Re: [Discuss-gnuradio] USRP Input Calibration / CIC Filter Response

[Eric Blossom]

On Thu, Mar 19, 2009 at 05:43:56PM -0400, Erich Stuntebeck wrote:
> Hi,
> 
> I'm trying to calibrate the USRP so I can get true dBm readings out of
> it. I'm using an LFRX card with 16 decimation and a center frequency
> of 2 MHz so I can look at the range from DC - 4 MHz. I hooked up a
> signal generator and swept it through the DC - 4 MHz range at 100 kHz
> intervals and generated the plot here:
> 
> http://ice.cc.gt.atl.ga.us/usrp_adc_response.jpg
> 
> The signal generator produced a -36 dBm signal for each frequency, and
> the y-axis of this plot is the output of the USRP's ADC in dB. (20 *
> log10 of the appropriate FFT bin for the frequency of interest, using
> an 8192 point FFT).
> 
> My question is what is causing the particular shape of this response.
> I'm guessing it has to do with the CIC filter, but I can't seem to
> reconcile my output with the plots from this post on the CIC filter's
> response:
> 
> http://lists.gnu.org/archive/html/discuss-gnuradio/2007-05/msg00191.html
> 

What is the daughterboard gain set to?
If you look at the time domain samples, what is their range?
You might try increasing the power a bit and see if anything changes.
Did you collect that data with usrp_rx_cfile.py?  If so, what was the
exact command line that you used?

What kind of a signal generator are you using?
Have you tried sweeping the same way into a spectrum analyzer?
If so, what did you see?

Eric