Re: [Discuss-gnuradio] saturation with multi_fft.py

[ematlis]

Eric-

using -d 160 the problem goes away with this latest build, but remains for 
higher decimation values.  For example, at decimation of 180, I can 
reproduce the problem.  As a picture is worth a thousand words, I've put 
two jpg's up online at:

www.nd.edu/~ematlis/z.gnuradio/multi_scope_180dec_1.3V.jpg

www.nd.edu/~ematlis/z.gnuradio/multi_scope_180dec_1.4V.jpg

The input signal is again a 10 kHz sine wave.  In the first image, the 
voltage level is 1.3 V p-p.  In the second, 1.4 V p-p.  In both cases I 
have zero db of gain and decimation of 180, with the LF_RX tuned to 0 Hz. 
As you can see, at 1.3 V P-P the result is perfect.  However at 1.4 V P-P, 
the signal is seriously corrupted and jagged.

The criteria you indicated, ie acquisition rate at 4x bandwidth, seems not 
to be the issue; I am measuring a 10 kHz sine wave at an effective 
sampling rate of 355 kHz and the problem remains.

In fact, the problem seems to be independent of the frequency of the 
incoming sine wave, as I get the same thing with a frequency of 1 kHz as 
opposed to 10 kHz:

www.nd.edu/~ematlis/z.gnuradio/multi_scope_180dec_1.4V_1khz.jpg

Any thoughts?

thanks,
eric

On Tue, 9 Oct 2007, Eric Blossom wrote:

> On Tue, Oct 09, 2007 at 05:11:52PM -0400, address@hidden wrote:
> > Thanks-
> >
> > I just downloaded, compiled and installed this latest revision.  When I
> > test multi_scope.py in the mutli-antenna examples directory with this
> > release I get the same issue.  Please note, I have the LF_RX
> > daughterboards, so I change the daughterboard ID from BASIC to LF in the
> > python code.  I also eliminate the second factor of 4 decimation in the
> > host (sw_decim = 4) by setting it to 1.  I then run with the following
> > command:
> >
> > multi-antenna]$ ./multi_scope.py -f0 -d250 -g0
> >
> > while acquiring a 10 kHz 0.3 V p-p signal.  The max value shown in the
> > waveform on the gui is just over 25,000, which is way too much compared to
> > the standard usrp_fft.py, which by the way is no longer in the examples
> > directory; I used one from a previous release.  If I increase the input
> > voltage level to 0.4 V p-p, I get a jagged waveform with sharp
> > discontinuities.
> >
> > So, my problem remains.  Any ideas?
>
> What's your test signal?  Is it your 100kHz wide signal?
> Can you try it with -d 400 as described below?
>
> Eric
>
> > On Tue, 9 Oct 2007, Eric Blossom wrote:
> >
> > > With the halfbands, it's flat to about 70% of the passband.
> > > decim = 320 gives 200kS/s complex baseband, a bit more than you'll
> > > need.
> > >
> > >
> > > Without the halfbands, I'd use about 4x the bandwidth of interest,
> > > then filter and decimate in the host for the final channel selection.
> > > In your case, I'd use decim = 160 giving 400kS/s.
>
> Eric