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

From: Gayathri Ramasubramanian
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Mon, 25 Aug 2014 01:17:39 -0700

Thank you for your note.
My questions here are just based on my previous measurements and your last mail. kindly clarify the same.

1) Is it possible that different USRPN210 devices with WBX boards have the different calibration factors. I set the channel gain to 0 and still get :
@ 400 MHz :  k = -62.8
@ 900 MHz :  k = -58.5
@ 1800 MHZ: k = -52.5
I checked the above with 3 USRPN210 +WBX devices and found the K value to be very close to the stated values. Your values seem to be different , Hence the question.

My method was that I first ran the flow graph with channel gain as 0 and k as 0. Then found the power value displayed for  the 'channel_power_dBm'.
I subtracted this value from my input signal value (from signal generator) and found out the difference. This is what I am taking to be the 'k' value. If I use k values (62.5 for 400 MHz) it in the block too, it give the 'channel_power_dBm' value to be very close to the input signal generator value.
I repeated this for 3 devices at the 3 frequencies.

Is this correct or am I doing something wrong. As my values seem to be almost 30 ~ 33 higher than ones you are getting from your tests. what could be causing this error/ discrepancy.

2) In you mails you say that we have to subtract RX gain from Calibration factor to baseline it for RX gain = 0 dB . Also in one of your previous mails you had sated that you get k = -34.5 wih RX gain of 15. So in view of these two stmts, was your 'k' value of  -34.5 for 1800 MHz.

This 1800 MHz is the only one satisfying this rule with previous data set: i.e -34.5 - 15 = 19.5 ( this is close to the 1800 MHz results you have written about in your last email). I just want to check if my understanding is correct and Im doing the math right.

3) Could you kindly paste a picture of your flow graph for spectrum calibration along with the xml file like last time. Using the xml to know which bock is specified is a little tough. Having a visual flow graph along with the xml would be more effective to understand and relate between them.

Please clarify above. Look forward to your response.


> On Aug 23, 2014, at 8:38 PM, madengr <address@hidden> wrote:
> 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
> sinks.
> 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:
> https://dl.dropboxusercontent.com/u/49570443/power_calc_2.grc
> 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.
> Thanks,
> Lou
> 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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