Microwave-photonic architecture for direction finding of LPI emitters: front-end analog circuit design and component characterization

Abstract

Direction finding (DF) systems are critical components in electronic warfare intercept receivers. Many existing radio frequency DF techniques use a non-linear frequency down-conversion process that generates spurious frequencies that can hide the signals-of-interest. In addition, these systems suffer from a limited bandwidth and poor resolution (e.g., > 2°). To overcome these limitations, a miniature microwave-photonic phase-sampling DF technique is investigated in this thesis. This front-end design uses a combination of integrated optical Mach-Zehnder phase detectors to measure the emitter's phase difference with the largest baseline being 23 cm. The front-end components were characterized to ensure accuracy in the resolved angle-of-arrival (AOA). The front-end design was constructed to be modular in order to facilitate testing and verification of analog components accuracy. Signal processing with a class IV laser required particular attention to ensure that any stray radiation or leakage was isolated. Testing with low probability of intercept (LPI) waveform modulation consisted of using a linear frequency-modulated continuous wave (FMCW) and a phase coded P4 modulation, both at 2.4 GHz, in an anechoic chamber. Measurements were taken to quantify the DF receiver sensitivity was Ȣʀ = 62.96 dBm. It was demonstrated that the system was capable of estimating the AOA for the FMCW signal with a root-mean square (RMS) error of 0.29° at < 1° resolution and a P4 modulation RMS error of 0.32° at <1° resolution.