Re: [Discuss-gnuradio] Sync 2x USRP2s

[Lyndon Evans]

Hi Doug,

Thanks for the quick reply.

In rx_timed_samples I basically do start_rx_streaming() right after 
set_time_at_next_pps(), so yeah, I'll try the sleep(1). What do you mean 
by "if the host PC doesn't have direct access to the PPS signal" (read 
the serial data from the GPS)?

Yes, I've read a bit about the phase offsets stuff, especially the post 
from Matt about ambiguities, but at the moment I'm in a learning phase, 
and a bit vague about what this means (and also no DSP guru!). I can see 
that the RFX boards PLL may start up at an (phase) offset regardless, 
but not sure how this would actually affect the sampling/ticks.

Also, as you say, you can split the signals, so why the MIMO cable? I'll 
do this then, its a lot simpler anyway.

Kind Regards,
David


Doug Geiger wrote:
> On 3/19/2010 10:48 AM, David Evans wrote:
> > Hi,
> >
> > I want to sync two USRP2s, and currently trying to modify the VRT 
> > rx_timed_samples...
> >
> > usrp2::clock_config_t cc;
> > cc.ref_source = usrp2::clock_config_t::REF_INT;
> > cc.pps_polarity = usrp2::clock_config_t::PPS_NEG;
> > cc.pps_source = usrp2::clock_config_t::PPS_SMA;
> > cc.provide_ref_to_mimo = false;
> > u2->config_clock(cc);
> >
> > // u2->set_time(....); // replaced with...
> > u2->set_time_at_next_pps(...);
> >
> > However, the code hangs at the start sampling, and manually applying 
> > a pulse to the PPS SMA does nothing.
> >
> > What is the correct voltage level and pulse shape required? Manually 
> > toggling anything between 0-1.5v and 0-5v does nothing. I hope 5V 
> > isn't too much!!!
>
> When do you start_rx_streaming()? I've found that it works best to 
> wait until the next PPS (e.g. sleep(1) if the host PC doesn't have 
> direct access to the PPS signal), and then calling 
> start_rx_streaming() with the desired parameters (i.e. start streaming 
> at current time + some offset) works.
>
> > Ultimately, I want to capture coherent samples with both USRPs, maybe 
> > with the MIMO cable, and a GPS reference + PPS. Would this be just a 
> > case of...
> > For USRP A (with ref + PPS attached)...
> >  cc.ref_source = usrp2::clock_config_t::REF_EXT;
> >  cc.pps_polarity = usrp2::clock_config_t::PPS_NEG;
> >  cc.pps_source = usrp2::clock_config_t::PPS_SMA;
> >  cc.provide_ref_to_mimo = true;
> > For USRP B...
> >  cc.ref_source = usrp2::clock_config_t::REF_EXT; ***MIMO???***
> >  cc.pps_polarity = usrp2::clock_config_t::PPS_NEG;
> >  cc.pps_source = usrp2::clock_config_t::PPS_SMA; ***MIMO???***
> >  cc.provide_ref_to_mimo = false;
> >
> > Kind Regards,
> > David
>
> I haven't tried the MIMO cable at all, but sync'ing by providing the 
> same clock/PPS to both USRP2's works pretty well.  You may also want 
> to review some of the recent discussions on mailing list about the 
> possible phase offsets you may get, even when all/both USRP2's are 
> locked to the same reference, and internal clocks are synchronized.  
> This is due to the implementation of the PLL on the individual 
> daughterboards, and thus will be dependent on what you're using.