Re: [Discuss-gnuradio] USRP Gain

[Brian Padalino]

On Mon, Sep 1, 2008 at 4:26 PM, Richard Jaeger
<address@hidden> wrote:
> I have been attempting to calibrate my USRP system.  I am running four
> channels and feeding the various channels to
> fft sinks following de-interleaving and channel filtering.  I am using the
> Basic RX boards, and the PGA in front of the ADC is set at 20 dB.
> For large decimation, the sensitivity of the system seems to much larger
> than I expected,
> and is a function of the decimation factor D.   Overall, I can't account for
> a gain of between 60 and 80 dB.
>
> For decimations below 96, the gain is fairly constant, changing +/- a couple
> of dB.  However, for decimations above about
> 96, the voltage gain in dB is growing approximately linearly with D:  (for
> example, when I change D from 100 to 160 the overall gain increases by about
> 5.5 dB. When I change D from 160 to 222, the voltage gain increases by
> another 5.5 dB).  So the gain itself is growing exponentially with
> decimation rate.

I'm not an expert by any means, but this CIC documentation is telling
me the gain is:

    g = (RM)^N

Since M and N are defined (M=1, N=4) and you're changing R between 160
and 220, finding the difference in gain in dB:

    10*[ log10(220^4) - log10(160^4) ]
    5.5 dB

Do you agree?

> I believe that the gain of a basic CIC filter is proportional to D (but not
> exponentially).  Is this the source of the gain variation that I am seeing,
> or is some scaling or normalization going on elsewhere that I am
> overlooking?
>
> The documentation indicates that 4-stage CIC filters are used, and the
> 4-channel configuration eliminates the half-band filters.
> Is there any information on how the decimation is distributed across the 4
> stages as a function of overall decimation?
>
> Thanks for the help.
>
> Dick...

Brian