Hi Arjun,
You mention that you have both units connected to ‘external PPS
source, and have a MIMO cable between them’. Not sure where the Vref is coming
from but I presume over the MIMO cable?
I wonder how you are setting the clock and time source but I had
an issue with my understanding of the behaviour of the MIMO cable in a
configuration of 2xN200 each with an SBX.
Firstly, while the documentation on MIMO doesn’t explicitly
mention/highlight it, the PPS is not distributed over the MIMO cable – the RTC
register of the master is copied to the slave which means that the slave
doesn’t actually ‘see’ a PPS over the MIMO cable. This was not a big issue for
me but it meant that operations such as get_last_pps and setting the ‘sync’ on
the slave to ‘unknown PPS’ both fail though, get_time_now works, so your timed
commands should be good.
Secondly, while it is obvious when you think about it, if the
slave is receiving Vref over the MIMO cable there will always be a fixed phase
offset based, at a minimum, on the physical length of the MIMO so the Vref
offset is a function of the frequency.
Once I understood especially the second behaviour, my system
performed as I would expect.
I know that your setup is different than mine (I was just
supplying the same signal through a splitter to each USRP and measuring the
phase offset), but wonder if this might explain your observations?
Kind Regards,
John
From: Bakshi, Arjun via USRP-users
Sent: Sunday, November 26, 2017 10:28 AM
To: [email protected] ; [email protected]
Subject: [USRP-users] Phase sync with different frequencies
Hi all,
I'd like to use the phase resync freature on the SBX to get reliable phase
alignment in the following setup:
link_freq1 = USRP-A TX @ Freq1 ---> USRP-B RX @ Freq1
link_freq2 = USRP-B TX @ Freq2 ---> USRP-A RX @ Freq2
Where link_freq1 should always have phase X, and link_freq2 should always have
phase Y. I measure the phase by first calculating the channel over a wire for
that link. I then look at the phase of the channel.
The USRPs are connected to an external PPS source, and have a MIMO cable
between them.
I've been able to use timed commands to get the same channel/phase every time
(after multiple restarts) in a single link setup like:
USRP-A TX @ Freq1 ---> USRP-B RX @ Freq1
But my code for phase sync in the 2-link setup doesn't seem to work(copied
below). I understand that the phase of each link's channel may not be the same
as the frequencies are different. However is it possible to set it up so that
the phase/channel over wire in link_freq1 is always X and that in link_freq2 is
always Y? Or is that impossible because of some hardware constraints?
Thank you,
AB
code snippet:
## 2-channel usrp source and sink have been initialized, everything except
## center feqs have been set
## uhd_usrp_source_0 is a multi usrp source block
## uhd_usrp_sink_0 is a multi usrp sink block
## both blocks have device args set to : "addr0=192.168.10.3,
addr1=192.168.10.2"
## the following code is added in the init() function:
time.sleep(1)
future_cmd_time = self.uhd_usrp_source_0.get_time_last_pps() +
uhd.time_spec_t(1.0)
##My guess is that both physical USRPs should see this time since MIMO+same
external PPS
self.uhd_usrp_source_0.set_command_time(future_cmd_time)
self.uhd_usrp_sink_0.set_command_time(future_cmd_time)
self.uhd_usrp_source_0.set_center_freq(uhd.tune_request(cf), 0)
self.uhd_usrp_source_0.set_center_freq(uhd.tune_request(cf2), 1)
self.uhd_usrp_sink_0.set_center_freq(uhd.tune_request(cf2), 0)
self.uhd_usrp_sink_0.set_center_freq(uhd.tune_request(cf), 1)
self.uhd_usrp_source_0.clear_command_time()
self.uhd_usrp_sink_0.clear_command_time()
This is in the init() function. I don't re-tune the frequencies again after
this.
--------------------------------------------------------------------------------
_______________________________________________
USRP-users mailing list
[email protected]
http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
_______________________________________________
USRP-users mailing list
[email protected]
http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
