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

[ematlis]

Thanks so much for clarifying that!  I see now that config.vh controls how 
the code is configured.

Regarding the loss of the half-band filters in the multi-antenna fpga 
configuration; what is the recommendation to avoid aliasing?  Say for the 
case of a 2 MHz carrier AM waveform with modulation out to 100 kHz.  What 
kind of filtering would I need to avoid aliasing?

thanks again,
eric

On Tue, 9 Oct 2007, Eric Blossom wrote:

> On Tue, Oct 09, 2007 at 01:02:18PM -0400, address@hidden wrote:
> > So there is no difference in the fpga code between the standard
> > usrp_fft.py and multi_fft.py found in the multi-antenna directory?  Then
> > why does the latter specify std_4rx_0tx.rbf?
>
> Because it wants 4 digital downconverters (it's a 4 antenna example).
> We generate two standard FPGA configurations:
>
>  std_2rxhb_2tx.rbf       # 2 DDCs w/ halfband filters and 2 Tx paths
>  std_4rx_0tx.rbf         # 4 DDCs w/o halfband filters and 0 Tx paths
>
> > And to reiterate my original concern, the multi-antenna versions of the
> > code will not accept more than between .3-.4 V p-p for decimation rates
> > greater than 128; greater than .4 V p-p and the result acts as if the A/D
> > chips are clipping.  It's not a hardware issue with my USRP because the
> > usrp_fft.py codes correctly measure up to 2 V p-p.
>
> To avoid confusion, let's make sure we've got our terminology
> straight.  There is no "multi-antenna" version of the FPGA code, only
> the two configurations described above.  They are built from the same
> source, and vary only in which of the .vh files is included.
>
> However, taking a look at the modification dates on the .rbfs in:
>
>   http://gnuradio.org/trac/browser/gnuradio/trunk/usrp/fpga/rbf/rev2
>
> It appears that the std_2rxhb_2tx.rbf was updated 3 months ago, but
> that the std_4rx_0tx.rbf was updated 6 months ago.  They should have
> both been generated and updated at the same time.  [A makefile to
> handle this would be a good idea.]
>
> I will rebuild them both and update the rbf's in svn.
> I'll post a note when this is done.
>
> > thanks for your help,
> > eric
>
> You're welcome,
> Eric