Long Microwave-Photonic Variable Delay of Chirped Radar Waveforms
Continuously-variable group delay is necessary for beam steering in phased array antennas. The delay of waveforms having high radio frequencies using either electrical cables or digital signal processing units is difficult to achieve. Alternatively, the use of optical techniques for the processing of radio-frequency waveforms, known as microwave photonics, brings about several potential advantages such as ultra-broad bandwidth and inherent immunity against electro-magnetic interference. Nevertheless, 'honest' microwave-photonic delay elements are restricted to delay variations of approximately 1 over the bandwidth of the processed signal. Such delays are often insufficient.
In this work, I examine the prospects of a significant relaxation in the objectives of the microwave-photonic setup. Rather than provide a universal delay of any input signal, an alternative processing scheme is proposed, which is suitable for a particular category of signals that are prevalent in radar systems: the chirped waveforms. While not strictly representing a group delay, I show through analysis, simulations and experiments that the processing scheme has a nearly identical effect on chirped waveforms. 500 MHz-wide signals are delayed by as much as 250 ns, more than two orders of magnitude longer than previous results. The method is applicable to waveforms of arbitrary durations, bandwidths, sweep profiles and central radio frequencies. Special emphasis is given to the fidelity metrics of the processed waveform.
* Research was carried out towards the M.Sc. degree, supervised by Dr. Avi Zadok