Thanks for the informative post. I've been searching for a way to turn off the dithering.
The I/Q plot show complex cross-correlation between two noise signals at 100 MHz center frequency (2.4 MHz bandwidth noise). The phase of the signal can be converted into time and Allan deviation, but I never did. I was just happy to see a straight line.
juha
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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