Hi, I guess I should respond, as I'm responsible for the blog posting that you linked.
I tested the relative phase stability using amplified noise that was fed via a splitter onto the two dongles. I cross-correlated the noise and found that there was a deterministic frequency difference between the channels. I recall this was a fraction of a radian per second. I just calculated what this small deterministic frequency difference was and removed it. I found that I could use the same constant during the whole experiment and it also didn't change after power cycling the dongles. This is how I produced the flat line IQ plot that is in the blog posting. Based on your plots, you are mostly also seeing a constant deterministic frequency difference (the linear slope on traces 3 and 4). Trace 3 has a phase jump for some reason and I don't know what is happening with trace 1. In my tests, I only measured data for several hours at a time. I didn't see any loss of lock or weird behavior during my experiments other than what I indicated above. I have heard from several people that they have managed to reproduce the dual rtl_sdr dongle configuration and to even use it for passive radar. I have even seen people taking the proper engineering approach and feeding a clock with a fanout buffer. I haven't seen any cross-correlation plots from these devices though. I have had issues with heating on the smaller dongles. In these cases the down converter stops working. A heat sink fixed this issue. Keep trying, I think you are almost there. Try recabling the clock differently or try using another pair of dongles -- there is a lot of variability between the dongles. Try looking at the clock signal with a scope to see what the levels are with a working individual dongle. I assume you are doing this for fun (that's why I did it at least). There are much easier ways to get multiple coherent channels into your computer with much better fidelity. juha On Thu, May 28, 2015 at 6:18 PM, <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 > 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 > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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