This paper seeks to reduce the confusion between phase noise, clutter suppression and their evaluation. Clutter filters operate based upon the low velocity returns of clutter. Thus the measurement of the Doppler shift is extremely important in the performance of the filtering algorithms.

The Doppler shift (radial velocity) is determined by measuring the phase shift of the received signal from an appropriately delayed sample of the transmitted signal. As the measurement is one of a shift in phase, any noise in the phase component of the signal (phase noise) will limit the velocity detection capability and any processes that rely on this ability, such as clutter suppression. Clutter filters rely on accurate frequency (phase) measurements for their operation. The more noise existing in a system, the less effective the filters will be at suppressing clutter.

Phase noise is an important performance characteristic, yet there remains confusion in specifications of a radar systems concerning phase noise characteristics and its relationship to clutter rejection. The operator is primarily concerned with the latter. The radar purist is concerned with the former. This paper draws upon our experience with the National Weather Service, Evansville radar program, the associated analyses and testing performed to achieve active clutter suppression greater than 65 dB. Major contributors to phase noise are identified and discussed. These include the transmitter (tube and high voltage power supply, synthesizer, and signal processor. Finally a mathematical model to estimate the clutter suppression available to a radar system is presented and evaluated.