[Discuss-gnuradio] dual receiver setup for interferometry measurements

[jean-michel . friedt]

I have a question concerning connecting two DVB-T dongles on the same clock
source for interferometric (or passive radar) measurements, as described at
http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html
I have assembled the same system with one dongle used as oscillator on a 28.8 
MHz resonator
and the second one as a slave to this clock. All works fine, solved the issue
when the oscillator would not start, now I have a reliable source of 
measurements.

Initial tests (these are R820T-based dongles) exhibits significant random phase 
drift
which I attributed to heating of the chips (they get above 50 degC when running 
continuously),
so after gluing two heat sink with heat-conducting epoxy, I more or less 
managed to
get a stable phase measurement when recording a same oscillator (200 MHz) with 
the
two dongles and displaying the phase as angle(conjugate(signal1)*signal2).

The question is as follows: at http://jmfriedt.sequanux.org/ph_tout.pdf I have 
shown one
graph, quite representative of all my experiments, displaying the evolution of 
the phase
difference between both dongles connected to the same 200 MHz oscillator. I 
*always* 
start with a quite stable phase difference (red curve -- inset in a zoom of 
this particular
measurement) after plugging in my USB hub fitted with the two dongles and 
starting gnuradio-companion
for recording the dongle I/Q stream (notice the abscissa sampling rate of 10 Hz 
=> the full
graph is about 1-hour long). After about an hour, I stop recording the red 
curve, and
all I do is switch off gnuradio-companion and start it back => green curve with 
a quickly falling
phase. Switch off again, disconnect-reconnect USB hub, restart an acquisition 
=> blue curve.
Same procedure => magenta curve.

Can anyone hint at an explanation as to why I always start with a reasonably 
stable phase
difference (yet not constant -- is the phase fluctuation indeed due to heating 
of the fractional
PLL in each RF frontend, drifting below the feedback loop time constant ?), but 
more worrisome
why I always get this huge drift after launching a new acquisition ? The fact 
that I always
get the same slope hints at a sofware/hardware communication issue, but how it 
is possible
since both dongles are clocked by the same source and receive the same commands 
from the
software ?

Thanks, JM

-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 
Besancon, France