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

From: Tomaž Šolc
Subject: Re: [Discuss-gnuradio] RMS value of a signal changes with PAPR
Date: Tue, 14 Apr 2015 12:59:31 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.6.0

Dear Lou,

On 13. 04. 2015 17:57, madengr wrote:
> 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.

Thank you very much for this suggestion. Indeed it appears that I was
taking an average of the log when doing power measurements with the
signal analyzer (technically my instrument is indeed a signal analyzer,
not a spectrum analyzer)

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.")

> 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.

Now everything indeed seems to point to the signal generator. I don't
have a RF power meter though. I did just measure the RMS of the output
signal with a digital oscilloscope (at a low frequency), and that also
shows that the RMS increases with PAPR, but that is not a very accurate

It's not the ALC loop I think, because I can switch it off and the
difference remains. Anyway, at least now I know where to look. Thanks.

> 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.

I tried several flow graphs for calculating signal power in GR (the
standalone "RMS" block, "Complex conjugate" + "Multiply", "Complex to
real" + "Complex to imag" + "Multiply", etc.). I haven't tried
complex_to_mag_squared, but others all come to within 0.1 dB of each other.

Best regards

> 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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