When evaluating the N9080B on the N9020A, it's been seen that the application in default mode for SEM measurements chooses Average-detector for in channel, and Peak detector for offset (out of channel).
To make a “True RMS detection”, that is requested by the 3GPP conformance testing, based on ITU-R SM.329, for a mix of noise-like & CW-like signals under test, use “Peak Detector with very narrow VBW” method instead of directly using the “RMS detector”.
Although it may not be intuitive compared to just using the RMS detector, the method measures CW-like signals (e.g. spurious) with the Peak Detector accurately and the noise-like portions to be RMS averaged with the narrow VBW settings.
A disadvantage of using this Peak Detector with the Narrow VBW setting is to make the sweep speed slower at a time, but it doesn’t require Avg/Hold ON, which is necessary for the RMS detector mode to make the result variance settled down.
MicroWaves& RF Magazine, July 2011
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