This paper describes a simple method of measuring instabilities in communication systems such as satellite links. The technique is based on the use of a modified commercial phase meter and frequency synthesizer. This can linearly measure the magnitude of phase variation of a source, in a bandwidth of 1 kHz with a resolution down to 0.5 deg peak to peak, relative to a stable reference. Frequency instabilities of less than 0.1 Hz can also be measured.The paper includes typical measurements of noise introduced by high stability frequency sources, medium and high-power travelling-wave tubes and klystrons, and low-noise parametric amplifiers.