# Conical inductors and mutual inductance

A

#### [email protected]

Jan 1, 1970
0
Hi gang,
We are using Coilcraft conical inductors for bias tees.

http://www.coilcraft.com/pdfs/bcl-s_.pdf

There are a lot of them on the board, right next to each other. A rule
of thumb I've used for inductors is to place them at 90 degrees from
each other, this is supposed to reduce mutual inductance, assuming the
inductors are the cylindrical coil type.

How do conical-wound coils interact? I think these are air-core
although the units I have have some sort of plug that the coil is
wound on, I don't know what it is.

I'm also guessing that coils placed on top and bottom of a standard 60
mils thick FR-4 PCB with two ground planes will still couple
magnetically, right?

T

#### Tom Bruhns

Jan 1, 1970
0
Hi gang,
We are using Coilcraft conical inductors for bias tees.

http://www.coilcraft.com/pdfs/bcl-s_.pdf

There are a lot of them on the board, right next to each other. A rule
of thumb I've used for inductors is to place them at 90 degrees from
each other, this is supposed to reduce mutual inductance, assuming the
inductors are the cylindrical coil type.

How do conical-wound coils interact? I think these are air-core
although the units I have have some sort of plug that the coil is
wound on, I don't know what it is.

I'm also guessing that coils placed on top and bottom of a standard 60
mils thick FR-4 PCB with two ground planes will still couple
magnetically, right?

To minimize magnetic coupling, you want to end up with cancellation of
fields going one way with those going the other way. The field around
a cylindrical coil is symmetrical about any plane through its axis and
also about a plane perpendicular to the axis through the center of the
coil (except that the field on either side of that plane will be in
opposite directions, which is where the cancellation comes in...).
The conical coil will have the symmetry about planes through its axis,
but not about a plane perpendicular to the axis.

To get a better "feel" for how the cancellation works, I connected a
couple identical coils, one to each port on a network analyzer, one in
a fixed position and the other on an insulating "wand" so I could move
it around and see just how much cancellation I could get, and how
sensitive it was to error in position versus separation. That was
worthwhile for me. I suggest you do the same with a couple of the
conical coils. One thing I especially got a better appreciation for
was that, though you can get good magnetic cancellation, when the
coils are close, you still have a capacitive coupling.

As for coils coupling magnetically when on opposite sides of a board
with ground planes: consider Faraday's law of magnetic induction.
There can be no time-varying magnetic field passing through a
perfectly conducting plane. Any field that gets from one side to the
other must be because it leaked around the edges, or because the
copper is less than a perfect conductor. I have several home-made RF
board on 5 of the 6 sides, but the 6th side is just open. These
filters show high attenuation in the stop bands, as predicted by a
model with no coupling between the coils. In those that are designed
for it, I get 120dB and more stopband attenuation. I'm quite sure
that would not be possible if there was significant coupling through
the partitions between the cavities.

Cheers,
Tom

A

#### [email protected]

Jan 1, 1970
0
To minimize magnetic coupling, you want to end up with cancellation of
fields going one way with those going the other way. The field around
a cylindrical coil is symmetrical about any plane through its axis and
also about a plane perpendicular to the axis through the center of the
coil (except that the field on either side of that plane will be in
opposite directions, which is where the cancellation comes in...).
The conical coil will have the symmetry about planes through its axis,
but not about a plane perpendicular to the axis.

To get a better "feel" for how the cancellation works, I connected a
couple identical coils, one to each port on a network analyzer, one in
a fixed position and the other on an insulating "wand" so I could move
it around and see just how much cancellation I could get, and how
sensitive it was to error in position versus separation. That was
worthwhile for me. I suggest you do the same with a couple of the
conical coils. One thing I especially got a better appreciation for
was that, though you can get good magnetic cancellation, when the
coils are close, you still have a capacitive coupling.

I see. I will try to set up an "appointment" with our high-speed test
instruments.
As for coils coupling magnetically when on opposite sides of a board
with ground planes: consider Faraday's law of magnetic induction.
There can be no time-varying magnetic field passing through a
perfectly conducting plane. Any field that gets from one side to the
other must be because it leaked around the edges, or because the
copper is less than a perfect conductor. I have several home-made RF

Thanks. The problem is that I don't know (yet) how low the frequency
is on the "DC" end of the coil.
It's supposed to be DC that changes once in a while to another DC
value.
But it's entirely possible someone will want to modulate the "DC" end
with sine waves of varying frequencies.
Could be anything from the audio band upwards.
Either way I'm already going into full paranoia mode by placing them
all at 90 degrees of each other. Of course every other one will be in-
line but hopefully the distance will be great enough.
The board is quite small and these coils will go all the way to the
edges. There is no plan for an enclosure yet.
board on 5 of the 6 sides, but the 6th side is just open. These
filters show high attenuation in the stop bands, as predicted by a
model with no coupling between the coils. In those that are designed
for it, I get 120dB and more stopband attenuation. I'm quite sure
that would not be possible if there was significant coupling through
the partitions between the cavities.

Cheers,
Tom

Thanks.

A

#### [email protected]

Jan 1, 1970
0
The conicals (originally designed by Piconics, who incidentally are
cheaper) are filled with ferrite epoxy stuff. They are so tiny that
I'd guess that coupling is minimal unless they are very close. Copper
planes should block through-pcb coupling above, very rough guess, 50
KHz.

But why don't you just test a couple?

Eh, tight schedule, no time for "play". I'm not very informed on
magnetics. I'll have to finish the job at hand first and then go back
to freshman-level physics...
Bias tees, especially super-wideband ones, are interesting. The
conicals are a good first inductor, albeit expensive, fragile, and
hard to handle.

Remember our consultant that recommended the staggered bypass
decoupling caps to stagger the "resonances" in the caps? I asked him
how come the AC coupling caps on bias tees, which are just plain ol
crappy X7R 100 nano jobs, are good enough to pass DC-6 gigabits?

I'm still waiting for an answer as to why we aren't putting in 5
different cap values for that bias tee cap...

J

#### JosephKK

Jan 1, 1970
0
I see. I will try to set up an "appointment" with our high-speed test
instruments.

Thanks. The problem is that I don't know (yet) how low the frequency
is on the "DC" end of the coil.
It's supposed to be DC that changes once in a while to another DC
value.
But it's entirely possible someone will want to modulate the "DC" end
with sine waves of varying frequencies.
Could be anything from the audio band upwards.
Either way I'm already going into full paranoia mode by placing them
all at 90 degrees of each other. Of course every other one will be in-
line but hopefully the distance will be great enough.
The board is quite small and these coils will go all the way to the
edges. There is no plan for an enclosure yet.

What do you mean every other one will be in the same direction? It
should be every third one; up-down, left-right, front-back.

J
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