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 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...
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