PCB Layout...loop areas?

[[email protected]]

Hi!

I've been reading some PCB layout design documens. When they say loop
areas, are they only referring to loops on one side of the PCB?

I looked at my design and noticed that there is a loop area on the Top
side. If I re-route some of the wires to the bottom side, will this
eliminate the loop area?

Thanks!

 

[Tim Wescott]

Hi!

I've been reading some PCB layout design documens. When they say loop
areas, are they only referring to loops on one side of the PCB?

I looked at my design and noticed that there is a loop area on the Top
side. If I re-route some of the wires to the bottom side, will this
eliminate the loop area?

Thanks!

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.

 

[[email protected]]

Hi!

I've been reading some PCB layout design documens. When they say loop
areas, are they only referring to loops on one side of the PCB?

I looked at my design and noticed that there is a loop area on the Top
side. If I re-route some of the wires to the bottom side, will this
eliminate 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.

 

[Terry Given]

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

 

[[email protected]]

thats only part of the story.

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.

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.
Agreed.

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

But there are printed circuit inductors and inductances ...

good pcb layout can be summarised in 6 words:

Current Flows In Loops. Minimise Them.

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

 

[John Larkin]

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

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

 

[Terry Given]

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.

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

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

John

Cheers
Terry

 

[Terry Given]

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

are you sure? try these:

Some Semblence Of Understanding Is Required

Cheers
Terry

 

[John Larkin]

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

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

 

[Rich Grise]

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

Here's one for the archives! ;-)

Cheers!
Rich

 

[colin]

Hi!

I've been reading some PCB layout design documens. When they say loop
areas, are they only referring to loops on one side of the PCB?

I looked at my design and noticed that there is a loop area on the Top
side. If I re-route some of the wires to the bottom side, will this
eliminate the loop area?

Thanks!

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 =^.^=