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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