For a low duty cycle pulse, the ac coupled signal will be approximately the
same as if it were dc coupled. Not sure I follow what you mean. There will
be only one rising edge for a narrow pulse ac coupled, as the falling edge
occurs much quicker than the HPF time constant.

On Friday, 16 September 2016, Bob kb8tq <kb...@n1k.org> wrote:

> Hi
>
> Most PPS signals these days are very low duty cycle. If you AC couple
> them, you can easily be triggering on the wrong edge. With the narrow pulse
> it may not be very obvious.
>
> Bob
>
> > On Sep 16, 2016, at 5:46 PM, Charles Steinmetz <csteinm...@yandex.com
> <javascript:;>> wrote:
> >
> > Bob wrote:
> >
> >> Set it to:
> >>
> >> 1) DC coupled (AC does not go low enough)
> >> 2) 50 ohms if your driving source will tolerate it, otherwise 1 meg ohm.
> >> 3) Manual trigger mode (Auto is to fast and it forgets where the
> trigger should be)
> >> 4) Trigger level around 1/2 the PPS P-P voltage
> >
> > I would just add the following:
> >
> > 1)  I'd be very surprised if AC coupling wouldn't work fine with a
> typical PPS pulse, which has very fast edges (low nS).  No LF response is
> required.  Indeed, AC coupling will keep any LF noise out (not that we
> expect much in this application).  This is true even if the PPS is a 50%
> duty-cycle square wave -- the spikes that get through every 500mS,
> alternating positive and negative, will have fast, accurate leading edges
> and will be way longer than necessary for proper triggering.
> >
> > 2)  If your source will not tolerate a 50 ohm load, buffer it.  Any
> significant length of cable between the source and a 1M termination will
> just slaughter your pulse.
> >
> > 4)  The relevant peak voltage is the actual voltage at the counter input
> connector -- which may be only 1/2, or possibly even less, of the nominal
> logic level, depending on the source impedance.
> >
> > Best regards,
> >
> > Charles
> >
> >
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