A system is described which directly measures the amplitude modulation to phase modulation conversion in a microwave amplifier. The system is of particular use for testing microwave amplifiers operating in the almost linear regime. It has a useful advantage over similar microwave measurement systems in that it is insensitive to low frequency jitter of phase shift or gain of the amplifier under test, in spite of its high sensitivity to a.m.-ph.m. conversion.A carrier frequency is modulated between two levels of amplitude before it is fed into the amplifier under test. The higher level signal is directly compared in phase with the lower level signal by temporarily storing the signal at one level in a delay line of appropriate length so that the two levels of signal arrive simultaneously at a sensitive phase bridge. Fast p-i-n diode switches enable this technique to be used successfully by providing pure amplitude modulation and switching the arms of the phase bridge alternately into a single detector to improve the accuracy of the bridge. This last technique is of interest for all types of microwave bridge.The modulation imposed on the carrier into the amplifier can also be used to measure the gain compression or distortion. The bridge successfully measured down to 0.1° phase shift, and 0.1 dB change in gain, per dB change in power level.