PCB Layout...loop areas?

No. If 1/2 of the loop is on the top and the other 1/2 is on the bottom it's still a loop. If you have a top trace directly over a bottom trace you'll minimize the loop area.

The "loop area" is interesting in as far as it is a measure of how much magnetic flux is linked to the loop - it is another way of saying "what is the mutual inductance between this conducting loop (secondary winding) and the various current-carrying loops on and around the board (primary windings) ta might excite it."

Any connection that completes the loop - on either side of the board - has to be taken into account.

thats only part of the story.

for a start, the loop is an antenna, so its is also quite capable of transmitting as well as receiving - IOW it can be the "exciting" loop.

then of course the inductance of the loop can (and usually does) have a deleterious effect on the behaviour of the associated circuit.

good pcb layout can be summarised in 6 words:

Current Flows In Loops. Minimise Them.

Cheers Terry

Mutual inductance is the whole story, but I agree that my choice of words did lay all the emphasis on the loop as a receiver.

Agreed.

But there are printed circuit inductors and inductances ...

unless you are deliberately making some kind of transformer or antenna ....

Sometimes a loop can be cancelled with an anti-loop. This works well for magnetically-coupled line-frequency hum pickup. Parts must be included in the loop, of course.

I think good PCB layout sometimes needs more than six words.

John

have you seen the trick of adding a tiny, air-cored tapped PCB inductor to a cap, designed such that -M cancels ESL? it works....but is only useful in filters, where one wants series L, except when there is a shunt cap.

thats the easy part. the tricky part is calculating the shape to get the right -M

:)

Cheers Terry

are you sure? try these:

Some Semblence Of Understanding Is Required

Cheers Terry

This is disgusting: make a loop on the board, run it through a trimpot with effective gain +-0.5, and inject the resulting signal into the signal path. Tweak pot for minimum hum.

There's an equivalent capacitive cancellation scheme, which I've done in big audio switch panels.

Gross!

John

Here's one for the archives! ;-)

Cheers! Rich

is it an open or a closed loop ? I came accross a problem with a ground plane with a big hole in the middle wich was efectivly a closed loop, soldering a wire accross the middle of it (surprisingly easily) reduced the amount of pickup from the nearby mains transformer to an acceptable level. (was an old VCR)

I wont mention the long earth loop in a microwave controller wich went all round the houses just to avoid a wire link ...

also keeping the signal lines in the same orientation as the ground lines minimises any pickup/radiation as the currents are in opposite directions and cancel. as said in other post its the area of the loop that is considered, there will probably be many loops all of wich may need to be considered not just the ground plane. loops with sensitive inputs shld be payed atention to (ie minimise the area) also those with high curents especialy with high frequencies. Ive seen several diferent gnd stratagies for avoiding ground problems, not all of wich are readily applied or even work well, as the off board IO connectors earth and chasis earth need to be consideredd too, can sometimes be dificult.

Colin =^.^=