Erik,
> Does anyone know if it is possible with the U6200A to measure every
period of a PPS?
Yes, use time interval mode instead of period or frequency mode. TI mode
is a 2-channel measurement so one channel is your GPS/1PPS and the other
channel is your Rb/1PPS. The resulting readings (phase difference) are
directly compatible with Stable32 / TimeLab.
> But why can't you measure the 1 s period of a PPS?
You can. The period of a 1PPS is simply 1 plus the delta between
successive pairs of TI readings.
If that's too succinct, consider two consecutive TI readings. Say TI[1]
= GPS[1] - Rb[1] and TI[2] = GPS[2] - Rb[2]. GPS period = GPS[2] -
GPS[1] which is (TI[2] + Rb[2]) - (TI[1] + Rb[1]) and since if your Rb
is accurate Rb[2] - Rb[1] is 1 so GPS period is simply 1 + TI[2] -
TI[1]. It's intuitive once you play with the data, on paper, in Excel,
or TimeLab.
> If the counter would be able to do gapless measurement
Well, yes, *if*. But check the U6200A manual. I don't see gapless period
measurements mentioned as a feature. It's not something often done. If
you want precise low rate pulse edge measurements you use time interval,
or timestamping instead. The U6200A, and the 53132A from which it is
cloned, are not timestamping counters. But both do time interval, so use
that. That's what almost all of us use.
Note that -- at 1/10th the price -- the TAPR/TICC can do all of this
because it is designed at its core to be a timestamping counter. It can
be used for frequency and period, but it does that based on the raw
timestamp data. The picPET and ProTIC are the same way.
> so I was surprised the U76200A could not do that
Period measurements usually start with one leading edge and count to the
next leading edge. Then there's the process of getting the binary data,
formatting to decimal, and communication. By the time the cycle resumes,
the 2nd pulse is already gone so the next period measurement starts with
the 3rd pulse ending with the 4th pulse. Thus the measurement rate is
half the 1PPS rate. Most period counters work this way. This issue is
not obvious with higher frequencies but very obvious with 1PPS. Using
auto-trigger makes it even worse.
All this is why time interval, or especially timestamping, counters are
a better way to go. You get gap-less / zero dead-time measurements for free.
/tvb
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