[USRP-users] Mechanism to determine phase offset of two nearly equal pieces of coax

Marcus D. Leech mleech at ripnet.com
Wed Jul 5 09:58:13 EDT 2017

On 07/05/2017 04:35 AM, John Shields via USRP-users wrote:
> I have a couple of N200 with a MIMO cable (and one has a GPSDO but I 
> don’t think I need to worry about that for this setup) and propose 
> feeding both cables to RX port on an SBX in each N200. The other end 
> of the coax will be connected to a splitter and the ‘sum’ port will be 
> connected to TX port on SBX (of which only the TX portion works) on a 
> USRP1 through 30dB attenuator.
> The phase imbalance spec of this splitter is less than 1 degree but if 
> I wanted, I could calibrate that out and I could live with it anyway.
> In order for this setup to allow me to determine the relative phase 
> offset ( whose length can be calculated from the Velocity Factor of 
> the RG 213) I need to start up the SBXs on the N200 with zero 
> differential phase offset.
> I have seen the section for SBX where I need to execute :
> //we will tune the frontends in 100ms from now
> uhd::time_spec_tcmd_time = usrp->get_time_now() + uhd::time_spec_t(0.1);
> //sets command time on all devices
> //the next commands are all timed
> usrp->set_command_time(cmd_time);
> //tune channel 0 and channel 1
> usrp->set_rx_freq(1.03e9, 0); // Channel 0
> usrp->set_rx_freq(1.03e9, 1); // Channel 1
> //end timed commands
> usrp->clear_command_time();
> So three questions:
> 1) am I missing something obvious?
> 2) is there a clever way in GRC to cause this code to be executed – 
> rather than in a .py file?
> 3) what is the most effective phase comparator to use? I expect to use 
> 400 Mhz to start with to minimise the chance of nx2*pi ambiguity.
>            Kind Regards,
>                      John

So, in GRC you'd have to select "Unknown PPS" in the SYNC field, and 
select MIMO for the clock and time sources for one of the devices.

Once that is done, you'll have to edit the generated code and wrap the 
timed-commands around the tuning commands emitted by GRC--there is
   currently no support for timed tuning in GRC itself.

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