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Re: [Discuss-gnuradio] Run USRP at alternate clock frequency?

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] Run USRP at alternate clock frequency?
Date: Thu, 6 Apr 2006 15:45:49 -0700
User-agent: Mutt/1.5.9i

On Thu, Apr 06, 2006 at 04:42:56PM -0400, Clark Pope wrote:
> Update on status:

Clark this is *very* cool.  Thanks for keeping us posted on your progress.

> Reran my custom dbs program and it worked fine EXCEPT:
> The default decimation is 4 so I'm recording at 11 megasamples per second. 
> (oddly enough I'm still able to demodulate my 802.11b packets fine even 
> though the chip rate is 11 mcps!?) When I try a decimation rate of 2 I get 
> the message:
> usrp_standard_rx::set_dim_rate: rate must be EVEN and in [4,256]
> I am using the std_4rx_0tx.rbf fpga design. Is the min decimation really 4?

No, with the slower xtal you should be able to decimate by 2.  The
gain scaling in the CIC might be off, but Matt thinks it'll work.
(The existing check was for USB sanity using the 64M xtal)

Just go ahead and modify the check in usrp_standard_rx::set_decim_rate

> If necessary I will go modify that fpga to just have a halfband so I can 
> record at 22 complex megasamples per second which is enough bandwidth to 
> support 802.11a/g. Can't beat more bandwidth at less power.:)

Sounds good, however you'll need to reimplement the halfband using
more multipliers if you do.  See the comments about timing in
fpga/sdr_lib/hb/halfband_decim.v   The current halfband code requires
a minimum decimation of 8.  4 upstream of it and 2 in it.


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