[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Sep 15 17:41:04 UTC 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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