[time-nuts] timestamps on downconverted data streams

[Bill Byrom]

Yes, you can do that. My employer (Tektronix) makes RF Signal Analyzers
which sample at a high rate then use a DDC (Digital DownConverter based
on decimation and digital filtering) to produce a much smaller output
I/Q file at a smaller bandwidth and lower time resolution. The
decimation and digital filtering delay acquired signals, so you have to
correct for the group delay in these operations.
--
Bill Byrom N5BB
 
 
 
On Sun, Dec 20, 2015, at 08:40 AM, 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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