On the flicker noise of ferrite circulators for ultra-stable oscillators

The flicker noise of the ferrite circulator is a critical element in ultra-stable microwave oscillators, in which the signal reflected from the input of the reference cavity is exploited to stabilize the frequency. This article explains why the circulator noise must be measured in isolation mode, proposes a measurement scheme, and provides experimental results. The observed flicker spans from -162 to -170 dB[rad/sup 2/]/Hz at 1 Hz off the 9.2 GHz carrier, and at +19 dBm of input power; in the same conditions, the instrument limit is below -180 dB[rad/sup 2/]/Hz. In addition, the scheme can be used as the phase detector of a corrected oscillator, and in the field of solid-state physics as an instrument for the measurement of random fluctuation in magnetic materials.