Re: [Discuss-gnuradio] multiple sources synchronization ?

[jean-michel . friedt]

> 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, address@hidden 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
> >
> 
> 
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-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 
Besancon, France