Holzworth Phase Noise analyzers offer a valuable External LO Mode feature. The External LO Mode function allows the user to connect two non-coherent frequency sources to the system to generate the test system LO frequencies. This feature is advantageous for testing DUTs that have a considerably better phase noise performance level than the internal, synthesized LO sources of the phase noise analyzer.

Phase noise measurements fall under two categories: absolute and residual (additive). Where absolute phase noise measurements are typical of oscillators and and other frequency sources, residual phase noise measurements are more specific to measuring the phase noise contribution of an individual component or subsystem.