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

[Erich Stuntebeck]

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

...particularly this plot:

http://www.nabble.com/file/8500/(0%20-%2032)MHz)%20CIC%20Frequency%20Response%20(CIC%20decimation%20%3D32).JPG

Can someone please explain what's going on here?

My end goal is to fit an equation to the dB "correction factor" I
calculate in order to normalize across the entire 4 MHz chunk of
spectrum.


Thanks,

Erich