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

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] saturation with multi_fft.py
Date: Tue, 9 Oct 2007 11:19:51 -0700
User-agent: Mutt/1.5.9i

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:


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,

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