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

[Ulrika Uppman]

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
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