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
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