Tom Van Baak
tvb at LeapSecond.com
Mon Oct 10 05:49:19 EDT 2016
Hi Magnus, two questions for you:
> I can shift the phase of the DUT intentionally, but if so I want to be
> able to compensate that shift in the software. Now, such a shift should
> be kept separate from the calibration factors which fills a different
If you can shift the DUT phase, you're all set. No need for most of the most of the discussion going on, right? What is the frequency error or stability of the DUT? A 500 ms shift should provide days to years of time before you'd hit the TIC zero.
> The trick that I then apply is to use a stop signal of higher frequency,
> but who's rising edge matches that of the PPS on the PPS occurence, and
> then let my DUT signal PPS be the start signal, as that will then
> defined the tau-rate. With a 100 Hz signal I now have 10 ms period and
> then from the last stop-time I have 990 ms for the counter to re-arm the
> start-channel, and thus hide the dead-time. This is the picket fence
> approach rather than having alternate counters to cover up each others
I've tried this. In general the picket fence method has no effect on you hitting the awkward region near 0 ns where the TIC waffles between - and +. In fact, a 100 Hz picket fence just means you will run into the TIC dead zone 100 times more often, but the effect the dead zone will be 100x less. To me this suggests that 2 Hz is all that is necessary, as PHK and I have mentioned. I almost hesitate to call 2 Hz a picket fence.
Or are we missing something? Is there some advantage to 10 Hz or 100 Hz over 2 Hz? It seems to me they all solve the problem where the counter start/stop accidentally gets turned into stop/start near the 0 ns region, which leads to sampling every 2 seconds instead of every 1 second for a while.
More information about the time-nuts