Power amplifier and power supply distortion of pulse compression radar chirps

This paper examines the effects of time-domain pulse droop on linear FM chirp signals used in pulse compression radar. The radar literature is oddly quiet on how to specify power amplifier droop for a desired level of radar performance, with the result that overly stringent droop specifications (-0.5 dB maximum droop, e.g.) are sometimes imposed on modern phased array radar antennas. Since this drives array cost, mass, and power consumption, it is worthwhile asking how much droop can be tolerated. The present paper analyzes the effects of droop on compressed linear FM chirp waveforms and derives criteria that maintain the fidelity of the matched filter output. The application of a spectral window to control time sidelobes is included in the analysis. We show the perhaps surprising result that chirps are robust to as much as 3 dB of droop. In fact, loss of sensitivity due to reduced energy on the target becomes an issue before significant distortion of the compressed chirp waveform occurs. This work should point the way to sensible droop specifications resulting in more cost effective phased array antennas.