The inductor can be tapped and the taps switched by pin diodes.. Made a n oscillator (for a synthesiser) years ago with varicaps to interpolate the tuning between tapped steps. The taps were switched when the varicap control voltage was above or below a window value. An alternative to a multiloop pll synthesiser for wide band., cant recall ssb noise results or total tuning range.
W> >
> I'm looking for a variable inductor (approx 10mm size) that
> > has a range of at least 2:1. This is for use in a 50-100MHz
> > tuned circuit, and I've calculated that Imin=185nH, and
> > Imax=366nH. Does anyone know if such a range is possible?
>
> For a 2:1 range in resonant tuned frequency you'd need
> a 4:1 range in variable inductance. That's more than
> most adjustable pot-core inductors, etc., provide. A
> quick review of various parts shows about 2:1 maximum.
>
> There are several ways to create variable inductors.
>
> One is to make an air coil and adjust a tuning slug in
> and out of the coil. This may be your best approach.
> Coilcraft has a nice selection of coils with tuning
> slugs, but most, like the Unicoil 10mm series, can't do
> a 4:1 range.
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> A second way is the classic variometer. Here the coil
> is split into two parts, wired in series, where one of
> the coils is capable of rotating within the other. If
> they are aligned the inductance is nearly doubled over
> a single coil. But, if the orientation of one coil is
> rotated with respect to the other the inductance will
> be reduced, and if they're reversed it can be very low.
>
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> If the coils are wound in spherical shapes, it's easier
> to nest one inside the other and orient it as desired.
>
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> It's easy to get > 10:1 ratios by this technique.
>
> You can also slide one coil in and out of the other.
>
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> A third method for obtaining wide variable inductance is
> with a rolling slider bar making a variable winding tap.
>
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> The links and pictures I have provided are for working
> at low frequencies with large inductors, but I imagine
> most of the concepts can be miniaturized.
>
> An interesting pot-core tuning technique that may be
> useful for you is to run a DC current through the coil
> to saturate the core, dropping its permeability. This
> scheme could probably be used to exceed a 4:1 ratio.
>
> An issue to evaluate is your required tuning stability.
>
>
> --
> Thanks,
> - Win