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

[Stefan Bruens]

On Thursday 19 March 2009 22:43:56 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:

I think there are two parts
- dampening at 0 and 4 MHz: This is most probably the typical CIC shape, 
[sin(x)/x]^4 (four CIC stages)
- "500kHz Ripple": this is most probably an artifact due to how you generate 
the response. There might be a small frequency offset between the USRP and 
your signal generator. The USRP oscillator is specified to be 64MHz +/- x ppm 
(parts per million, x either 20 or 50 depending on USRP revision). If you take 
the power just from a single bin, you may use the wrong bin.

One possibility would be to take several bins around the "correct" bin and sum 
up the powers of these bins. Actually, you dont need the fft at all, you can 
just calculate the power of the incoming signal in the time domain.

A somewhat related question: which firmware are you using - with or without 
halfband filter?

Regards,

Stefan

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