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

From: Clark Pope
Subject: Re: [Discuss-gnuradio] Run USRP at alternate clock frequency?
Date: Fri, 07 Apr 2006 10:08:49 -0400 




From: Martin Dvh <address@hidden>
To: Clark Pope <address@hidden>, gnuradio mailing list <address@hidden> 
Subject: Re: [Discuss-gnuradio] Run USRP at alternate clock frequency?
Date: Fri, 07 Apr 2006 00:06:55 +0200

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.
I plan to but probably later this year. Right now I'm collecting on linux, then processing on windows, and then running some utilities to get the packets into ethereal. My goal is to have a single net interface under linux that looks like any other.
I'd like to start an open source project on this maybe on sourceforge, but I've heard mixed experiences. Any suggestions on a host is appreciated. 


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. 



I will try today.


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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