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

Tom Miller tmiller11147 at verizon.net
Mon Sep 15 19:04:23 UTC 2014


So does adding ~80 pF per meter or 8 nF for 100 meters (RG58) to your output 
have any effect on the risetime? Because that is what it will see with an 
open cable.

I am sure you can make a case for some condition(s) where an unterminated 
cable will still work. But it is not something we have been shown necessary 
to make precise measurements (something this group strives for).

YMMV,
Tom


----- Original Message ----- 
From: "Mike S" <mikes at flatsurface.com>
To: <time-nuts at febo.com>
Sent: Monday, September 15, 2014 10:29 AM
Subject: Re: [time-nuts] Correcting jitter on the 1 
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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