Frequency Versus Time We think of frequency sources such as RF and microwave signal generators as well controlled sources of frequency. It is easy to verify signal source performance using standard test equipment such as spectrum analyzers, frequency counters, or signal source analyzers. These instruments are fine so long as the signal source is relatively stable at a fixed frequency. But what if you need to analyze what happens as the frequency source slews from one frequency to another such as in the case of a sweep generator, frequency synthesizer or simple VCO? You won't have much luck using conventional instrumentation to measure these. What is needed is an instrument that can measure frequency versus time.