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

From: madengr
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Mon, 25 Aug 2014 07:23:07 -0700 (PDT)

1)  What you are doing is correct.  What's the power of your signal
generator?  I was using -40 dBm, and as Marcus says, that is strong, but
you'll need to have a good SNR since the noise bandwidth is ~250 kHz. 
Several posts ago you stated running -8 dBm into the WBX.  You may have
damaged the WBX, since that 30 dB difference is about the isolation you may
get with a damaged amp.  I suggest sniffing down the chain of the WBX with a
spectrum analyzer, make sure the two amps are working.  Also as Marcus said,
you need to be using the same bandwidth and decimation, or that will change
the calibration factor.

I choose 250 kHz wince I was working with WBFM channels.

2) If the RX gain is changed, then the k must change to compensate.  See my
*.grc I linked in the last post.  I assigned RX gain to a QT slider then
added it to K.  The RX gain can be changed and the RX power stays about the

3)  I'm on travel now so I don't have access to my computer.  You ought to
update to GR 3.7; lot's of nice features, and it also preserves the flow
diagram even if the blocks of changed.


Gayathri Ramasubramanian wrote
> Hi
> 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.
> Thanks
> Gayathri

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