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SMPS LC filter... not!?

Hi all, I'm having a big fail when trying to make an LC filter for a CUI SPMS. There's nothing to the circuit. Here.

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The problem is that no matter what I do to the Land or C's the output always has the same amount, (shape and size) of ripple. Here's a 'scope shot.

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Channel 2 is the input Chan. 1 is output.

And here's the layout.

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(I was having the same problem in an earlier layout, but thought it might be the lack of a good ground plane.. hence today's modification.)

I'm doing something stupid, 'cause no matter what I do nothing changes.

George H.

Reply to
George Herold
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Aw snap.. never mind, it's some dang ground thing. I can drag the ground of the 'scope probe around the pcb and get all sorts of "answers".

George H.

Reply to
George Herold

First thought is to improve the scope probe grounding - you may be picking up signals in the ground lead.

I found that the best way to look for small signals is to hardwire coax (eg RG174) directly with the screen soldered to the ground plane very close to the measurement point - I used to keep some short lengths of coax SMA term inated on one end just for that purpose.

I can often see whether there is any signal in the probe ground just by gri pping the probe cable tightly with my hand - if there is any change in the signal on screen there must be signal in the ground that is corrupting it.

kevin

Reply to
kevin93

I

g up signals in the ground lead.

Quite Right!

eg RG174) directly with the screen soldered to the ground plane very close to the measurement point - I used to keep some short lengths of coax SMA te rminated on one end just for that purpose. Hmm yeah that works. But I've got to send all the power supplies out of th e box, with one ground connection. (This whole ground idea is over rated :^)

George H.

ripping the probe cable tightly with my hand - if there is any change in th e signal on screen there must be signal in the ground that is corrupting it .

Reply to
George Herold

I remember that I used a large diameter ferrite toroid as a common mode filter. It needs to be large enough to get the probe through the middle. Was I cheating?

Graham

Reply to
grahamholloway

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