Take a look at the phase rotator in the frequency translating FIR filter. Is it done with sine/cos lookups or is it a rotating phasor? Rotating phasors can lose amplitude due to finite precision effects.
Matt On Fri, Aug 14, 2015 at 1:57 PM, John Ackermann N8UR <j...@febo.com> wrote: > Still working with the polyphase channelizer program. While everything > "works", there is something very strange: the output amplitude slowly drops > the longer the program runs. As near I can tell, this happens in or > following the frequency translating FFT block. > > To test, I stripped the flowgraph down to the bare minimum and put a > frequency display at the output of the UHD block, and another at the output > of the fx fft block. > > By using the "max hold" feature of the display, I can easily monitor > amplitude drop during the program run. I'm using a signal generator to > feed the USRP a known signal level (162.475 MHz at -70dBm). > > In the attached screenshot, the left display is the UHD output, and the > right is the fx fft output. The flowgraph had been running for about 5 > minutes. The signal out of the fx fft has dropped 20+ dB while the UHD > signal level remains the same. The longer the program runs, the further > the fx fft output drops. > > I'm not seeing any error messages on the console to indicate overrun, > underrun, or other issues. Due to size limits, I attach only the fft > screenshot, but here are all the relevant files: > > http://www.febo.com/pages/gr-projects/amplitude_loss_test.grc > http://www.febo.com/pages/gr-projects/amplitude_loss_test.py > http://www.febo.com/pages/gr-projects/amplitude_loss_test_fft.png > http://www.febo.com/pages/gr-projects/amplitude_loss_test_flowgraph.png > > Any idea what could lead to this kind of problem? It seems like some sort > of accumulating error, but I'm lost as to what. > > Thanks, > John > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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