Re: [Discuss-gnuradio] dual receiver setup for interferometry measurements

[jean-michel . friedt]

Thanks for your reply. Indeed designing phase coherent receivers is my daily
job activity (partly), but the fun of hacking DVB-T receivers is to find ways 
of using 
these for applications they were never intended for. I have indeed read your 
posts 
concerning the difficulties in reproducing interferometric measurements, a great
source of inspiration. At the moment we are considering quantifying the phase
drift (switching from a reference oscillator to the unknown signal), but I 
wonder
nevertheless why some have achieved excellent coherence and some (including 
myself)
are failing. Nevertheless I'd like to actually identify the source of the drift
(PLL bandpass, temperature-related setpoint drift, software configuration ?).

Thanks, JM

> This is largely because this $10.00-apiece hardware was never designed
> for this class of application. When you're doings things that require
> phase-coherence, you have to design your radios to support it. 
> 
> There are at least two PLLs involved here--one on the R820T chip, and
> another, as far as I can tell, in the RTL2832U chip, which does the
> conversion to baseband from the low-IF of the R820T. 
> 
> I was never able to get my RTL receivers to be phase-coherent in any
> useful way, there was always a slow phase drift, that was unpredictable.
> 
> 
> On 2015-05-28 14:18, address@hidden wrote: 
> 
> > 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 [1]
> > 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 [2] 
> > 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
>  
> 
> Links:
> ------
> [1] http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html
> [2] http://jmfriedt.sequanux.org/ph_tout.pdf


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