Hi Said, Late reply... (been busy)
The purpose of the straight drive is to maximize the signal amplitude on the cable. The test intends to demonstrate that it is perfectly adequate if only the far end is matched, and yet you get twice the amplitude you would get with a 50 ohm drive signal. On the other hand, if the near end is driven like that and the far end is open, you get very significant ringing even with a modest bandwidth oscilloscope (100MHz in my case). The longer the cable, the lower the ringing frequency and potentially the worse the uncertainty on the rising edge timing. Tom asked how much cable reflections could bother a 1 PPS signal and the answer is that it can bother the leading edge a whole lot, no matter what the frequency is. Of course, if you receive the 1PPS with a serial port connected to a busy CPU with multiple uS of latency, it won't make much difference unless the cable is so long as to be impractical. Didier KO4BB On Thu, Sep 18, 2014 at 10:00 AM, Said Jackson via time-nuts < time-nuts@febo.com> wrote: > Hi Didier, > > You showed the effect of standard end-termination and no end-termination > both while driving the cable with a mis-matched low signal source > impedance. These are the same type of plots that Tom posted a link to last > week. > > The interesting plots will be when you set R1 to match the cable impedance > and then vary R2. > > Bye, > Said > > Sent From iPhone > > > On Sep 18, 2014, at 3:41, Didier <shali...@gmail.com> wrote: > > > > Hi Tom, > > > > This page will show you the kind of problems to expect: > > > > http://ko4bb.com/Test_Equipment/CoaxCableMatching.php > > > > Didier KO4BB > > > >> On September 15, 2014 8:43:21 AM CDT, Tom Van Baak <t...@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 > >> > >> _______________________________________________ > >> time-nuts mailing list -- time-nuts@febo.com > >> To unsubscribe, go to > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> and follow the instructions there. > > > > -- > > Sent from my Nexus 7 tablet. > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.