Photonic-assisted single system for microwave frequency and phase noise measurement

Deceptive jamming is one of the important electronic interference methods in modern warfare. By producing false or misleading target echoes, the radar will obtain false target information and make wrong judgments. In order to resist the deceptive jamming, radar receivers need to distinguish the real and false targets by measuring the parameters of the received signals, like frequency. Unfortunately, the electronic jamming equipment, empowered by the current digital radio frequency memory (DRFM) technology, is capable to produce false signals with the same carrier frequency of the real target echoes. As a result, it is difficult to identify real and false targets by simply monitoring the frequency.

Phase noise is an important parameter that characterizes the stability of a microwave signal. The probability of correctly identifying false targets can be significantly improved by simultaneously monitoring the frequency and the phase noise of the received signal. Considering the continuous expansion of the radar spectral coverage, the measurement of microwave frequency and phase noise in a wide spectral range is the key to improve the anti-deceptive jamming capability.

In recent years, the rapid development of microwave photonics has provided new solutions for broadband microwave frequency and phase noise measurements. Compared with the electrical methods, microwave photonic methods have the advantages such as high frequency and large bandwidth. However, the previous photonics-assisted microwave frequency measurement and phase noise measurement are physically isolated from each other.

In Chinese Optics Letters, Volume 18, Issue 9, 2020 (J. Z. Shi, et al., Photonic-assisted single system for microwave frequency and phase noise measurement), Prof. Fangzheng Zhang, Prof. Shilong Pan and their colleagues from the Microwave Photonics Laboratory in Nanjing University of Aeronautics and Astronautics report a photonics-assisted single system for measuring the microwave frequency and phase noise simultaneously in a wide spectral range. The system uses a photonics-assisted broadband in-phase and quadrature mixer to acquire the phase difference between the signal under test and its replica delay by a span of optical fiber together with a variable optical delay line. By adjusting the variable optical delay line, the frequency of the signal under test can be estimated through frequency-to-phase-slope mapping, and corresponding phase noise can be calculated from the power spectral density of the phase fluctuation under a specific time delay. In the experiment, frequency and phase noise measurement of microwave signals from 5 to 50 GHz is successfully demonstrated, of which the frequency measurement errors are controlled within ±150 MHz and the phase noise measurement errors at 10-kHz offset frequency are kept within ±3 dB.

Prof. Fangzheng Zhang from Nanjing University of Aeronautics and Astronautics believes that this work can provide important technical support for electronic anti-interference. Considering that the current experimental demonstration system only measures single-frequency signals, future work will focus on the parameter measurement of complicated signals such as linearly chirped microwave signals.

(a) Photonics-assisted single system for microwave frequency and phase noise; (b) frequency measurement errors versus frequencies from 5 to 50 GHz; (c) phase noise measurement results of signals tuned from 5 to 50 GHz.