Even with totally digital frequency generation (we had it on receivers designed in the 60's) the very selective RF amplifier section of a receiver can be a bit complicated.And the tuning of the RF amplifier tuned circuits is a bit complicated.
Well in a simple way a tuning circuit. The ideal would be a variable band-pass but as I wrote at the beginning I was trying to understand if it was possible to use in circuit with varactor and potentiometer instead of a variable capacitor.I think it time to tell us what you are trying to do if you want more specific help.
Yeah variable inductors are hard to find, except for one thing, almost every inductor is inherently variable even without any modification. The best would be those with magnetically active cores. Since the permeability is the slope of the BH curve and the curve tapers off to the horizontal closer to saturation, if you vary H you vary the inductance, and if you vary the DC bias current you vary H. So if you vary the DC bias you vary the inductance. For a small signal AC test signal, as you increase the DC bias current, the inductance may go up a little but then after some point it will start to go down and continue to decrease as more DC current is applied. Because there is always a DC current, you would have to be able to use it in a configuration where coupling capacitors are ok.
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by Duane Benson
by Robert Keim