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

Bob Camp kb8tq at n1k.org
Mon Sep 15 17:48:12 EDT 2014


These are saturated logic signals. When you terminate both source and load you get an interesting issue with compatible logic levels. 

For instance: 5V CMOS switches at roughly 2.5V. If you series terminate and load terminate, your destination now sees a 0 to 2.5V signal. Either it’s running 2.5V CMOS and switching at 1.25V or you have a problem.


On Sep 15, 2014, at 9:03 AM, Dave Martindale <dave.martindale at gmail.com> wrote:

> 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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