On 06/10/2021 04:50 AM, Skyvalakis Konstantinos wrote:
I set up the following flowchart:
Multi_usrp_source -----> complex_to_real -------> WX GUI Scope Sink
I repeated the phase identical test tone experiment multiple times and
I could clearly see that the phase offset between the 2 channels was
different almost every time.
Phase offsets were ranging from almost 0 degrees up to 90 degrees.
Thanks.
Gnu Radio does NOT emit code that wraps the tuning requests in timed
commands, so the synthesizers won't be properly
tuned for phase synchronization. ALSO the WX GUI Scope sink had (as
I recall) a bug that caused things to appear out
of phase even if they weren't. You should probably use the QT GUI
time sink instead--what version of GR are you using?
The WX GUI widgets have been deprecated for several years now, and
don't even appear in later versions of GR.
You'd have to modify the generated code to have it wrap the tuning
requests in timed commands.
------------------------------------------------------------------------
*From:* Marcus D Leech <[email protected]>
*Sent:* Wednesday, June 9, 2021 10:53 PM
*To:* Skyvalakis Konstantinos
*Cc:* [email protected]
*Subject:* Re: Phase Synchronize 2 USRP N200 w/ SBX cards
Sent from my iPhone
On Jun 9, 2021, at 3:52 PM, Skyvalakis Konstantinos
<[email protected]> wrote:
Οk, I know the way of the atan2 method you are proposing. I will try
it tomorrow when I will have access to the equipment again.
When you said injecting a phase identical test tone earlier you
meant, driving the signal from the generator to the RX2 port of each
one of the SBX daughterboards (using a splitter), with equal length
cables and then checking the phase offset between the 2 channels?
Exactly this, yes. You want to measure the phase offset that is
dominated by the receivers and not external path length differences.
Thanks.
------------------------------------------------------------------------
*From:* Marcus D Leech <[email protected]>
*Sent:* Wednesday, June 9, 2021 9:05 PM
*To:* Skyvalakis Konstantinos
*Cc:* [email protected]
*Subject:* Re: Phase Synchronize 2 USRP N200 w/ SBX cards
Your generated tone should be OK as long as the path lengths are the
same during phase calibration.
Measuring mutual phase offset can be done in a number of ways like
taking the atan2 of the two channels and averaging.
Sent from my iPhone
On Jun 9, 2021, at 1:50 PM, Skyvalakis Konstantinos
<[email protected]> wrote:
You assumed correctly. I am doing this in a single multi_usrp
object, so it is ok. I will also remove that sleep command.
I don't know how I can measure the phase difference between the two
RX channels. In my DoA experiment I am just transmitting a tone @
868MHz from an RF generator and I'm receiving it from the 2 RX channels.
Also could you elaborate more on the last thing you mentioned, the
one about the phase identical test tone....
On Jun 9, 2021 19:51, "Marcus D. Leech" <[email protected]> wrote:
On 06/09/2021 10:11 AM, Skyvalakis Konstantinos wrote:
I have 2 USRP N200 devices, each one equipped with an SBX
daughterboard.
I am providing 10MHz ref clock and 1 PPS input on both
devices with equal length cables, from an external signal
generator.
My application is DoA. Is it possible to do it, with 2 USRP
N200 and the SBX daughterboards?
1) Is the following timed commands code snippet correct for
frequency and phase synchronization of both SBX cards?
self.source = uhd.usrp_source(
",".join(("addr0=192.168.10.2, addr1=192.168.10.3", "")),
uhd.stream_args(
cpu_format="fc32",
channels=range(2),
),
)
self.source.set_clock_source('external', 0)
self.source.set_time_source('external', 0)
self.source.set_clock_source('external', 1)
self.source.set_time_source('external', 1)
self.source.set_samp_rate(2e6)
self.source.set_time_unknown_pps(uhd.time_spec())
self.source.set_gain(50, 0)
self.source.set_antenna('RX2', 0)
self.source.set_gain(10, 1)
self.source.set_antenna('RX2', 1)
time.sleep(1)
self.source.clear_command_time()
self.source.set_command_time(self.source.get_time_now() +
uhd.time_spec_t(0.1));
self.source.set_center_freq(uhd.tune_request(868e6,0), 0)
self.source.set_center_freq(uhd.tune_request(868e6,0), 1)
time.sleep(0.11)
self.source.clear_command_time()
2) Is there anyway to measure and compensate the offset
between the 2 SBX daughterboards?
3) Can this synchronization problem be tackled easier with a
MIMO cable (since I do have one) ?
Thank you in advance.
Konstantinos
Assuming that you're streaming two channels in a single
multi_usrp object, then this looks OK, except there's no reason
for that
time.sleep(0.11) *before* the self.source.clear_command_time().
What magnitude of residual phase-offset are you seeing, and how
are you measuring it?
A MIMO cable won't help with residual phase offset.
The usual way of calibrating this out is to inject a
phase-identical test tone into each channel, and measure the
phase offset when
starting up.
