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

[Martin Dvh]

Clark Pope wrote:

> Update on status:
>
> I purchased a 44 Mhz oscillator from digi-key (p/n 300-7254-1-ND, 
> CSX750FJC44)  for $3.38. The first attempt to remove the USRP 64 MHz 
> failed miserably with a pen iron. I purchased a heat gun from digi-key 
> (MA1008-ND) for $52, set it to the 1000 degree setting and the old 
> oscillator came up in about 10 seconds. Installed the new oscillator and 
> checked the 44 MHz clock with a scope.
>
> I then tried to run my customized dbs application that records 8-bit I/Q 
> data to a file for me from any of the 802.11b channels. Got all zeros in 
> the data. Then remembered eric told me to change the clock in db_dbs_rx.py.
>
> My code is a little older than current but here's my mods in 
> gr-usrp/src/db_dbs_rx.py:
> 1. change line 88 from
> self.refclk_divisor = 16
> to
> self.refclk_divisor = 11
>
> 2. change line 108 from
> return 64e6/self.refclk_divisor
> to
> return 44e6/self.refclk_divisor
>
> then in /gr-usrp I did
> make clean
> make
> make check
> make install
>
> 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:
Will you be sharing this code?
I think more people will be interested in doing 802.11b.

> 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?
>
> 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.:)
It will be interesting to see if you will be able to get complex 8 bit samples 
over the bus with decimation 2.
This will be 44 Mbyte/sec over the usb bus. This is within the theoretical limits of usb2 but until now I have not heard of anybody being able to sustain more 
then 32 Mbyte/sec over the bus using the usrp.

Greetings,
martin

> Thanks,
> Clark
>
>
> > From: Eric Blossom <address@hidden>
> > To: Clark Pope <address@hidden>
> > CC: address@hidden
> > Subject: Re: [Discuss-gnuradio] Run USRP at alternate clock frequency?
> > Date: Fri, 10 Mar 2006 13:48:52 -0800
> >
> > On Fri, Mar 10, 2006 at 11:21:08AM -0500, Clark Pope wrote:
> > >
> > > I would like to run my USRP off a 44 MHz clock instead of the 64 MHz 
> > clock
> > > that is currently used. A couple reasons:
> > > 1. the 64MHz rate exceeds the USB bandwidth even with byte-wide samples
> > > (64*8 = 512 mbps)
> > > 2. 44 MHz works much better for the wlan signals I'm after
> > >
> > > So the questions are:
> > > 1. can I generate a 44Mhz clock from the 64 MHz or do I have to 
> > change the
> > > crystal?
> >
> > You need to change the crystal.
> > You'll also need to change the definition of
> >
> >   /*!
> >    * \brief return frequency of master oscillator on USRP
> >    */
> >   long  fpga_master_clock_freq () const { return 64000000; }
> >
> > in usrp_basic.h  The rest of the code should follow, though I haven't
> > tested it.  YMMV, patches welcome ;)
> >
> > > 2. If I change over to 44 MHz will everything else still work? 
> > Especially
> > > the dbs and flex2400 boards?
> >
> > Not sure about those.  Paging Matt Ettus, paging Matt Ettus...
> >
> > Eric
>
>
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