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RE: [Discuss-gnuradio] Syncronous data aquisition using multiple USRP2 b

From: MarcW
Subject: RE: [Discuss-gnuradio] Syncronous data aquisition using multiple USRP2 boards
Date: Tue, 23 Feb 2010 06:54:42 -0800 (PST)

I am still at a complete unknown as to the source of the phase shifts of the
USRP2 clocks I obtain in my initial post. Looking at the input clock
reference signals to each board there is no phase offset and Matt said in
his post on USRP2 syncronisation that clocks on multiple boards must have no
phase offset. Does anybody know why I am getting these such large phase

Best regards,

MarcW wrote:
> Hi Ulrika,
> In my example the top sine wave in the 10MHz clock reference and the lower
> 3 plots are from the debug clock pins on different USRP2 boards all using
> this clock reference. All boards are started at approximately the same
> time and all 4 signals undergo no drift with respect to each other but
> have a clear fixed phase offset which can be up to half a clock cycle.
> The only modification I made is config_mimo(MC_LOCK_TO_SMA) and
> clocks_enable_test_clk(true,10) which is done in txrx.bin. The 10 refers
> to the divide by 10 of the clock.
> We get no jitter at the clock edges though. Are your probes good for the
> frequency?
> What cables do you use to connect your reference clock? We are currently
> using quite long BNC's so I think my next step is to try to reduce this by
> using much shorter SMA's.
> Best regards,
> Marc.
> Ulrika Uppman wrote:
>> Hi,
>> What do we see in your example? Have you divided the clock by 10?
>> Is the output stable or are the edges jittering anything?
>> When I look at our clock outputs (in a very quick measurement) they are
>> all approximately in phase but the edges are jittering quite a bit, and I
>> would assume that to different usrp2s could in a worst case be as far
>> apart as half a clock cycle...
>> Regards,
>> Ulrika
>>> -----Original Message-----
>>> From: address@hidden 
>>> [mailto:address@hidden
>>>  On Behalf Of MarcW
>>> Sent: Thursday, February 18, 2010 6:51 PM
>>> To: address@hidden
>>> Subject: [Discuss-gnuradio] Syncronous data aquisition using 
>>> multiple USRP2 boards
>>> I am trying to syncronise data acquisition for 3 USRP2 boxes 
>>> at 2.4GHz to aquire say 1M samples from each board and write 
>>> to a text file. So far I have managed to modify the firmware 
>>> to output the USRP2 clock to the debug pins and to call 
>>> config_mimo(WE_LOCK_TO_SMA) and sync_to_pps(). When I connect 
>>> a 10MHz 2v Pk-Pk sine wave to each board (No PPS signal) and 
>>> look at the clock output pin for each there is no drift in 
>>> waveforms with respect to one another and the external clock 
>>> reference. However, there is a phase offset between the clock 
>>> edges on each board which changes every time they are reset 
>>> (sometimes of half a clock period).
>>> Please see the example which includes the external clock 
>>> reference and 3 clock outputs.
>>> Wont this mean that I have unsyncronised sampling since the 
>>> ADC is controlled by the position of the clock edge? 
>>> Is there a way to overcome this problem?
>>> My current python script to read in parallel is derived from 
>>> the example /gr-utils/src/python/usrp2_rx_cfile.py using 
>>> gr.hier_block2 to create multiple flow graphs (usrp2 -> text 
>>> file replicated four times). Do I need any special 
>>> syncronisation functions in here or should the problem be 
>>> fully handled on the firmware/FPGA?
>>> Best regards,
>>> Marc.
>>> http://old.nabble.com/file/p27642862/scope_plot.jpg
>>> --
>>> View this message in context: 
>>> http://old.nabble.com/Syncronous-data-aquisition-using-multipl
>>> e-USRP2-boards-tp27642862p27642862.html
>>> Sent from the GnuRadio mailing list archive at Nabble.com.
>>> _______________________________________________
>>> Discuss-gnuradio mailing list
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