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.

 

Sure, you can create dynamic negative reactance (negative inductance and a capacitance) with an active circuit and coupled components to create a variable capacitor and/or inductor. It's usually a lot more practical to use traditional variable components unless there is some special capability needed.


https://en.wikipedia.org/wiki/Foster's_reactance_theorem#Non-Foster_networks

https://www.nature.com/articles/s41598-021-86973-x

 

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.

 

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.

 

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

 

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.

OK, right. I was thinking of a varistor.

 

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

And the tuning of the RF amplifier tuned circuits is a bit complicated.

 

@nsaspook seem a bit complicate. I need something simpler if possible but thanks.

@Audioguru again Even if varactors are matched pairs?
I know there are common cathode varactors. Maybe it is for this reason?

 

And the tuning of the RF amplifier tuned circuits is a bit complicated.

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.
http://www.navy-radio.com/rcvrs/r1051.htm
http://www.schmitzplace.com/R-1051.htm

 

@nsaspook seem a bit complicate. I need something simpler if possible but thanks.

Simple wasn't in your original requirement list. That's why actual variable components are still used.

 

@nsaspook i agree. I haven't problem to use a variable capacitor but a variable inductors with knob are harder to find.

 

I think it time to tell us what you are trying to do if you want more specific help.

Bob

 

There is an old device called the "Increductor,"
https://worldradiohistory.com/hd2/I...s/Radio-Electronics-1957-04-OCR-Page-0039.pdf
https://worldradiohistory.com/hd2/I...-IDX/IDX/60s/60/Pop-1960-07-OCR-Page-0084.pdf

 

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.

 

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.

 

Then there are these things. The one below is a TRF receiver.


Source: https://www.aavso.org/improved-gyrator-tuned-vlf-receiver

 

@DickCappels thanks.

R5 pot for tuning and R8 pot for gain right?

 

@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.

 

You wont find them because your looking under the wrong name. You want to look for "roller inductor"/ https://www.ebay.com/itm/264689764304

 

@KeepItSimpleStupid

what i want to understand is something to place on pcb pretty small because is only for receiving but must be a knob for tuning.