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

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] Run USRP at alternate clock frequency?
Date: Fri, 07 Apr 2006 00:06:55 +0200
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.5b) Gecko/20030823

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
self.refclk_divisor = 11

2. change line 108 from
return 64e6/self.refclk_divisor
return 44e6/self.refclk_divisor

then in /gr-usrp I did
make clean
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.



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...


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