A comprehensive method to synthesize negative group delay (NGD) in the microwave regime with a maximally flat response is proposed in this paper. This method is based on transversal-filter ...topologies; it will be shown that by choosing proper coupling coefficients of each tap of a transversal filter, we can realize NGDs with maximally flat characteristics at the output of the transversal filter. The desired coefficients to realize maximally flat NGDs with various amount of group delay are analytically derived and tabulated in this paper. Furthermore, the results are verified experimentally through microwave transversal-filter approaches in both passive and active ways using multi-section asymmetric directional couplers and distributed amplifiers.
Recent advancement in digital coding metasurfaces incorporating spatial and temporal modulation has enabled simultaneous control of electromagnetic (EM) waves in both space and frequency domains by ...manipulating incident EM waves in a transmissive or reflective fashion, resulting in time-reversal asymmetry. Here we show in theory and experiment that a digitally space-time-coded metamaterial (MTM) antenna with spatiotemporal modulation at its unit cell level can be regarded as a radiating counterpart of such digital metasurface, which will enable nonreciprocal EM wave transmission and reception via surface-to-leaky-wave transformation and harmonic frequency generation. Operating in the fast wave (radiation) region, the space-time-coded MTM antenna is tailored in a way such that the propagation constant of each programmable unit cell embedded with varactor diodes can toggle between positive and negative phases, which is done through providing digital sequences by using a field-programmable gate array (FPGA). Owing to the time-varying coding sequence, harmonic frequencies are generated with different main beam directions. Furthermore, the space time modulation of the digitally coded MTM antenna allows for nonreciprocal transmission and reception of EM waves by breaking the time-reversal symmetry, which may enable many applications, such as simultaneous transmitting and receiving, unidirectional transmission, radar sensing, and multiple-input and multiple-output (MIMO) beamformer.
A new type of reconfigurable negative group delay (NGD) circuits based on active transversal filter topology is introduced in this work. By incorporating varactors with the NGD circuit at the input ...and output ports of active transversal filter based on distributed amplifiers (DAs), one can notably enhance the tunability for both amplitude and phase responses, thereby enabling NGD characteristic synthesis with a wider reconfigurable range. In this article, the proposed reconfigurable NGD circuits are applied to create both tunable non-Foster elements and tunable squint-free beamforming networks, which are verified by experiment. Furthermore, a general <inline-formula> <tex-math notation="LaTeX">N </tex-math></inline-formula>-stage transfer function for the proposed reconfigurable NGD circuit is derived, in which a two-stage particular case is given as an illustrative proof-of-concept example. Based on the reconfigurable NGD circuits, a prototype of a tunable negative capacitor is fabricated and measured, of which the capacitance can be tuned from −1.0 to −4.0 pF over a bandwidth of 100 MHz at 1 GHz or around 10% relative bandwidth. In addition, such an NGD circuit is utilized to create a low-dispersion superluminal transmission line, which enables a reconfigurable series antenna feedline for squint-free beam steering. Experimental results show that the main beam of the NGD antenna array has a tuning range from −15° to 15° within a squint-free bandwidth of 100 MHz at 2 GHz or within a relative bandwidth of around 5%.
Microwave radar sensors have been developed for non-contact monitoring of the health condition and location of targets, which will cause minimal discomfort and eliminate sanitation issues, especially ...in a pandemic situation. To this end, several radar sensor architectures and algorithms have been proposed to detect multiple targets at different locations. Traditionally, beamforming techniques incorporating phase shifters or mechanical rotors are utilized, which is relatively complex and costly. On the other hand, metamaterial (MTM) leaky wave antennas (LWAs) have a unique property of launching waves of different spectral components in different directions. This feature can be utilized to detect multiple targets at different locations to obtain their healthcare and location information accurately, without complex structure and high cost. To this end, this paper reviews the recent development of MTM LWA-based radar sensor architectures for vital sign detection and location tracking. The experimental results demonstrate the effectiveness of MTM vital sign radar compared with different radar sensor architectures.
Owing to the data explosion and rapid development of artificial intelligence (AI), particularly deep neural networks (DNNs), the ever-increasing demand for large-scale matrix-vector multiplication ...has become one of the major issues in machine learning (ML). Training and evaluating such neural networks rely on heavy computational resources, resulting in significant system latency and power consumption. To overcome these issues, analog computing using optical interferometric-based linear processors has recently appeared as promising candidates in accelerating matrix-vector multiplication and lowering power consumption. On the other hand, radio frequency (RF) electromagnetic waves can also exhibit similar advantages as the optical counterpart by performing analog computation at light speed with lower power. Furthermore, RF devices have extra benefits, such as lower cost, mature fabrication, and analog-digital mixed design simplicity, which has great potential in realizing affordable, scalable, low latency, low power, near-sensor RF neural network (RFNN) that may greatly enrich RF signal processing capability. In this work, we propose a <inline-formula> <tex-math notation="LaTeX">2\times2 </tex-math></inline-formula> reconfigurable linear RF analog processor in theory and experiment, which can be applied as a matrix multiplier in an artificial neural network (ANN). The proposed device can be utilized to realize a <inline-formula> <tex-math notation="LaTeX">2\times2 </tex-math></inline-formula> simple RFNN for data classification. An <inline-formula> <tex-math notation="LaTeX">8\times8 </tex-math></inline-formula> linear analog processor formed by 28 RFNN devices is also applied in a four-layer ANN for Modified National Institute of Standards and Technology (MNIST) dataset classification.
A new architecture of self-injection-locked (SIL) quadrature radar sensor based on a metamaterial (MTM) leaky-wave antenna (LWA) to detect multi-target vital sign and location simultaneously is ...proposed for the first time. The angular information of targets is obtained by a 1-D MTM LWA with -50° to +30° beam-steering angle when the frequency varies from 1.85 to 2.85 GHz. Furthermore, the distance from the radar sensor to the targets is obtained by imposing the frequency-shift keying (FSK) modulation on the radar waveforms. Meanwhile, the vital sign signals of multiple targets can be extracted from the Doppler frequency spectrum. For the proof of concept, two frequencies, 2.24 and 2.45 GHz, are chosen to detect two targets at different locations, which correspond to the scanning angle at -10° and -40°, respectively. A first-order microwave differentiator followed by a quadrature coupler is employed to convert the radio frequency (RF) signals from the output of the MTM LWA to in-phase (I) and quadrature (Q) RF signals with frequency-dependent amplitudes, thereby eliminating the null point issues for radar detection. Experimental results show that the proposed SIL MTM radar sensor can detect the target location accurately, while the obtained vital sign results agree well with the ground truth.
A radar sensor architecture based on a super-regenerative oscillator (SRO) with very high sensitivity is presented for vital sign detection and motion sensing. As a proof of concept, the proposed ...SRO-based radar sensor architecture, operating in its logarithmic mode, incorporates a patch antenna as a frequency-selective network to transmit and receive radio frequency (RF) signals at 2.32 GHz to conduct target sensing. In this prototype, a common source heterojunction (HJ)-FET is adopted as a variable-gain amplifier in the positive feedback loop of SRO, where two quench signals, 200 Hz and 10 KHz, are employed respectively at the gate of the transistor to periodically modulate the oscillation condition. Furthermore, the theoretical analysis for the radar signal gain of the proposed SRO radar sensor is carried out, demonstrating a 20 dB/decade voltage gain with respect to the input signal reflected from the target. Thanks to the intrinsic automatic gain control characteristic of SRO, the voltage gain of radar signal can go beyond 100 dB in theory. To this end, the measured voltage gain of radar signals with different quench frequencies of the SRO is compared with the theoretical values in the logarithmic mode, exhibiting a maximum gain of 70 dB in the experiment. In addition, compared with the self-injection-locked (SIL) architecture, the experimental results show that the proposed SRO-based radar sensor can exhibit 34-39 dB voltage gain improvement, thereby detecting the actuator movement and vital sign signals of human target accurately at a longer distance with higher sensitivity and reduced circuit complexity.
In this article, we introduce a wireless nondestructive evaluation (NDE) technique that exploits intermodulation interrogation from a compact, fully passive nonlinear antenna sensor, so as to ...suppress electromagnetic interreferences, such as clutters, echoes, and self-jamming. The antenna sensor consists of a local oscillator and a passive rectifier/mixer, connected to a microstrip patch antenna whose resonance frequency is sensitively tuned by the surface crack level (i.e., cavity perturbation). This nonlinear antenna sensor receives a frequency-hopped radio frequency (RF) monotone from the interrogator and reradiates an intermodulation signal produced by mixing the input RF signal (2.4 GHz) and the low-frequency signal (1.0-1.2 MHz) generated by the local oscillator. Through the frequency-hopped spread spectrum (FHSS) interrogation, the spectral signal strength pattern can identify the crack level encoded in the resonance frequency shift of the microstrip antenna. Compared with other nonlinear wireless sensors, such as harmonic/subharmonic sensors, the proposed intermodulation-based antenna sensor not only enables clutter-resistive passive wireless sensing, but also provides several advantages. First, it can significantly reduce the size and cost of the sensor and interrogator by removing high-harmonic antennas and reducing the design complexity of the interrogator. Second, it provides digital memory-free identification by assigning the oscillation frequency as the sensor's identification (ID) key. Additionally, receiving intermodulation signals rather than harmonic tones can reduce the path loss and thus increase the wireless interrogation distance. The proposed fully passive intermodulation-based antenna sensor may open a new path to build portable, low-cost, low-noise, energy-efficient wireless pervasive NDE platforms.
This article presents a nonlinear leaky wave antenna (LWA) with frequency dependent parametric radiation based on a fundamentally slow‐wave transmission line (TL) structure. Unlike a conventional LWA ...that radiates at the excitation frequency, the radiation for the proposed travelling wave structure relies on the parametric frequencies based on the injected pump signals. The proposed nonlinear fundamentally slow wave structure utilizes a periodic sharply bend TL loaded by varactor diodes as nonlinear elements. By utilizing the n = −1 spatial harmonic, the fundamentally slow wave structure can enter the leaky wave region at higher frequencies, where the parametric radiation results from the bifurcation of the injected pump signals. Such TL‐based nonlinear LWA reduces the design complexity and fabrication difficulty. The resulting parametric frequency radiation can be used for beam steering, which provides additional degree of design freedom.
A new type of nonreciprocal composite right/left-handed leaky wave antenna (CRLH LWA) is introduced, which incorporates a distributed mixer (DM) for simultaneous transmit and receive. In this ...proposed design, the drain line of the DM is formed by the CRLH LWA, while the gate line consists of a conventional microstrip line. GaAs-based field-effect transistors (FETs) are used as mixing elements within the microstrip line. The local oscillator (LO) signal is injected through the input port of the microstrip line, connected to the FET gates. In the receive mode, the RF signal captured by the CRLH LWA on the drain side propagates in the same direction as the LO signal. Conversely, during the transmit mode, the RF signal is injected from the composite right/left-handed (CRLH) drain line port in the opposite direction to the LO signal. By leveraging the principles of the DM, it is demonstrated both theoretically and through measurements that the proposed CRLH DM exhibits a directional dependency in its mixing behavior. This characteristic allows for good isolation between the transmit and receive modes for the intermediate frequency (IF) signal by selecting an appropriate IF signal extracted from the other port of the CRLH drain line. This directional dependency in RF-to-IF conversion, which exhibits nonreciprocal property, provided by the CRLH DM enables simultaneous transmission and reception of the RF signal. The received information can be extracted from the downconverted IF signal.
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