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Re: [Discuss-gnuradio] USRP1 DBSRX 10MHz reference clock


From: Eric Blossom
Subject: Re: [Discuss-gnuradio] USRP1 DBSRX 10MHz reference clock
Date: Tue, 16 Jun 2009 11:41:52 -0700
User-agent: Mutt/1.5.18 (2008-05-17)

On Tue, Jun 16, 2009 at 11:24:31AM -0700, Kutik wrote:
> 
> Eric Blossom wrote:
> > 
> > On Tue, Jun 16, 2009 at 08:23:37AM -0700, Kutik wrote:
> >> 
> >> I would like to change the USRP master clock to 10MHz.
> >> 
> >> I call urx->set_fpga_master_clock_freq(10e6) prior to
> >> usrp_standard_rx::make.
> >> 
> >> I wonder how to set the REFCLK_DIVISOR in db_dbs_rx::_refclk_divisor().
> >> 
> >> From all examples I have seen the (master_clock/REFCLK_DIVISOR) is 4MHz.
> >> This would mean REFCLK_DIVISOR must be 2.5, but on the other hand it must
> >> be
> >> an integer..
> >> 
> >> Can I set the REFCLK_DIVISOR to 3 (resulting in Max2118 clock 3.33MHz)?
> >> Or
> >> would you recommend something else?
> > 
> > Can you tell us what you are really trying to do?  Why do you want to
> > change the master clock to 10MHz?  The USRP won't function with that
> > slow of a clock.  It won't be able to talk over the GPIF port to the
> > FX2 USB controller.
> > 
> > Eric
> > 
> 
> 
> Thanks for the answer,
> 
> I would like to acquire narrowband data (200 kHz). In this range I'll
> receive multiple CW signals. The maximal frequency inaccuracy is 2kHz.
> Therefore USRP1 has to be disciplined by precise reference clock.
> 
> I have 10MHz reference signal available, and hoped there will be no need to
> buy 64MHz oscillator (disciplined by 10MHz).
> 
> I've read (usrp_basic.h) the master clock range is 10-64MHz, thus I wanted
> to give it a try.

OK, thanks.

After some off-list discussion, I think you should be able to make
this work.  I expect that Johnathan or Matt will chime in with some
additional information.  (The GPIF bus uses a clock from the FX2, so I
was mistaken about that being a problem.)

Eric




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