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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: Mon, 13 Apr 2015 08:57:22 -0700 (MST)


Looks like you are measuring power on the spectrum analyzer with bandwidth
integration, and it's giving you -95 dBm in both cases, which is good if
it;s doing teh math, but what happens when you put the analyzer in linear
mode?  Or leave it in log mode, open up the RBW >> 100 kHz and put it in
noise marker mode?  Reason I'm suggesting is that the spectrum analyzer
(assuming it's not signal analyzer) is taking the video average of the log
detector output, versus the log of the average, and that equates to 1.4 dB
error for CW vs AWGN.

Maybe the ALC loop in your signal generator is also using a log detector,
which dives you a false power for AWGN, which the spectrum analyzer then
falsely corrects with it's log detector.  Maybe GR is giving you the correct
answer.  Need an amp and power meter to give you a true power measurement.

In GR I use complex_to_mag_squared block and integrator (with decimate) to
get the mean square power.  Maybe try that to see what it yields.


Tomaž Šolc wrote
> Dear all,
> I'm calculating the RMS value of a signal with the following setup
> RF vector signal generator -> USRP/rtl-sdr/... -> "RMS" block in GRC.
> Please bear with me - I'm not interested in the exact (absolute) voltage
> level.
> What I can't explain is why the calculated RMS value consistently shows
> a higher value when the signal is modulated (e.g. has a higher
> peak/average power ratio) compared to CW (unmodulated sine wave)
> For instance, RMS value shown in GRC for band-limited Gaussian noise is
> always around 2.5 dB _higher_ than RMS of CW of equivalent power
> (equivalent power according to the generator level setting and a
> spectrum analyzer with a power meter function). Similarly, 100% AM
> modulated signal shows around 1.3 dB _higher_ RMS.
> This effect appears with a USRP, rtl-sdr dongle as well as some custom
> hardware, so it doesn't seem to be something device-specific. Also, all
> hardware effects on the receiver side I can imagine result in _lower_
> gain for modulated signals. If anything, I would expect to see a _lower_
> RMS when turning on the modulation.
> Some more details are in this blog post:
> https://www.tablix.org/~avian/blog/archives/2015/04/signal_power_in_gnu_radio/
> Any ideas would be welcome. At this point I have a feeling I'm missing
> something obvious here...
> Thanks,
> Tomaž
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