Recording small frequency drifts of microwave oscillators

A method of recording small frequency drifts of ultrahigh-stability microwave oscillators by heterodyning without causing instability is proposed, a method which involves two frequency stepdowns with subsequent measurement of period fluctuation in the low-frequency signal by means of an electronic period counter. The latter contains an internal quartz oscillator and its relative error of period measurement, which determines the sensitivity of the method, depends on the period of its fluctuations and on the signal-to-noise ratio at its output as well as on the relative frequency instability of that quartz oscillator. When the signal-to-noise ratio is high, above 60 dB, then the fluctuation period of that quartz oscillator becomes the dominant factor and the sensitivity of the method is proportional to the low frequency squared. The instrumentation includes a balanced microwave mixer in the form of a hybrid ring of microstrip lines with a ferrite diode at each of the two inputs an amplifier on the output side in the first stage, a low-frequency mixer feeding through a tuned amplifier into the electronic period counter with a frequency synthesizer acting as heterodyne in the second stage, and a monitoring spectrum analyzer across the counter input.