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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jul 6 21:04:27 EDT 2013


Charles,

On 07/07/2013 12:30 AM, Charles P. Steinmetz wrote:
>
>> But what happens if, over a day, your DUT 1PPS wanders ahead and/or
>> beyond the REF 1PPS? This is common with GPS 1PPS boards or with
>> too-accurate house 1PPS references or when comparing poor quartz with
>> a GPSDO.
>
> I generated the "ARM" 1 pps (it's not a "REF" 1 pps) from one of the
> trigger signals. Its only purpose is to arm the counter at the
> approximate midway point through the period of the two signals being
> compared, so its accuracy is not a factor as long as it's not so bad
> that it wanders all the way around to between the two trigger events.
> One could use almost anything, including a 555 one-shot, for 1 pps
> signals. My scenario was drawn from the OP, who IIRC was comparing 1 pps
> signals from two GPSDOs. In that case, you know beforehand that the
> trigger events will always be relatively close together compared to the
> period of the events, so it is easy to place the ARM event far away (per
> period) from the trigger events -- far enough that neither trigger event
> will wander into an overlap with it. There are certainly plenty of other
> situations in which this would not work.

Arming from one of the triggers will make the arming and that trigger 
relate nicely, but it doesn't help for the other PPS.

> (You say wandering is commom with GPS 1 pps boards -- have you really
> seen them wander on the order of 500 mS with respect to "true" GPS pps?)

I never said GPS. I was looking at the general problem and it's corner 
cases. I did however say that there can be mechanisms which bounds the 
differences, which in that case could form a guarantee for it not to 
become a problem.

If one of the signal sources is not a GPS but a free-rolling clock, then 
you need to care about the full set of triggering details, which was 
what I was looking at.

>> The solution is either a time-stamping counter, or to deliberately
>> offset the DUT or REF by enough microseconds to avoid any sign changes
>> in TIC measurements ever. I'm curious if you've discovered a reliable
>> third alternative.
>>
>> Without reproducing your ARM/START/STOP scenario here myself, it sort
>> of sounds like you're moving the start ahead in time. True, this will
>> give nice valid positive TI measurements but your consistent tau is
>> now silently corrupted.
>
> The question posed was "why doesn't the 5370B do what I expect in +/- TI
> mode?" I wasn't trying to answer "how can I set up a 5370B so that every
> possible measurement can be made in +/- TI mode?"

True. I diverted inspired of that discussion, which I indicated.

> The scheme I described compares two signals of nominally equal period
> and reports results as +/- time intervals, as long as the absolute value
> of the TI is always less than 1/2 of the period of the signals. It is
> effectively the same as delaying one of the test signals by 500 mS [in
> the case of 1 pps signals]. In both cases, you have an ambiguity if the
> test signals can wander as far as 1/2 period in relation to each other.

Your stop signal needs to be well in advance of the arming, or the 
arming will miss as the counter was still in it's dead-time.

But I suspect you mean something else.

Cheers,
Magnus


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