Measurement of switching characteristics of agile frequency sources

With the continuing requirement for faster and more stable agile microwave frequency sources, there is a directly linked requirement for the characterisation of such sources. The need to measure switched frequency or phase stability and repeatability often goes beyond the accuracy represented by the digital algorithm of the reciprocal of the measurement time, and enters the area where frequency must be considered as rate of change of phase. The paper stemmed from a particular requirement, and it was in the light of this that two particular techniques were investigated, namely time-interval averaging and delay-line frequency discriminators. Time-interval averaging is a digital technique which repeatedly measures the frequency of a very short sample. This, therefore, gives the absolute frequency averaged over a large number of samples, but cannot measure a single or non-repetitive event. Delay-line frequency discriminators compare the phase of two closely taken frequency samples and will measure single frequency samples and will measure single frequency events, but can be subject to ambiguities. The relative merits and accuracy of these techniques are discussed and experimental results presented in the paper.