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Re: [Discuss-gnuradio] RMS value of a signal changes with PAPR

From: madengr
Subject: Re: [Discuss-gnuradio] RMS value of a signal changes with PAPR
Date: Tue, 14 Apr 2015 07:30:44 -0700 (MST)

Now that's disturbing that the analyzer is giving a false readings over
integrated BW measurements, even when it claims it's in RMS mode, although
that's maybe opposed to peak detector mode.  It's still probably defaulting
to a LOG detector, and you can measure multi-tone signals, but each tone
must fall within it's own RBW to get a correct RMS measurement, then
integrate all of those to get the total power.  I suppose the moral of the
story is traditional spectrum analyzers in log detector mode are only valid
for measuring single tones within a RBW, and use a thermistor based power
sensor to validate everything.

Maybe you can now try calibrating your setup to dBm by measuring a strong,
single tone then adjusting the K offset in LOG10 block to read the same as
the spectrum analyzer.  With the WBX card I can vary the source 40 dB and
the power computed by GR varies maybe +/- 0.1 dB.  I have not tried it with
AWGN so I'd be interested to see your results, and I'm guessing it will be
off due to shape of filter.  Maybe instead use the spectrum plot for
calibration near the center of band, then normalize by bin size to get a
dBm/Hz for noise.


Tomaž Šolc wrote
> After some poking around the menus I found the "Average Mode" setting.
> When setting it to "Power" instead of "Log" (which is the default), the
> power measurements match what I'm seeing in GRC.
> I didn't look for this previously, because the instrument has a special
> "channel power" measurement mode which I assumed set everything
> correctly (it automatically sets, for instance, the detector to "RMS"
> mode, optimal RBW and so on). I've gone through the manual, and it's a
> bit cryptic on this topic ("The average value is always correctly
> displayed irrespective of the signal characteristic.")

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