Forum: GNU Radio Measuring phase between channels

Bc825944dcf5627687ce0cc80d3739a9?d=identicon&s=25 David Miller (Guest)
on 2014-06-17 19:07
(Received via mailing list)
Hello,
I want to measure the phase difference between three synchronised USRP
channels (Common Ref & PPS), does software exist to do this, and to what
accuracy, please?
Sorry, this is a bit of a newbie question, and probably already covered!
Dave
B4ffbc711addde4c649b1ed526df6157?d=identicon&s=25 Martin Braun (Guest)
on 2014-06-17 20:30
(Received via mailing list)
On 17.06.2014 19:06, David Miller wrote:
> Hello,
> I want to measure the phase difference between three synchronised USRP
> channels (Common Ref & PPS), does software exist to do this, and to what
> accuracy, please?
> Sorry, this is a bit of a newbie question, and probably already covered!

Sure, the easiest way would simply be to put in a scope, and check by
looking -- of course, that's not very accurate.

There's enough math operators in GNU Radio to see the actual phase
difference, such as conjugate and multiply, then convert to phase.

Cheers,
Martin
D17685d174fee4ca258c75cce7bc2202?d=identicon&s=25 Marcus Müller (Guest)
on 2014-06-17 21:36
(Received via mailing list)
Hi David,

for starters, you could just divide one channel by the other and
calculate the result's argument. There are blocks for both operations :)

Short explanation:
Just consider the complex signals $s_1, s_2$ in polar coordinates
(instead of the usual real/imag representation)
$s_1(t) = a_1(t) * e^{j \phi_1(t)}$.
Dividing that by $s_2(t) = a_2(t) * e^{j \phi_2(t)}$ gives you
$p(t) = a_1(t)/a_2(t) * e^{j \phi_1 - \phi_2(t)}$, of which you can take
the argument to get the phase difference.

Greetings,
Marcus
7ca910fd345e9f6d2116b4019815eab3?d=identicon&s=25 Stephen Harrison (Guest)
on 2014-06-18 01:48
(Received via mailing list)
I did an experiment similar to this a year or so ago. I synced all the
USRPs to the same 10MHz and PPS. I had pretty good results using libgps
to
get the GPS time and then using UHD to start streaming on a particular
PPS
edge on all units. With LFRX boards this will ensure that the DDCs start
at
the same time and start from zero degrees. I seem to remember there was
some small random variation from run to run. If you are just looking for
relative changes in phase difference I found this approach works fairly
well.


On Tue, Jun 17, 2014 at 10:36 AM, Martin Braun <martin.braun@ettus.com>
Please log in before posting. Registration is free and takes only a minute.
Existing account

NEW: Do you have a Google/GoogleMail, Yahoo or Facebook account? No registration required!
Log in with Google account | Log in with Yahoo account | Log in with Facebook account
No account? Register here.