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

[Ulrika Uppman]

We used a couple of "home-modified" sma cables but extended them with bnc 
(totally around 0,5 m long) but we didn't divide the clock 
(clocks_enable_test_clk(true,1)). Maybe we should look over the cables to see 
if that might be the cause of the jitter? We do however not get any phase 
offset larger than a quarter of a 100 MHz clock cycle.
I will try to make another measurement later if I have time.

Regards,
/Ulrika

> -----Original Message-----
> From: address@hidden 
> [mailto:address@hidden
>  On Behalf Of MarcW
> Sent: Tuesday, February 23, 2010 11:37 AM
> To: address@hidden
> Subject: RE: [Discuss-gnuradio] Syncronous data aquisition 
> using multiple USRP2 boards
> 
> 
> 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.
> >> 
> >> 
> >> 
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> 
> --
> View this message in context: 
> http://old.nabble.com/Syncronous-data-aquisition-using-multipl
> e-USRP2-boards-tp27642862p27701371.html
> Sent from the GnuRadio mailing list archive at Nabble.com.
> 
> 
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