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Pulsed X-Band PM/AM Noise Measurement Systems
Craig Nelson of the NIST Phase Noise Group has developed a second generation dual-channel PM/AM noise measurement system that is designed to make low-noise PM or AM measurements of either pulsed or CW amplifiers or other devices. Several breakthroughs in the development program have provided significant improvements over conventional commercial approaches. Using the Group’s vast experience in low-noise frequency synthesis and PM and AM noise measurement strategies has resulted in a reduction by more than 30 dB in the measurement-system noise floor in pulsed mode, an improvement by a factor of 1000.
The measurement system has two nearly identical channels for measuring PM and AM noise. The correlated spectrum representing the phase noise of the device under test is extracted; uncorrelated noise from each PLL averages down, thus lowering the measurement noise floor and enhancing the precision in the estimate of phase noise. Furthermore, the pulsed measurement system uses especially fast FFT cross-spectrum analysis and precise blanking that reduces the test time from 45 min to less than 30 s. This reduction of test time by a factor of 100 plays a vital role in circumventing a costly test problem, namely fatigue or failure of an expensive device under test (DUT).
Each channel in the PM noise-measurement mode uses a double balanced mixer operating at quadrature by completely automated control. The phase between a low-noise synthesized reference source is split into signal and reference channels. The signal channel is routed to the input of the DUT which is typically an amplifier; however it could be any device that does not change the average signal frequency. For PM noise measurements, the output of the DUT is compared with the phase of the reference channel using a double balanced mixer. Once the phase between the two signals is set at approximately 90 degrees (quadrature), the system controller maintains quadrature throughout testing using electronically controlled phase shifters. In this way, the NIST system readily handles wide excursions in phase drift of the DUT, a problem that invalidates standard approaches that introduce spurious AM noise components.
Calibration of the mixer's phase-to-voltage conversion, the gains of the various amplifiers following the mixer, and the FFT spectrum analyzer is done simultaneously by using a SSB PM/AM calibration standard. The approach uses a calibrated amount of offset signal power that can be added to the reference and produces equal amounts of PM and AM modulation when the added signal power is small compared to carrier power. The system controller measures the peak reference signal power to determine the level of PM and AM sidebands generated by the addition of the calibration signal to the reference. The output signals detected at the FFT in the various channels, relative to the calibrated PM/AM signal, quickly provide a highly precise measurement of the gain of the entire PM or AM noise-detection process. Compared to conventional approaches, the uncertainty is reduced from +2 dB to +0.5 dB, an improvement by a factor of 3.