how to avoid mutual coupling between inductors

P

Philip Newman

Guest
Hi,

I've got a high pass filter working at about 500MHz using surface
mount passive components. Unfortunately, this filter isn't working as
I wish.

I've debugged the filter by observing the delay S22 coefficient for
each resonator and coupling capacitors and comparing it the simulated
response, making changes to the circuit to match the simulation. I've
spotted my problem, and there is coupling between two inductors,
causing my filter to malfunction.

How can I reduce this coupling effect?

I've made the inductors orthogonal, increased grounding and sheilded
them to little or no effect. the measured coupling is about k = 0.28.

They are coilcraft inductors, 1008HQ series, Q approx. 65 values 39nH
and 56nH.

Any ideas on how to reduce coupling?

Regards,

Phil Newman
 
I can't move them further apart, as there are other components on the
board in the way. I would have to do a complete redesign of the board,
which may be necessary in the end!
not sure what you mean by criss-cross methods?

Phil
 
Philip Newman wrote:
Hi,

I've got a high pass filter working at about 500MHz using surface
mount passive components. Unfortunately, this filter isn't working as
I wish.

I've debugged the filter by observing the delay S22 coefficient for
each resonator and coupling capacitors and comparing it the simulated
response, making changes to the circuit to match the simulation. I've
spotted my problem, and there is coupling between two inductors,
causing my filter to malfunction.

How can I reduce this coupling effect?

I've made the inductors orthogonal, increased grounding and sheilded
them to little or no effect. the measured coupling is about k = 0.28.

They are coilcraft inductors, 1008HQ series, Q approx. 65 values 39nH
and 56nH.

Any ideas on how to reduce coupling?

Regards,

Phil Newman
Click to expand...
Can you post to a web page, to alt.binaries.schematics.electronic or
email a photo of your layout? Are these inductors over a ground
plane?

I must say that your coupling is amazing. If I had to place two
surface mount inductors and there was a requirement that they have a K
= 0.28, I would be sweating to make sure that happened. I suspect
there is some other explanation for the performance of your filter.

--
John Popelish
 
they are about 2mm away from each other. it is a 9th order elliptic
filter. i can't get a photo i'm afraid. i've doctored the circuit
quite a bit so the layout I have is out of date. had to fix the circuit
so that the inductors were orthogonal to each other.

not really sure what else could be causing the filter to fail then. if
i'm looking at the delay S11 plot, there is a peak for the resonance of
1 resonant pair, and when I add the other resonant in, there is a
second peak, which the simulation says shouldn't be there, because both
resonances are very close together one hides the other. the main
resonance is at 390MHz (correct) where the odd one is at 500MHz.
 
Just a side question:

If I have inductors in the same orientation (ie facing same way) but in
different horizontal plane (i.e. one above the other, looking down on
the circuit) will it still couple in the same way if they were just
next to each other?
 
Just a side question:

If I have inductors in the same orientation (ie facing same way) but in
different horizontal plane (i.e. one above the other, looking down on
the circuit) will it still couple in the same way if they were just
next to each other?

I may have come up with a way to move the inductor right out of the
way, but want to make sure that it won't still couple.

Cheers,
 
I read in sci.electronics.design that Phil Newman
<phillenium2002@hotmail.com> wrote (in <1105375816.010615.198930@c13g200
0cwb.googlegroups.com>) about 'how to avoid mutual coupling between
inductors', on Mon, 10 Jan 2005:
Just a side question:

If I have inductors in the same orientation (ie facing same way) but in
different horizontal plane (i.e. one above the other, looking down on
the circuit) will it still couple in the same way if they were just
next to each other?

Yes, assuming they are solenoidal, but the distance between them is the
Click to expand...
'slant' distance, in the (non-horizontal) plane that contains both
inductors.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
"Philip Newman" <phillenium2002@hotmail.com> wrote in message
news:50bff1fe.0501100651.ee0c557@posting.google.com...
Hi,
Hi.
Click to expand...

I've got a high pass filter working at about 500MHz using surface
mount passive components. Unfortunately, this filter isn't working as
I wish.

I've debugged the filter by observing the delay S22 coefficient for
each resonator and coupling capacitors and comparing it the simulated
response, making changes to the circuit to match the simulation. I've
spotted my problem, and there is coupling between two inductors,
causing my filter to malfunction.

How can I reduce this coupling effect?

I've made the inductors orthogonal, increased grounding and sheilded
them to little or no effect. the measured coupling is about k = 0.28.
Click to expand...
That little or no effect, together with that value of k after
such efforts, makes me skeptical of your diagnosis. If
you have unbroken ground plane surrounding the parts,
that alone should get the coupling lower than you indicate.

So, before getting too excited about magnetic coupling, I
would revisit the method you used to deduce that number.

They are coilcraft inductors, 1008HQ series, Q approx. 65 values 39nH
and 56nH.
Click to expand...
I presume these are air core inductors from the picture
published by Coilcraft.

Any ideas on how to reduce coupling?
Click to expand...
If the coil axes are coplanar with the PCB, and you rotate one coil
around another (in the PCB plane) while maintaining their relative
orientations, you can observe 4 coupling nulls per rotation. [1]
When the coil axes are orthogonal, those nulls occur when either
part falls on the axis of the other. If your coupling is truly as high as
you state, you likely have got the parts placed well between those
nulls. So, one approach would be to do some lab work and find
better placements. For example, if the parts are placed with their
axes parallel, the nulls are about midway between the coil axis and
its perpendicular.

[1. This holds for normally wound inductors, formed as a
single solenoid. More complex winding patterns can be
used to create more nulls and to cause the field to fall off
faster with distance.]

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
 
"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote in message
news:uWzEd.14$hi6.823@news.uswest.net...
"Philip Newman" <phillenium2002@hotmail.com> wrote in message
news:50bff1fe.0501100651.ee0c557@posting.google.com...
Hi,
Hi.

I've got a high pass filter working at about 500MHz using surface
mount passive components. Unfortunately, this filter isn't working as
I wish.

I've debugged the filter by observing the delay S22 coefficient for
each resonator and coupling capacitors and comparing it the simulated
response, making changes to the circuit to match the simulation. I've
spotted my problem, and there is coupling between two inductors,
causing my filter to malfunction.

How can I reduce this coupling effect?

I've made the inductors orthogonal, increased grounding and sheilded
them to little or no effect. the measured coupling is about k = 0.28.

That little or no effect, together with that value of k after
such efforts, makes me skeptical of your diagnosis. If
you have unbroken ground plane surrounding the parts,
that alone should get the coupling lower than you indicate.

So, before getting too excited about magnetic coupling, I
would revisit the method you used to deduce that number.

They are coilcraft inductors, 1008HQ series, Q approx. 65 values 39nH
and 56nH.

I presume these are air core inductors from the picture
published by Coilcraft.

Any ideas on how to reduce coupling?

If the coil axes are coplanar with the PCB, and you rotate one coil
around another (in the PCB plane) while maintaining their relative
orientations, you can observe 4 coupling nulls per rotation. [1]
When the coil axes are orthogonal, those nulls occur when either
part falls on the axis of the other. If your coupling is truly as high as
you state, you likely have got the parts placed well between those
nulls. So, one approach would be to do some lab work and find
better placements. For example, if the parts are placed with their
axes parallel, the nulls are about midway between the coil axis and
its perpendicular.

[1. This holds for normally wound inductors, formed as a
single solenoid. More complex winding patterns can be
used to create more nulls and to cause the field to fall off
faster with distance.]

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.


As Larry says, I would revisit your measurements.
Click to expand...
One thing that caught me out once was to ignore the transmission
lines between components at these frequencies.

Regards
Ian
 
Philip Newman wrote:
Hi,

I've got a high pass filter working at about 500MHz using surface
mount passive components. Unfortunately, this filter isn't working as
I wish.

I've debugged the filter by observing the delay S22 coefficient for
each resonator and coupling capacitors and comparing it the simulated
response, making changes to the circuit to match the simulation. I've
spotted my problem, and there is coupling between two inductors,
causing my filter to malfunction.

How can I reduce this coupling effect?

I've made the inductors orthogonal, increased grounding and sheilded
them to little or no effect. the measured coupling is about k = 0.28.

They are coilcraft inductors, 1008HQ series, Q approx. 65 values 39nH
and 56nH.

Any ideas on how to reduce coupling?
Click to expand...
A nineth order elliptic calls for a compartemented build,
IMO.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
 
"Philip Newman" <phillenium2002@hotmail.com> wrote in message
news:50bff1fe.0501100651.ee0c557@posting.google.com...
Hi,

I've got a high pass filter working at about 500MHz using surface
mount passive components. Unfortunately, this filter isn't working as
I wish.
Click to expand...
At 500 MHz, they almost never do. Parasitic effects are too large to
ignore.
 
On 10 Jan 2005 06:51:10 -0800, phillenium2002@hotmail.com (Philip
Newman) wrote:

Hi,

I've got a high pass filter working at about 500MHz using surface
mount passive components. Unfortunately, this filter isn't working as
I wish.

I've debugged the filter by observing the delay S22 coefficient for
each resonator and coupling capacitors and comparing it the simulated
response, making changes to the circuit to match the simulation. I've
spotted my problem, and there is coupling between two inductors,
causing my filter to malfunction.

How can I reduce this coupling effect?

I've made the inductors orthogonal, increased grounding and sheilded
them to little or no effect. the measured coupling is about k = 0.28.

They are coilcraft inductors, 1008HQ series, Q approx. 65 values 39nH
and 56nH.

Any ideas on how to reduce coupling?
Click to expand...
Can you not make up your own toroids to those (admittedly small)
values? Micrometals make some very small doughnuts...
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
I don't really have the necessary experience or knowledge to do that!

I've moved the inductor, which didn't help to solve the problem, so
having tested magetic sheilding, minimising coupling by making
inductors orthogonal, moving them, increasing ground vias, the next
solution I think is to start removing some of the substrate (making
holes) between the tracks, and seeing if there is any propagation
between tracks. Er = 4.7 for the PCB which isn't that high, but I
think this is the next logical step. Everything else has been tested!!
I'll keep you updated!

Phil
 
On 11 Jan 2005 06:32:21 -0800, "Phil Newman"
<phillenium2002@hotmail.com> wrote:

I don't really have the necessary experience or knowledge to do that!
Click to expand...
I find that admission very surprising, given what you stated in your
original post.
It's really very simple to make up a couple of sub-minature toroids;
the formulae are widely available (probably just a handful of turns of
fine-gauge wire) and the nature of the doughnut form of inductor means
that there is no field radiated to speak of; it's self-contained so
you vastly lessen the risk of any mutual inductance arising.
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
What about the Q factor? these filters have quite a sharp transition
so need to have a fairly high Q, particulalry as these inductors
resonate at the first and second nulls.

I've only been fiddling about with these filters for a couple of months!
 
On 11 Jan 2005 07:34:01 -0800, "Phil Newman"
<phillenium2002@hotmail.com> wrote:

What about the Q factor? these filters have quite a sharp transition
so need to have a fairly high Q, particulalry as these inductors
resonate at the first and second nulls.

I've only been fiddling about with these filters for a couple of months!
Click to expand...
This should assist considerably:

http://www.micrometals.com/appnotes_index.html

--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
On 10 Jan 2005 06:51:10 -0800, phillenium2002@hotmail.com (Philip
Newman) wrote:

Hi,

I've got a high pass filter working at about 500MHz using surface
mount passive components. Unfortunately, this filter isn't working as
I wish.

I've debugged the filter by observing the delay S22 coefficient for
each resonator and coupling capacitors and comparing it the simulated
response, making changes to the circuit to match the simulation. I've
spotted my problem, and there is coupling between two inductors,
causing my filter to malfunction.

How can I reduce this coupling effect?

I've made the inductors orthogonal, increased grounding and sheilded
them to little or no effect. the measured coupling is about k = 0.28.
Click to expand...
Perhaps it's stating the obvious, but are you aware that you need both
orthogonality AND symmetry? Eg, the centre of one inductor on the axis
of the other (like a "T"). Looking at it another way, inductors
forming the legs of an "L" can be orthogonal, but still couple quite
well.

Tony (remove the "_" to reply by email)
 
OK, this may not come out as I wish, but this is how I had the
inductors...
_ _
| | _ | | _

so they were all in one line. they didn't share a common ground so
coupling was still an issue.

I've removed some of the substrate to no effect. now drilling the gaps
between the pads, to see if that makes a difference!
 
NO, the drawing didn't work (for me anyway)! I'm working through
google groups (the new beta version is very pretty, and much quicker!)
but still lacking in picture placements.

I'm thinking that if this coupling mechanism isn't inductive but
capacitative? I've magnetically sheileded it, attempted to reduce
coupling, reduced the effective permittivitty by drilling holes
(therefore lessening propagation and forcing the inductive path).
what to do....!
 
I've managed to get my four poles back on my filter, which is good.
Seem to have removed the coupling effects. Did this by laying the chip
capacitors on their sides (helps to increase Q) and putting parallel
capacitors right next to each other (helps to reduce inductance).
However, filter response still not good as there is a massive roll-off
and poles slightly out of place.

cheers for the help,
 
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