> There's nothing in your system that would make both RTL dongles and the > soundcard start sampling at the same time, so naturally there's a large > time offset between these. You will need to time align these signals > first, before you can use the sound card signal to determine in which > state (reference or unknown signal) your switch is; this is at least as > hard as the problem of aligning the RTL dongles themselves. > To be honest, I'd rather write an estimator that tells me, only from the > signal, whether each RTL dongle is observing the reference or the signal.
right, this is what I ended up doing yesterday: since the reference and measured signals exhibit different power, I am recording the phase difference as well as the magnitude from the dongle connected to the switch, and use the magnitude information to differentiate the two states of the switch. > How do you frequency-synchronize both dongles? Have you modified them to > use a common oscillator, or do you also plan to do that based on the > observation of the 50MHz tone? Yes I removed the quartz from one dongle and am using the 28 MHz output from the other dongle to clock the former. I believe that the slow phase slip is not important for passive radar applications since the cross-correlation reinitializes the phase, but for my interferometric measurement I need the long term phase drift information. Thanks for the answer, JM > > Best regards, > Marcus > > On 22.07.2015 09:51, jean-michel.fri...@femto-st.fr wrote: > > Possibly a stupid question, but might help me better understand how the > > gnuradio scheduler works: > > > > my objective is to make a low cost phase-referenced radiofrequency > > interferometer using two DVB-T dongles. > > Since I have observed that the PLL inside each dongle induces slow phase > > drift, I want to use an external > > RF switch to monitor a known (50 MHz) reference oscillator feeding both > > dongles, and then monitor the > > unknown signal. Current switching rate is about 50 Hz triggered by an > > external generator (which also > > synchronizes other events of the experiment, not relevant to this post). My > > idea for synchronizing the > > post-processing of phase extraction was to record on the one hand the two > > DVB-T dongle data flow > > (this I know works), and on the other hand the sound card microphone > > connected to the switch trigger > > signal. This process is summarized in the grc flowchart at > > http://jmfriedt.sequanux.org/damien_grc.png > > > > However, the sound card output shows a result completely out of sync with > > the phase measurements. I > > understand that the sound card and DVB-T dongles do not share the same > > clock sources, but considering > > the huge decimation factor (48*32 kHz for the DVB-T, 48 kHz for the sound > > card, and a phase output recorded > > at about 0.5 to 5 kHz, not shown on this grc chart), I would have expected > > the trigger signal to be more or > > less synchronized with the DVB-T outputs, which is not at all the case. > > > > Is the gnuradio scheduler unable to interleave two data sources as > > different as a sound card and the > > USB data flow from the two DVB-T dongles ? Is there a way I might tune my > > flowchart to achieve > > the expected result ? > > > > Thanks, JM > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio -- JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 Besancon, France _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio