Substitute for variable capacitors and variable inductors for tuning stage

Thread Starter

Zeno53

Joined Sep 23, 2020
60
Good evening everyone,

with what can I replace a variable capacitor and a variable inductor with more modern components? For the capacitor there are varactors but is there more?

Thank you.
 
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nsaspook

Joined Aug 27, 2009
8,394

MisterBill2

Joined Jan 23, 2018
8,808
A varactor will not be useful in an L-C tuning circuit. It would need a varicap, tuning diode. And the concept that the basic functions are obsolete is poor reasoning. All of the active components needed to do alternative tuning methods develop noise voltages, and noise is usually not welcome.
 

Audioguru again

Joined Oct 21, 2019
3,208
A varactor diode is a varicap tuning diode.
A radio has at least two variable capacitors for tuning and their tuning range must match. It is difficult to align a good match with varactor diodes.
 

BobTPH

Joined Jun 5, 2013
3,333
Modern radios are tuned digitally using a crystal and divider to generate a base frequency, and. PLL to multiply it to the desired frequency.

Bob
 

Thread Starter

Zeno53

Joined Sep 23, 2020
60
I think it time to tell us what you are trying to do if you want more specific help.

Bob
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.
 

Audioguru again

Joined Oct 21, 2019
3,208
For each radio band, a superhet radio has one or two variable LC tuned circuits at thee radio frequency plus another variable LC at the local oscillator frequency. They are aligned to track each other.
 

MrAl

Joined Jun 17, 2014
8,262
@nsaspook i agree. I haven't problem to use a variable capacitor but a variable inductors with knob are harder to find.
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.
Air core inductors may vary very little.
The effect with magnetically active core inductors will vary widely with different inductors you'd have to do some experiments to find out what would work in your application. Obviously you dont want to have to apply too much current. Theoretically the inductance can be varied from the value advertised for the product to nearly zero. How much current it takes to get that wide range would depend a lot on the physical size of the core as well as the core characteristics.

For a physical adjustment you can push a magnetically active core into an air core inductor to change the inductance. Many transistor radios used these things not sure if they do anymore though. They had a screw slot at the top you could turn to tune it. The core was in the shape of a screw so when you turn it it would go deeper in or farther out thus changing the inductance.
 
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