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Re: [Discuss-gnuradio] calibrating USRP


From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] calibrating USRP
Date: Wed, 10 Jul 2013 08:33:02 -0400
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I think that i am a little bit confused with calibration. Is there any way to determine the gain of the circuitry in front of the ADC? Is it correct enough to say that power level at the input of PGA in front of ADC is
just ((sampled_value*Vp-p)/2^12  ^  2 ) / Rinputpga?

The total gain in front of the ADC will vary from daughtercard to daughtercard, and with frequency.

Calculate the power inside the flow-graph, which is proportional to AVG(I*I + Q*Q).   For RMS power, there's an RMS block.

What you need to do is use a *calibrated-in-power* signal source, and use that to calibrate your receiving setup.  

You can't reliably backtrack from the values coming out of the USRP, because you don't know the total gain in front of the ADC to sufficient
  precision.



On Mon, Jun 3, 2013 at 5:05 PM, Marcus Leech <address@hidden> wrote:
I'd tune slightly-off the target frequency.  The gain isn't going to change that rapidly across the tuned frequency range.  If I were doing this, I'd probably calibrate every 20Mhz or so.

Since neither UHD nor Gnu Radio have the concept of calibration data, you can store it in whatever way is appropriate for your application. It's a simple matter of programming...
 
on Jun 03, 2013, Nemanja Savic <address@hidden> wrote:
Thank you Marcus for super fast answer. Well, for the begining I would like to skip temperature calibration, only power level calibration regarding frequency. I have following doubts:
1. To which frequency should be tuned USRP if for example generator generates 400 MHz. Should it be almost the same frequency, so that downconverted signal falls very close to DC or something else.
2. What is the best advice for storing calibration data.


On Mon, Jun 3, 2013 at 4:44 PM, Marcus Leech <address@hidden> wrote:
Indeed, the only way to do this is to use a signal generator with known power levels, and precision attenuators.  You'll have to repeat the process over the full tuning range of the device(s) in question, since
 effective gain will change a little with tuned frequency--that's just a natural property of analog RF components.  The gain will also change (a little) with ambient temperature as well, so it depends on how precise you want your calibrations to be.
 
 
 
on Jun 03, 2013, Nemanja Savic <address@hidden> wrote:
Hi all guys (yet again),
I have a question about calibrating received power level of my USRP, equipped with wbx, lftx and lfrx.
I am not sure whether it is apropriate to discuss that here.
Basically I would like to be able to determine absolute power level I am receiving. I suppose that basic procedure for doing this would be using super precise, expensive signal generator. Then tracing signal of know power to daughter board and measure power level in fft sink. And maybe also doing this for complete band of daughterboard usage. If the previous is correct, than I would like to ask you, how do you store calibrating data, inside .h file or ...?
Best and thank you,

--
Nemanja Savić


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--
Nemanja Savić



--
Nemanja Savić


-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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