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[Discuss-gnuradio] multi-rtl phase synchronization
From: |
jmfriedt |
Subject: |
[Discuss-gnuradio] multi-rtl phase synchronization |
Date: |
Fri, 7 Jul 2017 07:55:07 +0200 |
I am tackling again the coherence issue with multi-RTL dongles for
interferometric measurements.
I have two R820T2 based DVB-T dongles connected to the same USB bus (USB hub)
and am feeding
both dongles with a high quality 100-MHz source (-34 dBm output of a
Rohde&Schwarz SML03 synthesizer,
30 dB gain on the RF frontend of the dongles) -- setup shown at
http://jmfriedt.sequanux.org/DSC06032.JPG.
I am trying to reproduce the graph "IQ plot of the z_1/z_2 relative phase
signal over ~6000 seconds at
1 Hz sample rate" from
http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html
Actually I notice that this graph shows something in units in the tens of
thousands: what is
"Z1/Z2 relative phase" ? How can I convert this axis to angle in degrees ?
I have manually applied the patch described at
http://www.rtl-sdr.com/forum/viewtopic.php?f=1&t=531 to
turn off dithering (I have checked that the function is called when starting
gr-osmosdr -- whether
dithering is actually disabled is challenging to assess). The resulting phase
measurement -- recording
either with multi-rtl source or two osmosdr sources, multiplying one output by
the complex conjugate
of the other, complex to phase and saving the resulting phase at 200 Hz rate,
is at
http://jmfriedt.sequanux.org/allan.pdf (left is time domain plot, right is
Allan deviation). On the
short term we have sub-degree stability, on the long term several degree drift.
I have checked that
thermal transfer between the two dongles is of utmost importance: if I cool one
of the dongles and keep
the other at room temperature, I observe phase rotations of several tens of
degrees. I believe such
long term phase instability is of little concern for passive radar measurements
where coherence over
several ms only is needed, but I find conflicting conclusions on long term
phase stability.
Can other experimenters on this topic comment and assess whether
http://jmfriedt.sequanux.org/allan.pdf
can be improved, or if is in line with the expected behaviour of these dongles ?
Thanks, Jean-Michel
--
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000
Besancon, France
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