[time-nuts] timestamps on downconverted data streams

[Jim Lux]

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.