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

[ematlis]

Can you give me an example?  I've never worked with verilog code before.

Also-  this fix you are proposing- is it a simple scaling of gain that 
would be independent of decimation?  Because as you can see from the 
following figure, the amplitudes scale directly with the decimation for 
values above 128 on the 4 rcv rbf.

www.nd.edu/~ematlis/z.gnuradio/Amplitude_vs_decimation.jpg

Both rbf's produce a flat response below 128 decimation; greater than this 
is where the change occurs.  The standard half-band rbf does have a couple 
of dips but is basically flat.

thanks,
eric


On Wed, 10 Oct 2007, Eric Blossom wrote:

> 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