[time-nuts] Correcting jitter on the1 PPSsignalfromaGPSreceiver.

[Peter Reilley]

You need not have a properly terminated transmission line but you
must then worry about the bounce.   If you understand the size
of the bounce and have a system that will not suffer false triggering
then you will be OK.   

There are some worries however.  It is hard to predict the exact
size of the bounce since any measurements that you take necessarily
affect the impedance of the line and therefore the characteristics 
of the bounce.

If you change the components of your transmission line then you effect
the size of the bounce.   High quality (low loss) coax will have 
a larger bounce than crappy coax.

The terminating impendence that you do have affects the bounce.
Is the receiving device 1 M ohm?   Is it 1 K ohm.  The capacitance
also has an effect.  Is it 10 pF, 100 pF, 1000 pF?   Complex 
impedances are lots of fun to figure out in these situations.
A 50 Ohm terminator generally swamps any complex impedance effects
and they can generally be ignored,

The length of the coax will effect the size and position of the 
bounce.

Do you know the size of the bounce?   Do you know your system's 
trigger set point?   Can you adjust the set point?

If you use a properly terminated cable these worries mostly go away.
You can run without a termination, it is just easier and more reliable
with it.

Pete.
 

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Mike S
Sent: Monday, September 15, 2014 10:29 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Correcting jitter on the1
PPSsignalfromaGPSreceiver.

On 9/15/2014 10:01 AM, Tom Miller wrote:
> Fast risetime pulses _are_ RF and need to be treated as such.

You say that as if simply saying it provides an explanation, or even a
reason. Exactly what ill effect on a triggered measurement is there if one
does not terminate a PPS signal properly? Does/can termination increase the
slew rate or make the speed of propagation more consistent, which might make
the measurement more accurate?

Like Tom said, what comes after the leading edge of a PPS signal (which is
the measurement trigger) seems irrelevant.

A simple though experiment. If I take a high impedance measurement at a tap
1M from the source, and the cable ends another 100M away, how can the
termination or lack thereof at that end effect my measurement of a single
event? It's over 700 ns round trip away? If I repeat that event 1/sec, is it
any different? I can see where there would be a difference when I get close
to a 700 ns cycle time, and likely before because of "ringing." But for a 1
second cycle? Someone will have to provide more than a dismissive "just
because" to convince me.
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