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

[Eric Blossom]

On Wed, Oct 10, 2007 at 03:14:21PM -0400, address@hidden wrote:
> The figures I showed you previously used std_4rx_0tx.rbf.
> 
> To test with the standard std_2rxhb_2tx.rbf, I use the usrp_fft.py (with 
> -S).  I tested two cases, both with decimation 180, 1 kHz input, tuned to 
> 0 Hz with 0 db gain.  The first is with the voltage level of 1.4 V P-P 
> (which cuases the clipping on the std_4rx_0tx.rbf), the second figure is 
> at the full scale voltage of 2 V P-P.  Neither show clipping.  The 
> clipping is clearly dependent on which rbf file is used.
> 
> www.nd.edu/~ematlis/z.gnuradio/usrp_fft_180dec_1.4V_1khz.jpg
> 
> www.nd.edu/~ematlis/z.gnuradio/usrp_fft_180dec_2.0V_1khz.jpg
> 
> hope this helps clarify the situation....
> 
> thanks,
> eric

OK.  So it looks like the non-halfband case needs some adjustment to
reduce the gain so that it matches (more closely) the halfband case.
We don't have any multipliers.  What power of 2 would you like?

The fix goes on lines 83 and 102 of rx_chain.v.  Just change which
bits of hb_in_i are assigned to i_out and likewise for Q.

Send a patch when you're happy ;)

Eric


> 
> On Wed, 10 Oct 2007, Eric Blossom wrote:
> 
> >On Wed, Oct 10, 2007 at 12:50:35PM -0400, address@hidden wrote:
> >>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 second one is showing clipping.  Note how the peaks are folded in
> >on top of themselves.
> >
> >Which rbf are you using for these?  std_2rxhb_2tx.rbf (the default),
> >or std_4rx_0tx.rbf (the one without the halfband).
> >
> >Can you run the same experiment using the other rbf and post those
> >pictures too?
> >
> >The graphs are great.  Thanks for posting the links.
> >
> >Eric
> >