[time-nuts] 53132A triggering

Bob kb8tq kb8tq at n1k.org
Fri Sep 16 18:55:57 EDT 2016


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. 


> On Sep 16, 2016, at 5:46 PM, Charles Steinmetz <csteinmetz at yandex.com> 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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