Instrumentation for measurement of the short-term frequency stability of microwave sources

The short-term frequency stability of microwave sources employed as carrier frequency generators in coherent radar systems is often expressed in terms of the power spectrum of the microwave signal. It may also be expressed in terms of the equivalent modulation spectrum that would produce the microwave signal spectrum. This paper discusses apparatus capable of measuring the modulating signal spectrum corresponding to a sideband-to-carrier level of -115 dB/Hz at 1000 Hz and greater from the carrier frequency at X-band. A passive reference is employed, although use of an active reference source is considered. Two stability measuring instruments are discussed, one utilizing a reflection cavity resonator and the other a transmission cavity resonator. The latter unit provides a number of significant features; for example, the capability of direct calibration, direct measurement of the limiting sensitivity of the instrumentation, and measurement of intentional frequency modulation of the source under observation.