[time-nuts] +/- TI button on 5370B

[Magnus Danielson]

On 07/06/2013 08:53 PM, Charles P. Steinmetz wrote:
> Magnus wrote:
>
>> For +/- TI mode, using a separate ARM does not help either, since
>> either of the channels suffice as trigger, and the relative timing is
>> resolve dynamically by the counter. For most time, the dead-time will
>> be hidden, but for longer runs where A/B timing diverge, it can create
>> the same issues as the remaining time from the STOP to the START (as
>> the sequence now has been assigned and maintained) can become to short
>> to cover up the dead-time.
>
> But that is the thing about using ARM as I described -- the sequence is
> NOT maintained, it is generated fresh every period (every second, in my
> example). So every period, you get a + or - TI reading depending on
> which trigger event (START or STOP) occurs first after the ARM pulse. Of
> course, this depends on the START and STOP events always being much
> closer to each other than 1/2 the measurement interval (in my example,
> two 1 pps signals that are always within 100 mS of each other; in
> reality, this could even be 300 mS if the ARM pulse is accurately located).

True. But you need to either keep them "tight" or have ARMing at half 
rate. If you don't, you end up with issues when one of the PPSes occurs 
at the time of the arming. At that time the dead-time will make the ARM 
trigger being missed, and you have the same end-game. That's why you 
need to go sub-rate on the ARM trigger to make any reasonable guarantees.

*IF* there is systematic guarantees for reasonable relative tracking, 
things becomes easier. You need to know the corner cases to know when 
you can do what, with guarantees to achieve the expected result.

> I did not have any trouble with a dead zone. I think this is because if
> the counter misses a trigger, it will always be re-armed before the next
> terminating trigger event occurs, so it would just not report a time for
> that period.

Yes, and that causes you to miss a sample, which I do point out. It's a 
bad thing too.

> Or have I misunderstood what you were saying?

Not completely, but I was trying to point out a bunch of corner cases 
and their consequences.

I would have to make a whole bunch of diagrams to illustrate it all, so 
it may not come out very clearly in text.

Cheers,
Magnus