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 <[email protected]> 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 <[email protected] > <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 > > > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] <javascript:;> > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > > and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] <javascript:;> > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
