Enhanced range phase noise measurements of voltage-controlled phase shifters using carrier nulling

Carrier nulling techniques have been used to measure the phase noise exhibited by UHF voltage-controlled phase shifters obtained from two different suppliers. The use of the phase shifter (DUT) within the carrier nulling bridge results in enhancement of DUT phase (and amplitude) noise by an amount equal to the relative null depth. This, in turn, allows measurement of devices whose 1/f AM and PM noise levels are lower than those of the noise measurement apparatus. Using this technique, it has been established that the 1/f PM noise level exhibited by the phase shifters is quite low. The noise level actually measured is largely masked by the KTBF (white) noise of the post-nulling amplifier. In the measurement set-up used, the post-nulling amplifier PM noise corresponds to a DUT noise measurement limit of -175 dBc/Hz (noiseless DUT). The 1/f PM noise level measured for the phase shifters is approximately 15 to 25 dB lower than that measured for several types of TO-8, voltage-controlled, UHF PIN diode attenuators. One may conclude, therefore, that the use of varactor diodes in phase shifters and other variable reactance circuits will allow construction of voltage-variable attenuators having much lower 1/f PM noise than the UHF PIN diode attenuators evaluated by the author