| Microwave signal measurement technology is widely applied in astronomy,communication,navigation,transportation,vehicle control systems,uncrewed aerial vehicles,radar control systems,medical equipment,home appliances and many other fields.A radar system determines the speed and location of a moving object from the Doppler frequency shift(DFS)and angle of arrival(AO A)of the echo signal.In the complex and changeable electromagnetic environment,radar signal frequency is regarded as an essential criterion for signal sorting and identification,so instantaneous frequency measurement is of great significance for radar reconnaissance in electronic warfare.In recent years,the traditional measurement technology for microwave signals has faced the narrow bandwidth bottleneck of electronic devices.Microwave photonics realizes the measurement and processing of microwave signals through optical technology,which has the advantages of low transmission loss,wide operating bandwidth,parallel processing,strong antielectromagnetic interference,etc.It is a potential technology for realizing the DFS,AOA and frequency measurement of high-frequency broadband microwave and millimeter-wave signals.In this paper,the recent domestic and international research trends of photonics-assisted DFS,AOA,and instantaneous frequency measurement techniques for microwave signals are reviewed,and the research status and existing problems are analyzed in depth.Then,the photonic-assisted schemes for measuring the DFS,AOA,and frequency of a microwave signal are proposed.The main research work and innovation of this paper are as follows:(1)A high precision and unambiguous scheme for simultaneous measurement of DFS and AO A of a microwave signal based on polarization division multiplexing dual-parallel Mach-Zehnder modulator(PDM-DPMZM)is proposed.In this scheme,PDM-DPMZM is adopted to realize single sideband modulation for transmitted signal,echo signals,and reference signal.By adjusting the polarization controller and polarizer,the phase difference of the ac power of the upper and lower output lowfrequency signals generated by photoelectric conversion is 90°,two highresolution phase-ac power mapping curves are constructed,and a unique phase value can be determined in the phase measurement range from-179°to 179°,thus achieving high-precision AOA measurement without direction ambiguity.The simulation achieves an error within ±0.9°of the measured AOA in the range of-83.92° to 83.92°,the DFS measurement within±100 kHz is realized with an error of ±5 ×10-3 Hz,and the direction of DFS can be determined by the relative frequency value on the electrical spectrum analyzer.(2)A scheme for the simultaneous measurement of DFS and AOA of a microwave signals without filtering and polarization devices is proposed.The scheme adopts the structure of DPMZM and dual-drive Mach-Zehnder Modulator(DDMZM)in parallel,and utilizes the principle of photon mixer to convert the incident echo signal and reference signal into low-frequency signals,so a mapping relationship between phase difference and ac power ratio is constructed.Moreover,by adjusting the DC bias voltage of the main modulator of DPMZM,two phase-ac power ratio mapping curves with a phase difference of 120° are constructed,which improves the measurement accuracy in the range of phase from-60°～60°,and thus improves the measurement accuracy of AOA.In the simulation,the direction of DFS is determined by comparing the relative frequency of two low-frequency electrical signals.For DFS in the range of±100 kHz,the measurement error is within ±5 ×10-3 Hz,and the scheme achieves AOA measurement with an error within±0.6° from-83.92° to 83.92°.This scheme does not use optical filters and polarization multiplexing devices,optimizes the measurement link,and solves the problem of amplitude dependence of echo signal during AOA measurement.(3)The microwave signal instantaneous frequency measurement scheme based on frequency-phase mapping technology is proposed.The scheme divides the signal under test into two paths and introduces relative delay τ,and the fixed low-frequency reference signal and two microwave signals under test with relative delay τ are modulated onto the optical carrier via DPMZMs in parallel.The phase difference between the lowfrequency electrical signal generated by a low-frequency photodetector and the reference signal is related to the delay τ and frequency of the signal to be measured.Therefore,a linear frequency-phase mapping function with a high slope is constructed,which enables high-precision frequency measurements.This scheme has a compact structure and only requires a low-frequency photodetector,without using polarization devices and tunable optical filters.The simulation results show the error is within ±0.04 GHz in the frequency measurement range of 4.1 GHz to 40 GHz,and the effects of DC bias voltage drift,and phase unbalance introduced by the 90°hybrid and power divider on the frequency measurement accuracy are also analyzed and discussed. |
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