[time-nuts] Distribution amplifier (again!) - now mostly ok but has gain peaking
mattia.rizzi at gmail.com
Fri Feb 3 09:29:57 EST 2017
>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.
2017-01-29 8:11 GMT+01:00 David <davidwhess at gmail.com>:
> On Sat, 28 Jan 2017 13:58:27 +0200, you wrote:
> >The picture gallery also shows a pulse distribution amp for 1PPS. It has
> >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
> >to tune a bit with wires and 0R resistors - without very much success..
> >ideas for improving this - or just leave it at 200ps skew?
> 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.
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