[time-nuts] timestamps on downconverted data streams

Bill Byrom time at radio.sent.com
Sun Dec 20 15:17:31 EST 2015

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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