This paper first describes the fundamentals of two-port phase noise measurement for microwave amplifiers and other devices. On the basis of the fundamental circuit

we have set up a highly sensitive phase noise measurement system

which contains a carrier suppression circuit

a phase modulation signal generation circuit

an appropriate exciting source and a set of advanced test equipment. The operating frequencies of the system are at 3cm band. The system can be used to measure the residual phase noise of amplifiers under test which can be operated under the condition of CM and PM. The background noise of the syste-m is less than-130dBc/Hz at 1kHz offset frequency.