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Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements

From: madengr
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Sat, 23 Aug 2014 17:38:58 -0700 (PDT)

1) I set the gain of the WBX to 15 dB, which is why our values do not match. 
If I use 0 dB WBX gain I get the following for a -40 dBm tone:

400 MHz, k=-30.6
900 MHz, k=-25.5
1.8 GHz, k=-19.2

There should be no computational differences between the GUI and frequency

2)  The channel gain is the RX gain; it's the same thing.  The WBX receive
side has two fixed gain stages followed by 0 - 31.5 dB of digital
attenuation.  That's what you are setting for the gain in the UHD; a gain of
31.5 dB equates to 0 dB attenuation setting.  Yes, you have to offset the
computed power by RX gain if you want the reading to stay constant.  Note
the n*log10()+k block does not allow k to me changed in real-time, so you
have to multiply each sample by a constant.  You must do the same if you
want to calibrate the spectrum.  See example here:


I have no idea about the differences between the WX and QT frequency sinks. 
They should give identical results with identical settings.

3) The channel power and spectrum are totally different computations, so
they must have different calibration factors.  See my linked example.

4) For finding intermod levels, you use the same math for the channel power
calculation, but narrow the channel with a frequency translating filter and
decimation.  Either tune to each intermod product or have multiple
channelizers in parallel.  This is where it may be more efficient to do the
FFT and find the peaks, but it's harder to program that.


Gayathri Ramasubramanian wrote
> Hi
> Thank you for previous note.
> I removed the FFT block and made my flow graph similar to yours. It works
> now :)
> It is the first time I am working with the QT GUI widgets so kindly
> help clear the foll doubts.
> 1. I do not have the QT_FREQUENCY_GUI_SINK BLOCK you have used only
> the QT_GUI_SINK which gives the frequency spectrum. This is because of
> the
> lower version ( I suppose.
> For a USRPN210+WBX board I got the calibration factor to be as follows:
> @ 400 MHZ, K = - 62.8
> @ 900 MHz ; K = -58.8
> @ 1.8 GHz ; K = --52.5
> I tested for 3 devices (USRPN210+WBX) and got around same result . The
> Gain was set to 0. This does not match the value you had obtained with
> your test.
> Is there a major difference in the working b/w QT_Frequency_SINK and
> QT_GUI_SINK that would affect the final result?If so, Can you suggest
> a work around. .
> 2. The channel gain value seems to add to the transmitted power from
> signal generator. So the calibration factor also needs to have channel
> gain added to its value for getting same result.
> So it seems like the channel gain affects the signal power like a LNA
> and hence increases the strength of the signal wrt to the noise. Then
> how is it different from RX gain.? Im a little confused on
> where the gain parameters actually impact the value of the signal.
> 2. The QT block also seems to do an internal FFT default size of 256 and
> send out a signal. However this value does not match the value seen on WX
> FFT GUI ( the CUrsor on QT SINK actually gives a vlue very close to actual
> signal generator reading). is there major difference in signal processing
> of QT widgets as compared to WX widgets. I found some previous threads
> stating whats the difference as per the software processing level in GRC
> not about the way it processed the received signal.
> 3. Why is there a difference in the channel_power_dBm value and the value
> seen on the spectrum using the cursor. (refer the picture). The cursor
> shows -81.42 dB @ 0.0 Khz offset from centre freq, while the
> channel_power_dBm shows a value of -17.5 dBm
> 4. Since This gives the channel power, I suppose this cant be used to find
> the intermodulation's power with two tone test. Is my understanding
> correct?or is there a work around for it too?
> Kindly clarify the above doubts.
> Look forward to your response.
> Thanks
> Gayathri

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