[time-nuts] Why are 1PPS signals so skinny?

[Dave Martindale]

It is worth noting that skipping the end termination is probably a bad idea
when daisy-chaining a signal from one output to more than one device input.
 The input at the end of the cable will see a clean rise from zero to 5 V
(or whatever the driver's open-circuit voltage is), but the other inputs
along the length of the cable will not.  They will see an initial rise from
0 to 2.5 V as the series termination at the driver and the cable impedance
act as a voltage divider while the cable is being charged. Later, they will
see another step change from 2.5 V to 5 V as the reflection returns from
the open-circuit far end of the cable.  If the input threshold is
automatically set at half the input voltage swing, the input could trigger
on the outbound or the reflected pulse, or even somewhere in between.

This is in contrast to having a 50 ohm termination at the end of the cable
(plus the 50 ohm series termination at the source), where all inputs along
the length of the cable see a single edge transition from 0 to 2.5 V.  They
will each see the edge at a different time due to propagation delay, but
all will see a clean edge.

     Dave

On Tue, May 15, 2012 at 4:00 PM, <SAIDJACK at aol.com> wrote:

>
> To make this work without the unnecessary power consumption simply remove
> the end-termination resistor, and use it as the series termination resistor
> (R1  in your schematic)! Done.
>
> Attached are two plots of a series terminated (~55 Ohms) high-speed  1PPS
> transmission from our CSAC GPSDO board zoomed-in  and zoomed-out to show
> the
> actual rise-time, and a longer time frame  view.
>
>