Re: [Discuss-gnuradio] Can a host control an USRP1 and an USRP2 with the same clock?

[Juha Vierinen]

> If the USRP1 and the USRP2 can work on the same host at the same time, can
> the USRP1 and  the USRP2 work synchronously (can they have the same clock or
> common time reference?)?  Some discussions mentioned the synchronization of
> two USRP2, but I cannot find discussions about  synchronization of a USRP1
> and a USRP2.

You can timestamp USRP1 data by inserting a syncronization pulse
(e.g., GPS PPS) to a digital i/o pin. You need the gr-gpio firmware
for this. This will have approximately one sample accuracy with a very
small known constant offset. We have been using this for nearly a year
with good success. You can easily get less than 1 us absolute timing
accuracy this way. I'm guessing that similar functionality will soon
appear for USRP2 as well, so you can keep these two in sync to some
extent.

Regarding relative sample clock coherence, Martin DvH had a nice idea
about syncronizing the relative clock frequencies by inserting a PPS
or 10 MHz signal into the digital i/o pin. This allows you to estimate
relative clock frequency to a reference 10 MHz or a 1 PPS signal.

However, if you buy some more hardware, you can drive both USRP2 and
USRP1 from the same clock. The USRP2 takes a 10 MHz standard, but the
USRP2 needs a 64 MHz clock. I have used either a 6x multiplier  or a
cheap DDS board to generate the USRP1 clock (60 MHz and 64 MHz). I
don't really know a source for 6x clock multipliers (if somebody
knows, please tell me). The DDS I'm using to generate a 64 MHz clock
for USRP1 is this:

http://www.eskimo.com/~novatech/PDF_files/lpo400ads.pdf

It even fits nicely into a USRP1 box, taking the space of one
daughterboard slot.

Hope this helps.

BR,
juha