Joel Koltner wrote:
I've made some modifications...

-- Cutting the ground trace that exits R5 and heads horizontally to the ground
test point/via and instead using a fat wire connection from R3's ground over
to pin 4 of U1
-- Cutting the ground trace from R2 and instead running it back to pin 4 of U1
(with wirewrap wire)
-- Changed R1 and R2 to 270k so that with worst case leakage the part doesn't
try to change modes (fixing design error that Joerg pointed out)

(I also checked that the digital scope's display is believable and that the
source voltage wasn't sagging.)

This improves things some: there's noticeably less jitter, although if you
look closely it's still there (interestingly, it's easier to *not* notice on a
Tek 2465 than a digital scope... hmm...). ...


Sure the DSO doesn't switch to equivalent time? I can't use mine above
25nsec/div on switchers because they didn't toss me the firmware keys to
disable that "feature". Here is a pretty sure-fire method to verify jitter:

Hook up a spectrum analyzer but make sure not to fry it's input (use a
really loose coupling and attenuate). Find the carrier, switch to peak
hold and then vary the input voltage. Jitter should produce a pronounced
broadening around the carrier.


... I don't think there's a lot left to
do on this particular PCB, but I'm thinking the results are good enough that
it's safe to put the design on a 4-layer boad and see what happens.

I'm amazed that a switcher at "only" 1.4MHz on a board that's 3/4" square can
be so difficult to tame.


Remember that the mods above are still kludges. Ideally the traces
should be on the other side of the ground plane.

--
Regards, Joerg

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