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, jean-michel.fri...@femto-st.fr 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 _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio