Hello David, >200ps of skew is consistent with long traces on board material like standard FR4 because of uneven dielectric constant
Can you elaborate it? What do you mean by "long traces"? I saw the PCB done by Anders, the track length is about 10 cm. Using the microstrip formula, even for a big dielectric change (Dk from 4 to 4.6, worst scenario), the propagation speed change is about 5% (30ps). Indeed, you'll get also reflections, but the effect is dwarfed by the "s" done by Anders for length matching. My gut-feeling is that most of the 200ps skew (80-85%) is coming from 74AC04's unmatched outputs, but since two persons already replied suggesting PCB issues, I'm very interested. cheers, Mattia 2017-01-29 8:11 GMT+01:00 David <[email protected]>: > On Sat, 28 Jan 2017 13:58:27 +0200, you wrote: > > >... > > > >The picture gallery also shows a pulse distribution amp for 1PPS. It has > an > >LT1711 comparator feeding an 74AC14 buffer with length-matched traces to > >74AC04's at the outputs. So far my length-matching didn't give zero > >output-skew between the outputs - I see around 150-200ps skew which I > tried > >to tune a bit with wires and 0R resistors - without very much success.. > any > >ideas for improving this - or just leave it at 200ps skew? > > > >cheers, > >Anders > > 200ps of skew is consistent with long traces on board material like > standard FR4 because of uneven dielectric constant produced by uneven > fiberglass weave. One way to ameliorate this is to route traces at a > diagonal compared to the fiberglass weave. > _______________________________________________ > 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.
