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View all threadsPhase jumps in USRP B210 with GPSDO
Hi all,
I'm trying trying to use multiple USRPs B210 (that have GPSDOs mounted)
for tasks requiring good phase coherence between the devices (i.e.
phased antenna array) and there is some problem. In the recorded signal
I have found abrupt phase jumps that make B210s unusable for performing
phase coherent tasks (and that generally might be a problem).
The devices were synchronized with use of Octoclock-G providing PPS and
10MHz ref signal. To check phase coherence I used a signal generator
(generating sinusoid: -40dBm at 690MHz with USRPs tuned to 690.1MHz with
sample rate 1MHz, PPS and ref signals taken from external ports, version
of UHD 3.9.2). See the attached picture in which I removed expected
phase of the sinusoid (recorded with one of the channels) from the phase
of the recorded signal. In this example phase jumps happen almost
periodically (about every 10 seconds).
Now is the funny part: after taking out GPSDO the problem goes away
(checked with 3 USRPs B210). Hence the very probable theory is that
GPSDO's interference causes the problem.
Have you observed this? And more importantly: do you a have solution?
(Taking GPSDOs out from all our B210s is not one of them as there are
many valid tasks that require both GPSDOs and good quality of signal
signal phase measurement. I still have to check if the same thing
happens without Octoclock-G.)
(in the next episode: slow start of transmission in USRP B210 :) )
--
Best Regards,
Piotr Krysik
You talk about board-mounted GPSDOs in each of your B210s, but then talk
about using an Octoclock-G.
In the Octoclock-G example, you are explicitly selecting "external" for
your refclock and time source?
It is fully-expected that no two GPSDOs will precisely agree on
frequency and phase, and will have some amount of mutual phase-noise.
Even when fed with the same antenna.
On 2017-09-21 10:54, Piotr Krysik via USRP-users wrote:
Hi all,
I'm trying trying to use multiple USRPs B210 (that have GPSDOs mounted)
for tasks requiring good phase coherence between the devices (i.e.
phased antenna array) and there is some problem. In the recorded signal
I have found abrupt phase jumps that make B210s unusable for performing
phase coherent tasks (and that generally might be a problem).
The devices were synchronized with use of Octoclock-G providing PPS and
10MHz ref signal. To check phase coherence I used a signal generator
(generating sinusoid: -40dBm at 690MHz with USRPs tuned to 690.1MHz with
sample rate 1MHz, PPS and ref signals taken from external ports, version
of UHD 3.9.2). See the attached picture in which I removed expected
phase of the sinusoid (recorded with one of the channels) from the phase
of the recorded signal. In this example phase jumps happen almost
periodically (about every 10 seconds).
Now is the funny part: after taking out GPSDO the problem goes away
(checked with 3 USRPs B210). Hence the very probable theory is that
GPSDO's interference causes the problem.
Have you observed this? And more importantly: do you a have solution?
(Taking GPSDOs out from all our B210s is not one of them as there are
many valid tasks that require both GPSDOs and good quality of signal
signal phase measurement. I still have to check if the same thing
happens without Octoclock-G.)
(in the next episode: slow start of transmission in USRP B210 :) )
--
Best Regards,
Piotr Krysik
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USRP-users@lists.ettus.com
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W dniu 21.09.2017 o 17:19, mleech@ripnet.com pisze:
You talk about board-mounted GPSDOs in each of your B210s, but then
talk about using an Octoclock-G.
In the Octoclock-G example, you are explicitly selecting "external"
for your refclock and time source?
It is fully-expected that no two GPSDOs will precisely agree on
frequency and phase, and will have some amount of mutual phase-noise.
Even when fed with the same antenna.
I explicitly written that I configured USRPs to take PPS and 10MHz ref
from external inputs. We just happen to have board mounted GPSDOs in all
USRPs B210 that we have, but in this example they were not used for
synchronization.
--
Piotr Krysik
W dniu 21.09.2017 o 19:16, Piotr Krysik via USRP-users pisze:
W dniu 21.09.2017 o 17:19, mleech@ripnet.com pisze:
You talk about board-mounted GPSDOs in each of your B210s, but then
talk about using an Octoclock-G.
In the Octoclock-G example, you are explicitly selecting "external"
for your refclock and time source?
It is fully-expected that no two GPSDOs will precisely agree on
frequency and phase, and will have some amount of mutual phase-noise.
Even when fed with the same antenna.
I explicitly written that I configured USRPs to take PPS and 10MHz ref
from external inputs. We just happen to have board mounted GPSDOs in all
USRPs B210 that we have, but in this example they were not used for
synchronization.
--
Piotr Krysik
Also take into account that I'm not comparing phase signals observed by
two different USRPs. What I have shown is for single channel of one USRP
in which such jumps are observed (I'm comparing phase of digitally
generated sinusoid with phase of recorded one). Same thing, however, can
be observed when recording signal with two USRPS in situation where one
have board mounted GPSDO and another one doesn't (with simple
unwrap(angle(x1.*conj(x2))) where x1 and x2 are signal vectors). Both
USRPs synchronized with use of Octoclock-G.
To make the matter simpler to gasp I will check if the same problem can
be observed on one USRP B210 with board mounted GPSDO - without any
additional hardware.
--
Piotr Krysik
On 09/21/2017 02:27 PM, Piotr Krysik via USRP-users wrote:
--
Piotr Krysik
Also take into account that I'm not comparing phase signals observed by
two different USRPs. What I have shown is for single channel of one USRP
in which such jumps are observed (I'm comparing phase of digitally
generated sinusoid with phase of recorded one). Same thing, however, can
be observed when recording signal with two USRPS in situation where one
have board mounted GPSDO and another one doesn't (with simple
unwrap(angle(x1.*conj(x2))) where x1 and x2 are signal vectors). Both
USRPs synchronized with use of Octoclock-G.
To make the matter simpler to gasp I will check if the same problem can
be observed on one USRP B210 with board mounted GPSDO - without any
additional hardware.
I use B210s for interferometry. Phase coherence is stable basically
"forever" between two B210 RX channels, regardless of the reference
used, since the
LO is shared between the two receive channels. So, absent bugs in
the FPGA digital processing, I'd be very surprised to see phase-hits
within a single
B210.
W dniu 21.09.2017 o 21:06, Marcus D. Leech via USRP-users pisze:
On 09/21/2017 02:27 PM, Piotr Krysik via USRP-users wrote:
--
Piotr Krysik
Also take into account that I'm not comparing phase signals observed by
two different USRPs. What I have shown is for single channel of one USRP
in which such jumps are observed (I'm comparing phase of digitally
generated sinusoid with phase of recorded one). Same thing, however, can
be observed when recording signal with two USRPS in situation where one
have board mounted GPSDO and another one doesn't (with simple
unwrap(angle(x1.*conj(x2))) where x1 and x2 are signal vectors). Both
USRPs synchronized with use of Octoclock-G.
To make the matter simpler to gasp I will check if the same problem can
be observed on one USRP B210 with board mounted GPSDO - without any
additional hardware.
I use B210s for interferometry. Phase coherence is stable basically
"forever" between two B210 RX channels, regardless of the reference
used, since the
LO is shared between the two receive channels. So, absent bugs in
the FPGA digital processing, I'd be very surprised to see phase-hits
within a single
B210.
Hi,
The problem appears only when synchronizing USRPs B210 that have board
mounted GPSDO with use of Octoclock-G. This configuration doesn't work
correctly.
Marcus - I suppose that you haven't seen this problem yet because you
didn't try to use B210s in such configuration.
I've checked today signal phase on a single USRP B210 with GPSDO, but
without connecting external synchronization source. In this situation
everything is fine. So what is good is that it is not board mounted
GPSDO alone that causes the problem.
In short the solution for now is: remove GPSDO from your USRP B210 if
you want to synchronize it with use of external PPS and 10MHz ref
source. (The solution is not perfect as these GPSDOs have delicate pins
at the connector.)
Best Regards,
Piotr Krysik
Hi Piotr,
And do you set both the Clock and Time source to the external ones
when using it in that configuration ?
I just had a look at both the schematic and VHDL and there is actual
switches ... the GPSDO 10M and PPS should basically be completely
disconnected when the reference for time and clock is external.
Also, are you using an external power supply ? Maybe the issue is the
added load on the supply rails when the GPSDO is plugged in.
Cheers,
Sylvain
Hi Sylvain,
W dniu 22.09.2017 o 11:44, Sylvain Munaut pisze:
Hi Piotr,
And do you set both the Clock and Time source to the external ones
when using it in that configuration ?
I think I have written this at least twice before. I will try again to
describe it again as clear as I can. Recipe to reproduce the problem:
- take a USRP B210 with GPSDO inside,
- connect external source (i.e. Octoclock-G) of PPS and 10MHz ref to it,
- set sources of PPS and 10MHz ref to "external",
- record sinusoid,
- look at the sinusoid's phase (one simple solution is to uwrap phase,
fit first order polynomial to it and remove it from the original).
I just had a look at both the schematic and VHDL and there is actual
switches ... the GPSDO 10M and PPS should basically be completely
disconnected when the reference for time and clock is external.
I suppose they are not enough. Anyway problem with combo of USRP B210
equipped with GPSDO and Octoclock-G was confirmed yesterday by mixbit on
usrp-users list (he said they have it in their bug tracker).
Also, are you using an external power supply ? Maybe the issue is the
added load on the supply rails when the GPSDO is plugged in.
External supply doesn't help. Only removing GPSDO from USRP B210 helps.
Which is not bad.
p.s. Now we are fighting with frequency offset between two USRPs B210
synchronized with octoclock. It causes phase drift of about 1
degree/second, and changes with time. When I will know more I will
describe it in a new thread on the mailing list.
Best Regards,
Piotr Krysik