[time-nuts] Correcting jitter on the 1 PPS signalfrom a GPS receiver.

Dave Martindale dave.martindale at gmail.com
Mon Sep 15 13:43:23 EDT 2014

I will agree that the end termination is optional if you are delivering a
pulse signal to just one input, which is at the far end of the coax.

However, I think there's still a problem with series-only termination when
the pulse signal is daisy-chained through multiple inputs.  When you apply
5 volts through a 50 ohm terminator to a 50 ohm cable, the instantaneous
voltage on the coax is only 2.5 V.  A pulse of amplitude 2.5 V travels down
the cable, and reflects from the open far end.  The reflection travels back
along the cable to the source, raising the voltage from 2.5 to 5 V as it

A device input located at the far end of the cable sees a single edge of 5
V amplitude, so it's happy.  But anything located somewhere along the cable
run sees two edges: one from 0 to 2.5 V, then a constant 2.5 V for a period
equal to twice the delay of the remaining cable, then another edge from 2.5
to 5 V.  Depending on the input threshold, this in-between device might
trigger reliably on the first edge, the second edge, or not reliably on

Having proper far-end termination is critical for analog video, where
daisy-chaining is common, and a reflection that's even 1% of the amplitude
of the original signal is likely to be visible as a ghost image.  With
pulse signals, maybe it makes more sense to use one cable per device input,
input plus lots of distribution amplifiers and splitters.

- Dave

On Mon, Sep 15, 2014 at 1:13 PM, S. Jackson via time-nuts <
time-nuts at febo.com> wrote:

> Hi Dave,
> yes there is a reason.
> The "standard" 1PPS signal termination (Thunderbolt etc) used to be 5 Ohms
> or less series termination into a 50 Ohms coax (yikes), then end-terminate
> to  get rid of all the undesired reflections.
> Your example below is properly terminating a 75 Ohms coax with a 75 Ohms
> series termination. The end-termination then becomes optional and affects
> the
>  signal level at the sink. So if a higher signal level is desired, simply
> leave  off the 75 Ohms end termination.
> But in the case of the Thunderbolt they don't use a 50 Ohms output
> impedance, they use something around 5 Ohms. That is the problem here: the
> total
> impedance mismatch from the very low source impedance into the 50 Ohms
> coax.
> The reason they do that is so that they can generate a "proper" signal
> level that is approaching 5V across the 50 Ohms end termination so that the
> signal remains CMOS compatible. Otherwise if they properly terminated the
> driver  with 50 Ohms they would have a voltage divider and would only
> generate
> 2.5V  at the sink.
> bye,
> Said
> In a message dated 9/15/2014 06:04:34 Pacific Daylight Time,
> dave.martindale at gmail.com writes:
> Is there  any reason (other than cost) not to both series-terminate the
> source and  parallel-terminate the sink?
> When I was dealing with analog video, the  standard distribution method was
> :
> 1. Buffer amplifier with high input  impedance, very low output
> impedance, and a gain of 2 (so 1 V P-P input  becomes 2 V P-P out)
> 2. A series 75 ohm resistor from the amp output to  each individual video
> output.  This formed a 2:1 voltage divider with  the 75 ohm coax to give
> 1 V P-P on the cable.  It also isolates the  loads from each other.
> 3. A single video signal could be looped through  multiple high impedance
> loads.
> 4. 75 ohm parallel termination at  the far end of the signal path
> (usually on the last device).
> This  way, every device along the way saw an undistorted copy of the
> signal.  The buffer amplifier sees a simple resistive load.  And  any
> reflections are absorbed at both ends of the cable.
> -  Dave
> On 15/09/2014 02:04, Fuqua, Bill L wrote:
> > A lot of devices  have a low output impedance so that the signal can be
> split using a TEE  adapter with little loss or need for a distribution
> amplifier.
> >  However, the cables must be impedance matched at far end, scope input,
> to  prevent reflections which are the source of the ringing.
> > You can match  the impedance at the source and you will get a reflection
> which will then be  absorbed by the source resistance. One way to do this
> > is to get a  small 15 turn pot about 100 Ohms put it, in series with the
> input source and  adjust it until the ringing is gone or you can put it at
> the far end
> >  ,input of the scope, to ground and do the same.  But the best solution
> is  to get a good feed thru 50 Ohm terminator and put it on the input of
> the
> scope.
> >  Bill
> >
> >
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