. divide So, thethe tworeverse sets signals of signals can be intofurther four suppressed. sets of signals And with − equivalentthe remained power. Then,signals the will signals be reflected will beback combined to the baluns at the at port baluns 1+ and with 1− antiphase. Finally, the at remained port 2+ and 2 signals will be output from port 1+ and 1−. However, the output signals have been suppressed − respectively. Finally, the signals will be output from port 2+ and 2 . significantly. −

Signal+ Signal+

Balun Balun Balun 1+ 2+ Balun 1+ 2+ Signal- Signal- Signal- Signal-

Balun Balun Balun Balun 1- 2- 1- 2-

Signal+ Signal+ 0 Ts/4 time Electronics 2019, 8, x FOR PEER REVIEW 5 of 13 (a) Signal+

Signal- Balun Balun Balun Balun 1+ 2+ 1+ Signal+ 2+

Signal- Signal- Signal-

Balun Balun Balun Balun 1- 2- 1- 2-

Signal+ Signal+ 0 Ts/4 time

Signal-

Signal+ Balun Balun Balun Balun 1+ 2+ 1+ Signal- 2+

Signal+ Signal- Signal-

Balun Balun Balun Balun 1- 2- 1- 2-

Signal+ Signal+ Ts/2 3Ts/4 time

(b)

Figure 3.FigureTransmission 3. Transmission procession procession of of the the signals signals inin the isolator. isolator. (a) ( aInput) Input from from port port1+ (1− 1);+ (b(1) Input); (b ) Input − from portfrom 2+ port(2 2+). (2−). − 3. Simulation Verification of the Device The harmonic balance module in the Keysight Advanced Design System (ADS) simulation software was used to get the spectrum results of the device, to verify ultra-broadband properties. Assuming all the transmission line segments with no dispersion and have a delay time of Ts/4 = 0.5 ns. The phase relationships between the clock signals are the same as shown in Figure 1. The frequency range of the input signal is from 0.1 to 1 GHz, with a step size of 0.1 GHz. The input power is set as 10 dBm.

3.1. Circulator For the DTC, the results shown in Figure 4 indicate that the transmission direction of the DTC is 1-2-3-4-1. Throughout the simulation band, the insertion loss is lower than 0.1 dB along the transmission direction. The reverse isolation is higher than 50 dB. And it is increased gradually with the frequency. The second isolated port has isolation higher than 80 dB throughout the simulation band.

(a) (b)

Electronics 2019, 8, x FOR PEER REVIEW 5 of 13

Signal+

Signal- Balun Balun Balun Balun 1+ 2+ 1+ Signal+ 2+

Signal- Signal- Signal-

Balun Balun Balun Balun 1- 2- 1- 2-

Signal+ Signal+ 0 Ts/4 time

Signal-

Signal+ Balun Balun Balun Balun 1+ 2+ 1+ Signal- 2+

Electronics 2019 8 Signal+ , , 862 Signal- Signal- 5 of 12

Balun Balun Balun Balun 1-When a pair of differential signals are2- input from1- port 2+ and 2 , respectively, the signals2- will − be combined in phase at the balunsSignal+ at port 1+ and 1-, respectively.Signal+ So, the signals can be regarded Ts/2 3Ts/4 time as common mode signals. The common mode signals will be suppressed significantly by the baluns. Assume that small parts of signals are reflected back to the baluns at port 2+ and 2 , the signals will be (b) − combined in phase at the baluns twice. So, the reverse signals can be further suppressed. And the remainedFigure signals 3. Transmission will be reflected procession back of th toe thesignals baluns in the at isolator. port 1+ (aand) Input 1 .from Finally, port the1+ (1 remained−); (b) Input signals − will befrom output port 2+ from (2−). port 1+ and 1 . However, the output signals have been suppressed significantly. − 3. Simulation Simulation Verification Verification ofof thethe DeviceDevice The harmonic balance module in the Keysight Advanced Design System (ADS) simulation software was used to get the spectrum results of the device, to verify ultra-broadband properties. properties. Assuming allall thethe transmissiontransmission line line segments segments with with no no dispersion dispersion and and have have a delaya delay time time of Tsof /Ts/44 = 0.5 = 0.5 ns. ns.The The phase phase relationships relationships between between the clock the signalsclock sign areals the are same the as same shown as in shown Figure 1in. TheFigure frequency 1. The frequencyrange of the range input of signal the input is from signal 0.1 tois 1from GHz, 0.1 with to a1 stepGHz, size with of 0.1a step GHz. size The of input0.1 GHz. power The is input set as 10power dBm. is set as 10 dBm.

3.1. Circulator Circulator For the DTC, the results shown in Figure 44 indicateindicate thatthat thethe transmissiontransmission directiondirection ofof thethe DTCDTC isis 1-2-3-4-1. ThroughoutThroughout the the simulation simulation band, band, the insertionthe insertion loss is lowerloss is than lower 0.1 dBthan along 0.1 the dB transmission along the transmissiondirection. The direction. reverse isolation The reverse is higher isolation than is 50 higher dB. And than it 50 is increaseddB. And it gradually is increased with gradually the frequency. with theThe frequency. second isolated The second port has isolated isolation port higher has isolat than 80ion dB higher throughout than 80 the dB simulation throughout band. the simulation band.

Electronics 2019, 8, x FOR PEER REVIEW 6 of 13 (a) (b)

FigureFigure 4. 4. SimulationSimulation results ofof thethe DTC.DTC. ( a(a)) Input Input from from port port 1; 1; (b )(b Input) Input from from port port 2; ( c2;) Input(c) Input from from port port3; (d 3;) Input (d) Input from from port port 4. 4.

When the device is used as a 4-port circulator, the transmitter, receiver, and antenna can be connected to port 1, port 2, and port 3, respectively; the matching load is expected to be connected to port 4 so that the transmitter and receiver have enough isolation. In order to verify the advantage of the DTC in anti-interference performance, two noise modules were built up, shown in Figure 5. As the noise is usually generated from a single noise source, a common mode interference is added at port 1 of the DTC, and a single noise source is added at port 1 of the single-ended transmission circulator (STC), which is constructed by a DBG. The power of the noise interference is set as 10 dBm. The frequency range of the common mode interference is from 0.1 to 1 GHz, with a step size of 0.1 GHz. From the results shown in Figure 6a. The common mode interference can be suppressed over 60 dB throughout the simulation band, which is similar to the situation as a gyrator. As for the STC, the results are shown in Figure 6b which indicate that the interference cannot be suppressed. Thus, the anti-interference performance can be improved compared to the STC. Besides, the DTC has a higher power capacity than the STC. By using ultra-broadband baluns, which can be regarded as power splitters (combiners), the DTC can theoretically raise 3 dB power capacity compared to the STC. Thus, the circulator can be better protected.

Noise 12Balun Noise 12 source source DTC DBG Balun 3 4 Balun 3 4

(a) (b)

Figure 5. Noise module of the circulators. (a) Noise module of the DTC; (b) Noise module of the STC.

Electronics 2019, 8, x FOR PEER REVIEW 6 of 13

Figure 4. Simulation results of the DTC. (a) Input from port 1; (b) Input from port 2; (c) Input from port 3; (d) Input from port 4.

Electronicsadded at2019 port, 8 ,1 862 of the single-ended transmission circulator (STC), which is constructed by a DBG.6 of 12 The power of the noise interference is set as 10 dBm. The frequency range of the common mode interference is from 0.1 to 1 GHz, with a step size of 0.1 GHz. From the results shown in FigureWhen 6a. The the common device is mode used interference as a 4-port circulator,can be suppressed the transmitter, over 60 dB receiver, throughout and antenna the simulation can be connectedband, which to portis similar 1, port to 2,the and situation port 3, respectively;as a gyrator. theAs matchingfor the STC, load the is results expected are to shown be connected in Figure to port6b which 4 so that indicate the transmitter that the and interference receiver have canno enought be isolation. suppressed. Thus, the anti-interference performanceIn order can to verify be improved the advantage compared of the to DTC the STC. in anti-interference performance, two noise modules wereBesides, built up, the shown DTC has in Figurea higher5. power As the capacity noise is th usuallyan the generatedSTC. By using from ultra-broadband a single noise source,baluns, awhich common can modebe regarded interference as power is added splitters at port (combi 1 of theners), DTC, the and DTC a single can theoretically noise source raise is added 3 dB at power port 1 ofcapacity the single-ended compared to transmission the STC. Thus, circulator the circulator (STC), which can be is better constructed protected. by a DBG.

Noise 12Balun Noise 12 source source DTC DBG Balun 3 4 Balun 3 4

(a) (b)

Figure 5.5. NoiseNoise modulemodule of of the the circulators. circulators. (a )(a Noise) Noise module module of theof the DTC; DTC; (b) Noise(b) Noise module module of the of STC. the STC. The power of the noise interference is set as 10 dBm. The frequency range of the common mode interference is from 0.1 to 1 GHz, with a step size of 0.1 GHz. From the results shown in Figure6a. The common mode interference can be suppressed over 60 dB throughout the simulation band, which is similar to the situation as a gyrator. As for the STC, the results are shown in Figure6b which indicate that the interference cannot be suppressed. Thus, the anti-interference performance can be improved Electronicscompared 2019 to, 8 the, x FOR STC. PEER REVIEW 7 of 13

(a) (b)

FigureFigure 6. 6. SpectrumSpectrum simulation simulation results results of of the the DTC DTC and and the the STC. STC. ( (aa)) DTC; DTC; ( (bb)) STC. STC.

3.2. IsolatorBesides, the DTC has a higher power capacity than the STC. By using ultra-broadband baluns, which can be regarded as power splitters (combiners), the DTC can theoretically raise 3 dB power For the isolator, spectrum simulations have been done to verify it, which have the same capacity compared to the STC. Thus, the circulator can be better protected. simulation settings as mentioned above. The simulation results are shown in Figure 7. Throughout the3.2. simulation Isolator band, the insertion loss is lower than 0.1 dB along the transmission direction. The reverse isolation is higher than 70 dB, which is increased gradually with the frequency. So, it can be For the isolator, spectrum simulations have been done to verify it, which have the same simulation used as an ultra-broadband isolator. settings as mentioned above. The simulation results are shown in Figure7. Throughout the simulation band, the insertion loss is lower than 0.1 dB along the transmission direction. The reverse isolation is higher than 70 dB, which is increased gradually with the frequency. So, it can be used as an ultra-broadband isolator.

(a) (b)

Figure 7. Spectrum simulation results of the isolator. (a) Input from port 1+ (1−); (b) Input from port 2+ (2−).

A common mode noise source has been added to the isolator to verify the anti-interference performance. The noise module is shown in Figure 8. Two situations are considered: (1) The common mode noise is input from port 1+ (2+) and 1− (2−) of the isolator, respectively. (2) The common mode noise is input from one side of the two reciprocal transmission line segments and DBG, respectively.

Electronics 2019, 8, x FOR PEER REVIEW 7 of 13

(a) (b)

Figure 6. Spectrum simulation results of the DTC and the STC. (a) DTC; (b) STC.

3.2. Isolator For the isolator, spectrum simulations have been done to verify it, which have the same simulation settings as mentioned above. The simulation results are shown in Figure 7. Throughout the simulation band, the insertion loss is lower than 0.1 dB along the transmission direction. The reverse isolation is higher than 70 dB, which is increased gradually with the frequency. So, it can be Electronics 2019, 8, 862 7 of 12 used as an ultra-broadband isolator.

(a) (b)

Figure 7. Spectrum simulation resu resultslts of the isolator. (a) Input fromfrom portport 11++ (1−);); ( (bb)) Input Input from from port − 2+2+ (2(2−).). − A common mode noise source has been added to the isolator to verify the anti-interference performance. TheThe noisenoise module module is shownis shown in Figurein Figu8. Twore 8. situations Two situations are considered: are considered: (1) The common (1) The commonmode noise mode is input noise from is input port 1 +from(2+ )port and 11+ (2(2+)) ofand the 1− isolator, (2−) of respectively. the isolator, (2) respectively. The common (2) mode The − − Electronicscommonnoise is input2019 mode, 8, from x noiseFOR one PEER is sideinput REVIEW of from the two one reciprocal side of th transmissione two reciprocal line segmentstransmission and DBG,line segments respectively.8 ofand 13 DBG, respectively.

Balun DBG Balun DBG Balun 2+

Noise Noise Balun Balun source source

1- 2- 2- Balun Balun Balun

(a) (b)

Figure 8. Noise modulemodule ofof thethe isolator. isolator. ( a()a Input) Input from from port port 1+ 1+and and 1 ;(1−b; )(b Input) Input from from the the DBG DBG and and two − tworeciprocal reciprocal transmission transmission line segments,line segments, respectively. respectively.

The simulation simulation results results are are shown shown in in Figure Figure 9.9. Th Thee spectrum spectrum results results show show that that the the isolator isolator has has a significanta signiﬁcant suppression suppression of of the the common common mode mode signals signals over over the the entire entire simulation simulation band band in in these these two situations.situations. The The suppression suppression of of the the common common mode mode si signalsgnals is is higher higher than than 70 70 dB when the common mode noise is input from port 1+ and port 1 throughout the simulation band. The suppression is mode noise is input from port 1+ and port 1− throughout the simulation band. The suppression is higher than 75 dBdB whenwhen itit isis input input from from the the DBG DBG and and two two reciprocal reciprocal transmission transmission lines lines throughout throughout the thesimulation simulation band. band. The The average average suppression suppression of the of situation the situation (2) is about(2) is 40about dB higher40 dB thanhigher the than average the averagesuppression suppression of the situation of the situation (1). (1). In summary, the device is easy to be integrated without the use of ferrite materials. Besides, according to the simulation results, the device has ultra-broadband working bandwidth, well anti-interference, and high isolation for the reverse signal. However, the device needs very accurate clock control signals and the delay lines with precise length to ensure the performances.

(a) (b)

Figure 9. The spectrum of the common mode noise of the isolator. (a) Input from port 1+ and 1−; (b) Input from the DBG and two reciprocal transmission line segments, respectively.

In summary, the device is easy to be integrated without the use of ferrite materials. Besides, according to the simulation results, the device has ultra-broadband working bandwidth, well anti-interference, and high isolation for the reverse signal. However, the device needs very accurate clock control signals and the delay lines with precise length to ensure the performances.

4. The Experiments of the Magnetic-Free Nonreciprocal Device A prototype was made to verify the broadband and anti-interference properties, as shown in Figure 10, which is also inspired by the previous work [9]. In this design, the GQSS in the DBG were made of 1.6-mm FR-4 printed circuit broad (PCB), and four Minicircuits' single-pole-double-throw (SPDT) switches. The typical transition time of the swithes is 4 ns. The balun was made of 1.6-mm FR-4 PCB and a Minicircuits' ADTL1-12+ RF transformer. The working band of the balun is 20~1200 MHz. The transmission line segments were realized by four 7-meter coaxial cables. The matching loads were realized by 50Ω end caps. The four clock signals were generated by four synchronized Tektronix AFG31023C signal generators, and the frequency of the clock signals in this design was

Electronics 2019, 8, x FOR PEER REVIEW 8 of 13

Balun DBG Balun DBG Balun 2+

Noise Noise Balun Balun source source

1- 2- 2- Balun Balun Balun

(a) (b)

Figure 8. Noise module of the isolator. (a) Input from port 1+ and 1−; (b) Input from the DBG and two reciprocal transmission line segments, respectively.

The simulation results are shown in Figure 9. The spectrum results show that the isolator has a significant suppression of the common mode signals over the entire simulation band in these two situations. The suppression of the common mode signals is higher than 70 dB when the common mode noise is input from port 1+ and port 1− throughout the simulation band. The suppression is higher than 75 dB when it is input from the DBG and two reciprocal transmission lines throughout theElectronics simulation2019, 8, 862band. The average suppression of the situation (2) is about 40 dB higher than8 ofthe 12 average suppression of the situation (1).

(a) (b) Electronics 2019, 8, x FOR PEER REVIEW 9 of 13 Figure 9. TheThe spectrum spectrum of of the the common common mode mode noise noise of of the the isolator. isolator. (a)( aInput) Input from from port port 1+ and 1+ and 1−; ( 1b); − chosenInput(b) as Input from6.9 fromMHz. the theDBG The DBG and N5244A and two two reciprocal reciprocalnetwork transmission transmissionanalyzer lineis lineused segments, segments, to measure respectively. respectively. out the S parameters. The insertion loss of the coaxial cable is increased with frequency. Therefore, the signals higher than 1.2 4. The Experiments of the Magnetic-Free Nonreciprocal Device GHz Inis summary,not obtained. the Moreover,device is easy limited to beby integratedthe bandwidth without of thethe baluns, use of ferritethe measurement materials. Besides,band is accordingchosenA prototypeas 10to ~1200 the wassimulation MHz. made toresults, verify thethe broadbanddevice hasand ultra-broadband anti-interference working properties, bandwidth, as shown well in anti-interference,FigureThe 10 ,measured which isand also S-parameters high inspired isolation by are thefor previoustheshown reverse in work Figuresignal. [9]. However, In11. this From design, thethe device theresults, GQSS needs the in theverytransmission DBG accurate were clockdirectionmade control of 1.6-mmof the signals circulator FR-4 and printed the is delay1-2-3-4-1. circuit lines broad Thewith (PCB),results precise andshow length four that to Minicircuits’ ensurethe return the performances.loss single-pole-double-throw is higher than 10 dB (SPDT)throughout switches. the band The typicalof 200~1200 transition MHz. time So, of the the circulator swithes is has 4 ns. broadband The balun wasproperty. made ofThe 1.6-mm minimum FR-4 4.insertionPCB The and Experiments aloss Minicircuits’ of the ofcirculator the ADTL1-12 Magnetic-Free is about+ RF 7 transformer. dB. Nonreciprocal It is generally The workingDevice increased band with ofthe the balun frequency. is 20~1200 The delay MHz. errorThe transmissionA of prototype the transmission was line segmentsmade line tosegments verify were the realized and broadband duty by cy fourcle and error 7-meter anti-interference of the coaxial clock cables.signals properties, Theare the matching asmain shown reason loads in Figurewerethat increase realized 10, which bythe 50is Ωinsertionalsoend inspired caps. loss. The by Besides, fourthe previous clock the signals transmissionwork were [9]. In generated this line design, segments by four the GQSS synchronized and inthe the baluns DBG Tektronix werealso madeAFG31023Cinfluence of 1.6-mm thesignal insertion FR-4 generators, printedloss. The circuit and reverse the broad frequencyisolation (PCB), is of andhigher the four clock than Minicircuits' signals 36 dB throughout in thissingle-pole-double-throw design thewas measurement chosen as (SPDT)band.6.9 MHz. The switches. The average N5244A The deviation typical network transitionof analyzer the simulation istime used of toth and measuree swithes the experiment out is 4 the ns. S The parameters. is balun25 dB was within The made insertion the of band1.6-mm loss of FR-40~1000the coaxial PCB MHz, and cable which a Minicircuits' is increased is caused with ADTL1-12+ by frequency.the error RF of Therefore, transforthe clockmer. thecontrol The signals workingsignals higher and band than the 1.2of amplitude the GHz balun is not andis obtained.20~1200 phase MHz.imbalanceMoreover, The limitedoftransmission the baluns. by the line bandwidth segments of thewere baluns, realized the by measurement four 7-meter band coaxial is chosen cables. as The 10~1200 matching MHz. loads were realized by 50Ω end caps. The four clock signals were generated by four synchronized Tektronix AFG31023C signal generators, and the frequency of the clock signals in this design was

Figure 10. TheThe prototype prototype of the magnetic-free nonreciprocal device.

The measured S-parameters are shown in Figure 11. From the results, the transmission direction of the circulator is 1-2-3-4-1. The results show that the return loss is higher than 10 dB throughout the band of 200~1200 MHz. So, the circulator has broadband property. The minimum insertion loss of the circulator is about 7 dB. It is generally increased with the frequency. The delay error of the transmission line segments and duty cycle error of the clock signals are the main reason that increase the insertion loss. Besides, the transmission line segments and the baluns also inﬂuence the insertion loss. The reverse isolation is higher than 36 dB throughout the measurement band. The average

(a) (b)

Electronics 2019, 8, x FOR PEER REVIEW 9 of 13

chosen as 6.9 MHz. The N5244A network analyzer is used to measure out the S parameters. The insertion loss of the coaxial cable is increased with frequency. Therefore, the signals higher than 1.2 GHz is not obtained. Moreover, limited by the bandwidth of the baluns, the measurement band is chosen as 10 ~1200 MHz. The measured S-parameters are shown in Figure 11. From the results, the transmission direction of the circulator is 1-2-3-4-1. The results show that the return loss is higher than 10 dB throughout the band of 200~1200 MHz. So, the circulator has broadband property. The minimum insertion loss of the circulator is about 7 dB. It is generally increased with the frequency. The delay error of the transmission line segments and duty cycle error of the clock signals are the main reason that increase the insertion loss. Besides, the transmission line segments and the baluns also influence the insertion loss. The reverse isolation is higher than 36 dB throughout the measurement band. The average deviation of the simulation and the experiment is 25 dB within the band of 0~1000 MHz, which is caused by the error of the clock control signals and the amplitude and phase imbalance of the baluns.

Electronics 2019, 8, 862 9 of 12

deviation of the simulation and the experiment is 25 dB within the band of 0~1000 MHz, which is caused by the error ofFigure the clock 10. The control prototype signals of the and magnetic-free the amplitude nonreciprocal and phase device. imbalance of the baluns.

Electronics 2019, 8, x FOR PEER REVIEW 10 of 13 (a) (b)

FigureFigure 11. 11.The The measured measured S S parameters parameters of of the the magnetic-free magnetic-free nonreciprocal nonreciprocal device. device. ( a(a)) Port Port 1 1 and and port port 2. (b2.) Port(b) Port 1 and 1 and port port 2. ( c2.) ( Portc) Port 1 and 1 and port port 2. (2.d )(d Port) Port 1 and1 and port port 2. 2.

The broadband balun chose in this paper is about 10 dB throughout the band of 20~1200 MHz. The broadband balun chose in this paper is about− −10 dB throughout the band of 20~1200 MHz. Also,Also, due due to to the the phase phase and and amplitudeamplitude imbalanceimbalance ofof thethe baluns, the return loss loss of of the the structure structure is is not not high.high. The The return return loss loss of of the the DTC DTC can can be be increased increased by by designing designing the the balun, balun, which which is well-matched is well-matched and hasand low has amplitude low amplitude and phase and phase unbalance unbalance in the in broadband the broadband range. range. TheThe delay delay error error of of the the coaxialcoaxial cablescables andand thethe duty cycle error of the clock clock signals signals have have significate signiﬁcate impactsimpacts onon thethe insertioninsertion lossloss ofof thethe DTC.DTC. TheThe insertioninsertion loss can be reduced by by using using advanced advanced integrationintegration technology technology to to make make subnanosecond subnanosecond switches, switches, precise precise delay delay lines, lines, and produce and produce high quality high clockquality signals. clock signals. ToTo verify verify the the anti-interference anti-interference performanceperformance ofof thethe circulator, replace one balun balun in in the the circulator circulator withwith a Wilkinsona Wilkinson power power splitter splitter to generate to generate a pair a ofpa co-phaseir of co-phase equal amplitudeequal amplitude signals assignals the common as the modecommon signal. mode The signal. prototype The isprototype shown in is Figure shown 12 .in The Figure working 12. The band working of the Wilkinson band of the power Wilkinson splitter ispower 380~2500 splitter MHz. is 380~2500 Two ports MHz. with Two baluns ports in with the circulatorbaluns in arethe connectedcirculator are with connected two 50Ω withend capstwo respectively.50Ω end caps The respectively. other two portsThe other are connected two ports withare connected the PNA with network the PNA analyzer. network The analyzer. measurement The resultsmeasurement are shown results in Figure are shown 13. Thein Figure result 13. shows The thatresult the shows suppression that the ofsuppression the common of the mode common signal ismode higher signal than is 40 higher dB throughout than 40 dB the throughout band of 20~1200 the band MHz, of 20~1200 which indicatesMHz, which that indicates the circulator that the can eﬀcirculatorectively suppresscan effectively common suppress mode interference common mode within interference the broadband within range. theThe broadband average deviationrange. The of theaverage simulation deviation and theof the experiment simulation is and 44 dB the within experi thement band is 44 of dB 400~1000 within the MHz. band of 400~1000 MHz.

Figure 12. Anti-interference testing module.

Electronics 2019, 8, x FOR PEER REVIEW 10 of 13

Figure 11. The measured S parameters of the magnetic-free nonreciprocal device. (a) Port 1 and port 2. (b) Port 1 and port 2. (c) Port 1 and port 2. (d) Port 1 and port 2.

The broadband balun chose in this paper is about −10 dB throughout the band of 20~1200 MHz. Also, due to the phase and amplitude imbalance of the baluns, the return loss of the structure is not high. The return loss of the DTC can be increased by designing the balun, which is well-matched and has low amplitude and phase unbalance in the broadband range. The delay error of the coaxial cables and the duty cycle error of the clock signals have significate impacts on the insertion loss of the DTC. The insertion loss can be reduced by using advanced integration technology to make subnanosecond switches, precise delay lines, and produce high quality clock signals. To verify the anti-interference performance of the circulator, replace one balun in the circulator with a Wilkinson power splitter to generate a pair of co-phase equal amplitude signals as the common mode signal. The prototype is shown in Figure 12. The working band of the Wilkinson power splitter is 380~2500 MHz. Two ports with baluns in the circulator are connected with two 50Ω end caps respectively. The other two ports are connected with the PNA network analyzer. The measurement results are shown in Figure 13. The result shows that the suppression of the common mode signal is higher than 40 dB throughout the band of 20~1200 MHz, which indicates that the circulator can effectively suppress common mode interference within the broadband range. The Electronics 2019, 8, 862 10 of 12 average deviation of the simulation and the experiment is 44 dB within the band of 400~1000 MHz.

Electronics 2019, 8, x FOR PEER REVIEW 11 of 13 Figure 12. Anti-interference testing module.

Figure 13. TheThe measured measured S parameters of the anti-interference testing module.

With the limitation of the experimental conditions,conditions, the delay error of the transmission line segments and duty cycle error of the clock signals cannot be avoided. The two kinds of errors have significantsignificant impactsimpacts on on the the insertion insertion loss. loss. The signals The signals pass through pass thethrough DBG, andthe reciprocalDBG, and transmission reciprocal transmissionline segments, line respectively, segments, are respectively, hard to be combinedare hard withto be equal combined amplitude withat equal the baluns. amplitude As a at result, the baluns.the magnetic-free As a result, nonreciprocal the magnetic-free device nonreciprocal as an isolator device is diffi ascult an to isolator be verified is difficult by using to be a PCBverified circuit. by usingThe experiment a PCB circuit. of the The isolator experiment is expected of the toisolator be realized is expected in the futureto be realized by using in precise the future clock by signals using preciseand advanced clock signals processing and advanced technology processing in the integration technology circuit. in the integration circuit.

5. Conclusions Conclusions A magnetic-freemagnetic-free device device has has been been designed designed in this in paper,this paper, which which can be can used be as used a gyrator, as a circulator, gyrator, circulator,or isolator or by isolator only adjusting by only the adjusting clock control the clock signals control to achieve signals multifunctional to achieve multifunctional nonreciprocal nonreciprocalfeatures. Simulation features. works Simulation have beenworks done have to been verify done the to ultra-broadband verify the ultra-broadband property of property the device. of theBesides, device. the Besides, anti-interference the anti-interference performance performanc of the devicee of as the a circulatordevice as anda circulator isolator isand also isolator verified. is alsoThe commonverified. modeThe noisecommon can bemode significantly noise can suppressed be significantly by using suppressed differential transmissionby using differential structures. transmissionMoreover, the structures. power capacity Moreover, can bethe improved power capa bycity 3 dB can theoretically be improved compared by 3 dB to thetheoretically previous workcompared [15]. to The the magnetic-free previous work nonreciprocal [15]. The devicemagnetic-free as a broadband nonreciprocal circulator device has as been a verifiedbroadband by circulatorthe experiment. has been Besides, verified broadband by the experiment. anti-interference Besides, property broadband is also anti-interference verified. The properties property is of also the verified.magnetic-free The properties circulators of andthe magnetic-free isolators based circul on SDLators are and displayed isolators inbased Tables on 1SDL and are2, respectively. displayed in TableFrom Table1 and1 ,Table compared 2, respectively. to the ultra-broadband From Table 1, circulator compared (UBC) to the and ultra-broadband high linearity circulatorcirculator (HLC)(UBC) and high linearity circulator (HLC) in [15], and single-ended transmission circulator (STC) in [16], the DTC proposed in this paper has ultra-broadband, well anti-interference performances at the same time. Besides, the DTC has the largest power capacity. From Table 2, the DTI proposed in this paper has both ultra-broadband and well anti-interference performances. Moreover, the DTI has the largest power capacity compared to the other two isolators shown in Table 2. The delay error of the clock signals, delay lines, and the duty cycle error of the clock signals, which have significant impacts on the insertion loss, can be reduced by using precise clock signals and advanced processing technology in the integration circuit. Moreover, exploration of new metamaterials can be considered to improve magnetic-free nonreciprocal devices.

Table 1. The properties of the device as a circulator.

DTC in this paper UBC in [15] HLC in [15] STC in [16] Bandwidth Ultra-Broadband Ultra-Broadband Limited Ultra-Broadband Anti-interference Well Limited Well Limited performance Power capacity Largest Smallest Medium Smallest

Electronics 2019, 8, 862 11 of 12 in [15], and single-ended transmission circulator (STC) in [16], the DTC proposed in this paper has ultra-broadband, well anti-interference performances at the same time. Besides, the DTC has the largest power capacity. From Table2, the DTI proposed in this paper has both ultra-broadband and well anti-interference performances. Moreover, the DTI has the largest power capacity compared to the other two isolators shown in Table2. The delay error of the clock signals, delay lines, and the duty cycle error of the clock signals, which have signiﬁcant impacts on the insertion loss, can be reduced by using precise clock signals and advanced processing technology in the integration circuit. Moreover, exploration of new metamaterials can be considered to improve magnetic-free nonreciprocal devices.

Table 1. The properties of the device as a circulator.

DTC in this Paper UBC in [15] HLC in [15] STC in [16] Bandwidth Ultra-Broadband Ultra-Broadband Limited Ultra-Broadband Anti-interference performance Well Limited Well Limited Power capacity Largest Smallest Medium Smallest

Table 2. The properties of the device as an isolator.

DTI in this paper UDI in [15] FCI in [15] Bandwidth Ultra-Broadband Ultra-Broadband Limited Anti-interference performance Well Limited Well Power capacity Largest Smallest Medium

Author Contributions: Methodology and original draft preparation, F.W.; writing—review and editing, Y.Z. and Y.F.; writing—review and editing, supervision, Y.F. Funding: This research received no external funding. Acknowledgments: The authors would like to thank Nagulu and Krishnaswamy for valuable discussions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Dong, H.; Smith, J.R.; Young, J.L. A Wide-Band, High Isolation UHF Lumped-Element Ferrite Circulator. IEEE Microw. Wirel. Compon. Lett. 2013, 23, 294–296. [CrossRef] 2. Zeng, L.; Tong, C.E.; Blundell, R.; Grimes, P.K.; Paine, S.N. A Low-Loss Edge-Mode Isolator With Improved Bandwidth for Cryogenic Operation. IEEE Trans. Microw. Theory Tech. 2018, 66, 1–7. [CrossRef] 3. Liberal, I.; Engheta, N. Erratum: Near-zero refractive index photonics. Nat. Photonics 2017, 11, 264. [CrossRef] 4. Pacheco-Peña, V.; Beruete, M.; Rodríguez-Ulibarri, P.; Engheta, N. On the performance of an ENZ-based sensor using transmission line theory and eﬀective medium approach. New J. Phys. 2019, 21, 043056. [CrossRef] 5. Wang, S.; Lee, C.H.; Wu, Y.B. Fully integrated 10-GHz active circulator and quasi-circulator using bridged-T networks in standard CMOS. IEEE Trans. Very Large Scale Integr. Syst. 2016, 24, 3184–3192. [CrossRef] 6. Mung, S.W.Y.; Chan, W.S. Active Three-Way Circulator Using Transistor Feedback Network. IEEE Microw. Wirel. Compon. Lett. 2017, 27, 476–478. [CrossRef] 7. Popa, B.I.; Cummer, S.A. Nonreciprocal active metamaterials. Phys. Rev. B 2012, 85, 205101. [CrossRef] 8. Estep, N.A.; Sounas, D.L.; Alu, A. Magnetless Microwave Circulators Based on Spatiotemporally Modulated Rings of Coupled Resonators. IEEE Trans. Microw. Theory Tech. 2016, 64, 502–518. [CrossRef] 9. Kord, A.; Sounas, D.L.; Alu, A. Pseudo-Linear Time-Invariant Magnetless Circulators Based on Diﬀerential Spatiotemporal Modulation of Resonant Junctions. IEEE Trans. Microw. Theory Tech. 2018, 66, 2731–2745. [CrossRef] 10. Qin, S.; Xu, Q.; Wang, Y.E. Nonreciprocal Components With Distributedly Modulated Capacitors. IEEE Trans. Microw. Theory Tech. 2014, 62, 2260–2272. [CrossRef] Electronics 2019, 8, 862 12 of 12

11. Reiskarimian, N.; Zhou, J.; Chuang, T.H.; Krishnaswamy, H. Analysis and Design of Two-Port N-Path Band-Pass Filters with Embedded Phase Shifting. Circuits and Systems II: Express Briefs. IEEE Trans. 2016, 63, 728–732. 12. Reiskarimian, N.; Zhou, J.; Krishnaswamy, H. A CMOS Passive LPTV Nonmagnetic Circulator and Its Application in a Full-Duplex Receiver. IEEE J. Solid State Circuits 2017, 52, 1358–1372. [CrossRef] 13. Dinc, T.; Nagulu, A.; Krishnaswamy, H. A Millimeter-Wave Non-Magnetic Passive SOI CMOS Circulator Based on Spatio-Temporal Conductivity Modulation. IEEE J. Solid State Circuits 2017, 52, 3276–3292. [CrossRef] 14. Dinc, T.; Tymchenko, M.; Nagulu, A.; Sounas, D.; Alu, A.; Krishnaswamy, H. Synchronized conductivity modulation to realize broadband lossless magnetic-free non-reciprocity. Nat. Commun. 2017, 8, 795. [CrossRef][PubMed] 15. Nagulu, A.; Dinc, T.; Xiao, Z.; Tymchenko, M.; Sounas, D.L.; Alù, A.; Krishnaswamy, H. Nonreciprocal Components Based on Switched Transmission Lines. IEEE Trans. Microw. Theory Tech. 2018, 66, 4706–4725. 16. Lu, R.; Krol, J.; Gao, L.; Gong, S. A Frequency Independent Framework for Synthesis of Programmable Non-reciprocal Networks. Sci. Rep. 2018, 8, 14655. [CrossRef][PubMed] 17. Fengchuan, W.; Yuejun, Z.; Fang, Y.; Yunqi, F. Magnetic-Free Isolators Based on Time-Varying Transmission Lines. Electronics 2019, 8, 684.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).