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N9080B SEM measurement defaults choose Average-detector for in channel, and Peak detector for offset (out of channel). 3GPP 36.141 V12.12.0 (2016-06) seems to state “True RMS” (Average) should be used for both. Is N9080B doing something it shouldn’t?

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.

The technical details are described in an article written by Joe Gorin, our very experienced SA R&D engineer in Santa Rosa, and published in MicroWaves & RF magazine (July 2011, page 54).

This article can be viewed (in web text) via the link below: 

One keyword here is to minimize “Scalloping error” of CW-like spur which may occur (max 3dB lower than the reality) with the RMS detector when the spur comes to in between a pair of contiguous detector buckets.

In addition, a seemingly disadvantage of using this Peak Detector with Narrow VBW setting is to make sweep speed slower at a time, but in fact it doesn’t require Avg/Hold ON which is necessary for the RMS detector mode to make the result variance settled down.