Re: [Discuss-gnuradio] multiple sources synchronization ?

[Marcus Müller]

Hi Jean-Michel!

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.

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?

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