Short-term stability for a Doppler radar: Requirements, measurements, and techniques

Short-term frequency stability is an important parameter affecting resolution and range of a Doppler radar. This paper describes system and circuit requirements found in a typical airborne Doppler radar designed for operation in a severe vibration and acoustic environment. The characteristic of a Doppler radar which leads to short term stability requirements is its use of a narrow-band receiver to detect a Doppler-shifted target return which is weaker than clutter. The system short-term stability requirements are determined by the following two points: 1) Target return linewidth has a direct effect on sensitivity and velocity resolution; it determines the minimum useful Doppler filter bandwidth. 2) Transmitter and receiver local oscillator noise sidebands appearing on clutter determine the maximum possible sub-clutter visibility. Short-term stability for a Doppler radar is defined in terms of linewidth and spectrum. Oscillator and crystal requirements are derived from the system requirements. Measurements of linewidth and spectral purity under quiescent and environmental conditions are described, and vibration characteristics of quartz crystals are considered.