[time-nuts] TimeLab

[Azelio Boriani]

In the real world of TICs is not possible to implement a stop pulse
that occurs before its start pulse. When a regular start-stop (stop
pulse after start, positive delay) is followed by a negative delay
(stop pulse before the start) the sample is lost because the start has
not yet occurred. The only way is to intentionally delay the stop so
that it will never occur before the start. The delay must be known and
very stable, of course.

On Sun, Oct 9, 2016 at 1:32 PM, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:
> Fellow time-nuts,
>
> I don't know if it is me who is lazy to not figure TimeLab out better or if
> it is room for improvements. I was considering writing this directly to
> John, but I gather that it might be of general concern for many, so I
> thought it be a good topic for the list.
>
> In one setup I have, I need to measure the offset of the PPS as I upset the
> system under test. The counter I'm using is a HP53131A, and I use the
> time-interval measure. I have a reference GPS (several actually) which can
> output PPS, 10 MHz, IRIG-B004 etc. In itself nothing strange.
>
> In the ideal world of things, I would hook the DUT PPS to the Start (Ch1)
> and the reference PPS to the Stop (Ch2) channels. This would give me the
> propper Time Error (DUT - Ref) so a positive number tells me the DUT is
> ahead of the reference and a negative number tells me that the DUT is behind
> the reference.
>
> Now, as I do that, depending on their relative timing I might skip samples,
> since the counter expects trigger conditions. While TimeLab can correct for
> the period offset, it can't reproduce missed samples.
> I always get suspicious when the time in the program and the time in real
> world does not match up.
>
> I could intentionally shift the PPS output of my DUT to any suitable number,
> which would be one way to solve this, if I would tell TimeLab to withdraw
> say 100 ms. I might want to do that easily afterhand rather than in the
> setup window.
>
> To overcome this, I use the IRIG-B004 output, which is a 100 Hz signal with
> a stable rising edge aligned to the PPS to within about 2 ns. Good enough
> for my purpose. However, for the trigger to only produce meaningful results,
> I will need to swap inputs, so that the PPS from DUT is on Start/Ch1 and the
> IRIG-B is on Stop/Ch2. This way I get my triggers right. However, my
> readings have opposite sign. I might have forgotten about the way to correct
> for it.
>
> However, TimeLab seems unable to unwrap the phase properly, so if I have the
> condition where I would get a negative value of say -100 ns then the counter
> will measure 9,999,900 ns, so I have to force a positive value as I start
> the measurement and then have it trace into the negative. I would very much
> like to see that TimeLab would phase-unwrap into +/- period/2 from first
> sample. That would be much more useful.
>
> I would also like to have the ability to set an offset from which the
> current zoom window use as 0, really a form variant of the 0-base but
> letting me either set the value or it be the first value of the zoom. I have
> use for both of these. I often find myself fighting the offset issues. In a
> similar fashion, I have been unable to change the vertical zoom, if I don't
> care about clipping the signal then it forces me to zoom in further than I
> like to. The autoscale fights me many times in a fashion I don't like.
>
> OK, so there is a brain-dump of the last couple of weeks on and off
> measurement experiences. While a few things might be fixed in the usage, I
> wonder if there is not room for improvements in the tool. I thought it
> better to describe what I do and why, so that the context is given.
>
> Cheers,
> Magnus
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