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:[email protected]] On Behalf Of Mike S Sent: Monday, September 15, 2014 10:29 AM To: [email protected] 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. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
