Re: [Discuss-gnuradio] Problems with the USRP/LFRX at lower frequencies

[Brian Padalino]

On Wed, Nov 12, 2008 at 11:00 PM, dan s <address@hidden> wrote:
> Hello,
>
> I'm having a ton of trouble with the USRP/LFRX daughterboard setup.  I am
> trying to calibrate the counts that are stored as integers when I use the
> usrp_rx_cfile.py script.
>
> I am using an HP waveform driver with 50 ohm output impedance to feed single
> frequency sinewaves to the LFRX channel A.  I am generating sinewaves with
> an amplitude of 50 mV peak to peak.  I tried measuring several frequencies
> from 5 to 250 kHz.  I am using a decimation of 128 (500 kHz) and the default
> gain of 10dB.
>
> I recorded the data for each frequency and used matlab to perform an FFT
> (normalized) to receive my peak counts for each case.   The number of counts
> I measure is not the same for all the frequencies.  I noticed the number of
> counts stays constant from 5 to 20 khz, then drops from 20 to 150 khz, then
> the number of counts rises slightly from 150 to 240 kHz.
>
> I'm puzzled why the number of counts I measure is not constant even though I
> have a constant amplitude sinewave signal fed into the LFRX.
>
> I also tried performing the same tests with an HP spectrum analyzer.  The
> analyzer measured a response of 16 mVrms (which corresponds to 50mV peak to
> peak) at each of the frequencies.  I am 100 percent confident the signal I
> am feeding into the LFRX is perfect.

The signal is being sampled by the ADC, filtered and decimated through
a CIC filter and then it goes through a decimating halfband filter.

Since you're doing a decimation of 128, you are really doing a 64:1 in
the CIC then a 2:1 in the halfband filter.  Without further filtering
at the host, I am pretty sure you will get this type of distortion due
to the lack of compensation for the CIC "droop" by the halfband
filter.

Have you tried plotting the response to see if you get that typical
droop, or if it's some other type of transformation?

Hope this helps.

Brian