I passed this to one of the RF gurus here at Axonn, and he provided the
following practical advice on the subject.

Keep in mind that this technique is definitely for high-volume production.
You can save US$.25 per inductor or so, so it can add up to a couple of
dollars in parts cost per board for a radio.  Bob says to expect it to take
at least 3 or 4 turns of copper to get it right, so you will burn more in
prototypes and engineering time than you will save unless your production
quantities are pretty high.

The technique works well in the 1GHz region, less well at lower frequencies
because the inductors needed are too big.  At really high frequencies like
2GHz, they start acting more and more like waveguides and striplines, and
that's an entirely different set of math to deal with.

-----Original Message-----
From: Bob Davis
Sent: Thursday, November 30, 2000 12:30 PM
To: Don Hyde
Subject: RE: [EE]: Coil Imprinted on PCB


Hi Don,

There are a few simulators that may do the trick, unfortuneately most
cost heavy bucks.

What the fellow needs is a modelling and simulation package that handles
spiral and/or microstrip inductors on a PCB.

There are some free packages that may do the trick here:

http://sss-mag.com/

Ansoft also is allowing a free download of their harmonica RF simulator
on their www sight.  I believe it's
http://www.ansoft.com/about/academics/index.cfm
the serenade and ensemble packages are awesome if he really wants to get his
hands dirty.

I am sure I have seen some java based calculators for such things on the
internet too.

Here's a quick and dirty, if you have a 15 mil wide line, on FR4 (G10), with
a board thickness
of 62 mils, and a groundplane on the bottom, the line is rougly 17
nH/inch....  keep the ground on
the top of the board at least 3X the board thickness away from the line, and
try to keep the line
much shorter than 1/4 of the wavelength of the frequency of interest
(effective wavelenght is
1/(SQRT(Dielectric Constant)) )  Dielectric for FR4 varies, but 5.0 is a
good SWAG.

Hope this helps, tell him to use http://www.metacrawler.com and search on
spiral inductor.

'73
bob


-----Original Message-----
From: Don Hyde
Sent: Thursday, November 30, 2000 10:27 AM
To: Bob Davis
Subject: FW: [EE]: Coil Imprinted on PCB


This question was posted to the PIC mailing list.  (He made a typo, he
actually meant nH, not uH).  I remember that you used some software package
that you thought reduced the amount of cutting and trying.  What was it?
Would you recommend it?

-----Original Message-----
From: Werner Soekoe [mailto:WernerS@FSL.GOV.ZA]
Sent: Thursday, November 30, 2000 1:07 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: Coil Imprinted on PCB


Hi

I'm designing an application that requires a few small coils with low values
1uH to 150uH. I've seen a few designs where the coil is part of the layout
on the PCB, and was wondering if anyone has any formulas for these coils. Or
is it a try your luck and see what value pops out thing?

Thanks!

Friendly Regards,
Werner Soekoe
WernerS@fsl.gov.za

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