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

[SAIDJACK at aol.com]

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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