On Mon, 4 Feb 2008 13:05:30 -0800, "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...). 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.

I'm not amazed at the difficulty. Any switcher has really fast edges
which make the layout that much harder. Magic rule is the small signal
grounds get hooked up directly to the ground pin of the IC - never to
a ground plane unless you can guarantee that there will be no high
current signals passing by.