Most PPS signals these days are very low duty cycle. If you AC couple them, you
can easily be triggering on the wrong edge.
??? All of the triggers I'm familiar with have slope selection (rising
edge/falling edge), and in my experience HP counters do not have
problems mistaking a falling edge for a rising edge. I suppose there's
always a first time, but....
See the attached image, which shows both the DC (green) and AC (red)
trigger channels. The value I used for C1 came from the 5370B input
amplifier. I shortened the pulse to 10uS just because 10 more uS of 0v
baseline didn't really add anything useful. I've shown two trigger
levels, 1.25v (half peak) and 0.5v. I would generally use 0.5 to 0.75v
with a 2.5v logic pulse, unless experience showed that a different value
worked better in the particular instance.
In both the DC and AC cases, the next rising edge that transits the
trigger points is the rising edge of the following PPS pulse.
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