Re: [SI-LIST] : Transmission line inductance

[email protected]
Fri, 26 Mar 1999 13:55:32 EST


I see a basic problem with your request for a printed inductor.

1. You should not use a transmission line structure (i.e., linear PCB trace)
as the effective inductance as seen from the chip power return lead will vary
as a function of frequency. That is, the ground plane on the PCB presents a
short to the end of your T-line. At any frequency up to 1/4 wavelength (or
90-degrees of the input frequency), the T-line looks inductive; however, the
magnitude of the inductor varies like a tangent function (zero at 0-degrees,
infinity at 90-degrees). As the frequency goes higher, it looks capacitive.
Then, beyond 180-degrees, it again looks inductive,...and so on. Read up on
Smith charts for more detail on the effects.

2. Use of a printed spiral inductor can be acceptable, but it will generate
fields that may not be contained by your enclosure (which is unknown to me).
Three-circle inductors are pretty efficient as regards inductance-to-real
estate ratio; however, the size of the inductor may still be prohibitive.

I recommend you check out TDK SMT inductors (or equivalent). Or do an
internet search for SMT inductors.

Mike Conn
Owner/Principal Consultant
Mikon Consulting

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