Subharmonic sampling for the measurement of short-term stability of microwave oscillators

The use of digital techniques for the precise measurement of medium- and short-term frequency stability of oscillators requires good resolution of the frequency counter. In the case of microwave oscillators, this is often preceded by some form of down-conversion. This paper is concerned with the theoretical principles, description, operation, and performance of a down-conversion system based on the subharmonic sampling technique. The system is intended for use on microwave oscillators with good frequency-stability performance and has as main features the use of a single frequency-stable reference oscillator and a pulse generator-sampler which allows power optimization of the subharmonic selected. The complete experimental system is presented and the limitations are discussed.