8903B, SINAD Measurement, Average and RMS Detectors – Technical Support Knowledge Center Open

The most suitable detector to use for measuring SINAD with the Keysight 8903B is the rms detector. This is because the SINAD measurement is made in the presence of noise. This noise is quite high relative to the signal. Keep in mind that the SINAD is measured when the reading is around 20 dB. This is different from a distortion measurement or a signal to noise measurement, where the signal is well above the noise level. The 8903B overcomes this somewhat by using smoothing filters when in the SINAD mode, and also by rounding off the display so that it changes in 0.5 dB increments instead of 0.1 dB.

The other type of detector is the average detector. This average-responding detector is calibrated to indicate the rms value of a signal (1.11 times the average value of the rectified waveform). However, this gives a true rms value ONLY when the signal is a pure sinewave. When the signal is a complex waveform, or when noise is present, the average detector is almost always lower than a true rms detector. With noise or harmonics present, the ratio of true rms-to-average is no longer the same.

As for sampling rate, it is the same for both average and rms detector, and for the fast or slow mode of either detector; it is approximately 4 samples per second. The slow mode changes the time constant of the circuit, but does not change the actual sampling rate.

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