Application of the interferometric noise measurement technique for the study of intrinsic fluctuations in microwave isolators

An ultrasensitive noise measurement system has been used to study the intrinsic fluctuations in microwave circulators and isolators. The key element of the measurement system is a carrier suppression interferometer which contains the device under test in one of its arms. The interferometer is used to cancel the carrier at the input of a low noise microwave amplifier in the readout system. Fluctuations in the device under test are then registered by the readout system which has an effective noise temperature of 360 K and is almost equal to its physical temperature. This corresponds to a phase noise of -193 dec Hz-l with 20 dBm of microwave power incident on the device under test. We show that this is sufficiently low to measure intrinsic fluctuations in low noise microwave components such as ferrite circulators and isolators. The noise of the microwave isolators was shown to be independent of both, microwave power and temperature in the region +10 to +20 dBm and -10 to +50 degrees C. We have also measured the phase shift in microwave isolators as a function of external magnetic fields and temperature to determine the effects of environmental fluctuations.