Hi David,
I've seen something similar on USRP B210 when I tried to use external
10MHz reference clock and there was GPSDO inside of the device. There
was longer time between phase jumps:
https://lists.ettus.com/empathy/attachment/277949
Look at the "Phase jumps in USRP B210 with GPSDO" topic from 2017 for
more:
https://lists.ettus.com/empathy/thread/5N6RUTKSFPCL3C47WQMAAZKTD6FMSWAT?hash=6RMPOPLWPZXSBKXMUV7YAP5DLGD6WGGM#6RMPOPLWPZXSBKXMUV7YAP5DLGD6WGGM
Removing GPSDO module solved the issue for me. Disabling GPSDO on E320
might be much harder as it is built-in to the device.
Best Regards,
Piotr Krysik
W dniu 02.09.2022 o 17:21, David Raeman pisze:
Hi all,
I'm working on a project that involves modeling an incoming signal's
phase as a stochastic process, and I'm seeing a weird phase artifact
on the E320. It looks like a slow periodic phase perturbation – my
best guess is something pulling a PLL, because it always returns back
to a settled state. It occurs with any external clock, but not when
using the internal clock. I either need to find a way to correct the
behavior, or to understand the root cause so I can confidently
consider a different USRP that won’t exhibit this behavior.
I confirmed the same behavior on 3 different E320 radios, first using
an external OCXO (3Vpp bipolar sinewave) and then using a benchtop
function generator to create 10MHz square or sinewave clocks. In all
cases with external clock, the phase artifact can be observed.
I am using only UHD utilities, two radios, and simple offline
processing of the samples:
(1) Cable radio A (transmitter, an E320) to radio B (receiver, any
USRP) with 30dB inline attenuation. Determine appropriate gains on
both radios to ensure the receiver will receive a robust, unsaturated
signal level.
(2) Radio A uses UHD’s tx_waveforms utility to send a 150kHz sine wave
with 400MHz carrier frequency and 500kHz sampling rate, where
reference clock can be internal (no problem) or external (problem).
(3) Radio B uses UHD’s rx_samples_to_file utility to capture 10
seconds of data at the same frequency and sampling rate, always using
internal clock.
This is repeated for various clock options on the transmitter,
everything else held constant. In a theoretically ideal system, the
unwrapped phase of the received baseband sinewave should be a line,
but in reality it'll wander due to imperfect clocks, noise, and other
systems effects. I want to see the wander, so my processing is:
(1) Compute the unwrapped phase over the 10 seconds of the captured
I/Q samples.
(2) Compute the best-fit linear trend line of the unwrapped phase, and
subtract it
(3) Plot the unwrapped phase residuals
Here are some images showing internal clock, external OCXO, and
external function generator square wave: <
https://imageshack.com/a/u1YW7/1 >. For all three cases I’m showing
the unwrapped phase residuals over the full 10 seconds, and then plot
zoomed into two seconds to show more detail. You can clearly see the
periodic phase issues on both the external clock cases, but not the
internal clock case.
Is this a known issue? Any speculation on what might cause this effect
when using an external clock? I can't figure out what the internal
TXCO does that is distinct here-- they both feed into the same ADF4002
PLL. The internal clock runs at 20MHz, but I was able to try an
external clock at that rate (required a 2-line patch to UHD) and it
didn't make a difference. The only other USRP I have on hand is an
N320, and this issue does not seem to happen on that radio model when
I use the same OCXO.
Thank you,
--
David Raeman
Synoptic Engineering
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