[time-nuts] timestamps on downconverted data streams

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 20 17:42:36 EST 2015


Jim,

It really depends on the complexity of your system.

For some you can analyze it or measure it with fairly simple tools.

For other systems, you need to calibrate it with a signal of known 
timing and compare the known timing out of the same box with that of 
your system. This is how GPS/GNSS receivers is typically calibrated, 
then building on the calibration and analysis of the synthesis box 
rather than the receiver.

GPS receivers is one example where absolute calibration is, ehm, tricky 
compared to a cable, filter or something.

Cheers,
Magnus

On 12/20/2015 03:40 PM, Jim Lux wrote:
> Here's an interesting problem.
>
> You have a fast sampler that is collecting samples off-the-air (e.g. the
> end of LORAN) with a fairly wide bandwidth: say 10 Megasamples per second.
>
> Those samples get post processed in a digital downconverter (not
> necessarily in real time) to a narrower band representation at a lower
> sample rate.
>
> You know when the input samples were acquired: e.g. you've got a good
> oscillator, and a reliable sync pulse.  For instance, your handy GPSDO
> (or the ensemble of H2 masers in your garage) might give you a 1pps tick
> good to, say, 20 ns, so you know when your 10 MHz samples were taken (to
> 20ns)
>
> Is there a consistent (and standardized) way to calculate and report the
> time of the output samples.
>
> Each output sample is composed of information from multiple input samples.
>
> One could test the system by digitizing a signal with known timing (e.g.
> a 1 MHz sine wave, where the zero crossing is "on the second") and then
> look for the zero crossing in the downconverted output.  Depending on
> the filtering in the downconverter, there's some time vs frequency
> characteristic that could be used to back out any deltas for other
> frequencies.
>
> So you could report the time of the low rate output samples in terms of
> the time of the input sample, at least for the 'center frequency' of the
> downconverter.
>
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