On Thu, 4 Mar 1999, Paul B. Webster VK2BZC wrote:

> Walter Markiw wrote:
>
> >  I need to build a coil to provide an inductance in the range of 0,001
> > to 0,1 Henrys.
> > It is for a frequency filter I am planning to build.
>
>   OK, enough is enough.  They've wagged you *mercilessly* on this one.
>
>   If you *really* want to build a *fixed* or *tapped* coil in this
> range, that is do-able, but you would usually use a laminated (soft)
> iron core to achieve the value.  An air-cored coil is rather impractical
> and a variable one, quite impractical, at least over the range you
> specified.
>
>   Audio (it is audio, isn't it?) frequency filters use active "gyrator"
> circuits to synthesise inductance with op-amps and capacitors.  That's
> how variable inductance is emulated.

Yeah! 'Gyrating Impedance Converters' or GICs have less component
sensitivy than other opamp topologies such as the Sallen-Key. The down
side though is that they require 2 opamps per complex pole (where as S-K
requires only one). Another benefit is that GIC's can be designed such
that all of the capacitors are the same value - the resistors control the
pole (or possibly zero) locations.

I guess if you used a Pic to vary the inductance you'd have a PIC-GIC :)