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variable inductor, variable capacitor

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Feb 12, 2006
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I am not quite sure weather I should put this question in this part or not, it’s partly digital and partly analogue, I am looking forward to design a variable inductor and a variable capacitor, i.e. I want to have a variable inductor and a variable capacitor whose values could be adjusted by the data from microcontroller. Price is not a factor in this design, but board space and relative simplicity is my main concern. I was originally thinking of using a number of separate fixed value inductors and capacitors which could be selected by the microcontroller via some analogue switches, but this will certainly take up a lot of board space and is virtually impractical. Any ideas? Any hints as to where I should start from?

for a variable capacitor (low capacitance) you can use a combination of varicap capacitive diode) and d/a converter?

What are the value ranges for both the caps and inductors?

For cap variable cap and switch bank of constant cap maybe a solution, but for inductor switch bank maybe the only solution.

Hi behranghm,

A circuit that accomplish that function is a reactance modulator (search for it). It basically consist of a transistor with two associated conponents (a C and a R). The value of the reactance is modulated by the transconductance of the transistor (i.e. by the current across it). The location of the C and the R determines whether the reactance is inductive or capacitive.


The problem you run into, if you're talking RF, is that inserting a switch
degrades Q by its series R, and pulls frequency by its shunt C. You can
design to this somewhat. But particularly with inductors switching the
coils is liable to make them worthless (you're talking ohms on the switch
vs tens of milliohms in an IC-scale inductor, less in a wire-wound).

If you have a limited quantity of frequencies of interest it may be
better to make the tanks the "element" and switch outside the core
LC loop.

If you don't care about compactness or solid state implementation
you could (as makers of high end load-pull test gear still do) use a
moving core inductor with stepper motor control, similarly you can
still find mechanical variable capacitors that could be fine-positioned.
Of course these are bulky, prone to microphonics and so on. But you
want to think about your "values" and what will be practical for you.

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