[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.

SAIDJACK at aol.com SAIDJACK at aol.com
Mon Sep 15 13:41:04 EDT 2014


I found the schematics of the Mini-T output circuit. It actually is a  
single 10 Ohms series resistor (R32) driven by six parallel 74AC04 gates.
 
Thus only a single resistor change on the Mini-T would fix the issue. Since 
 the gates probably have about 2 Ohms equivalent impedance, simply changing 
R32  to a 47 Ohms resistor would fix the problem.
 
Also, another issue with the end termination happens when driving very long 
 coax cables: RG-142 for example has about 60 Ohms center conductor 
resistance  and 7.5 Ohms shield resistance at 1km length.
 
So the cable DC resistance is almost 70 Ohms at 1km, which would make the  
voltage across the end termination drop to around 2V from the desired 5V.  
In this scenario the series terminated system would generate a nice 5V at the 
 end of the cable after the reflections have calmed down (within some us).
 
bye,
Said
 
 
 
 
In a message dated 9/15/2014 07:24:27 Pacific Daylight Time,  
tshoppa at gmail.com writes:

Some  good refs on coax driving showing scope traces for ringing  etc:

http://www.ti.com/lit/an/snla043/snla043.pdf

http://www.ti.com/lit/an/sboa075/sboa075.pdf


On  Mon, Sep 15, 2014 at 9:43 AM, Tom Van Baak <tvb at leapsecond.com>  wrote:

> How important are all these cable / termination / impedance  issues for
> 1PPS signals? I know ringing and reflections are  undesirable in many
> applications. But for 1PPS?
>
> I  often use pick whatever cable, termination, and trigger level gives  
the
> cleanest edge, the best risetime. What happens to the signal tens  or
> hundreds of nanoseconds after the edge seems irrelevant to me.  Could one 
of
> you RF experts comment?
>
>  /tvb
>
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