Off Topic: is there such a thing as an integrated circut chip as variable inductor?

[john doe]

Posted on December 12, 2017 at 05:06

I'm trying to learn antenna matching and I'm as much a 'do the math' guy as I am 'hack it out through trial and error' kind of person.  Usually it's both, when I'm not getting it.  And I'm not getting tank circuits right and I dont have a large set of inductors to play with.  Is there a chip that is a variable inductor with a digital interface?  I should have just built the oatmeal container radio and moved on to motor controls.

:)

[John F.]

Posted on December 13, 2017 at 12:58

You may be thinking of a gyrator. Google 'gyrator circuit' ... but don't expect it to work at RF frequencies!

[waclawek.jan]

Posted on December 13, 2017 at 13:32

There are motorized tunable inductors around, but fine matching is usually achieved with electrically tunable capacitors.

You did not specify what frequency and power ranges are you talking about. For starter you might want to have a look at

http://www.st.com/en/emi-filtering-and-signal-conditioning/smart-antenna-tuning.html

JW

[S.Ma]

Posted on December 13, 2017 at 13:36

For antenna tuning, there is this component which is the tunable AC capacitor: (instead of inductor)

http://www.st.com/en/emi-filtering-and-signal-conditioning/rf-tunable-capacitors.html

[AvaTar]

Posted on December 13, 2017 at 14:53

I would have guessed the VHF range, of about 100MHz.

The simulation of adjustable inductances by opamp is not new, but mostly for the audio range.

Electronically tunable capacitors are usually implemented via varactor diode, with a (high) bias voltage commutes the capacity.

[waclawek.jan]

Posted on December 13, 2017 at 15:10

Electronically tunable capacitors are usually implemented via varactor diode, with a (high) bias voltage commutes the capacity.

Yes that's the usual way; but the STPTICs are different, based on ferroelectric effect. Another interesting option are

http://www.psemi.com/products/digitally-tunable-capacitors-dtc

capacitors, basically a bank of capacitors in series with switches, integrated with decoding logic.

There are then also motorized capacitors, too; it's not that unusual to use motorized vacuum variable capacitors for certain high-powered applications.

JW

[AvaTar]

Posted on December 13, 2017 at 15:28

Motorized caps are probably a bit oversized for a radio applications ...

And yes, varactors are around for a few decades now.

These varactors, together with NVM, allowed for an electronic radio/TV station memory - no more turning complex and skidding cable-operated varicaps ...

[Oliver Sedlacek]

Posted on December 14, 2017 at 11:15

RF circuits are usually electronically tuned with varactors, capacitors that vary in value with DC bias. You could achieve a similar effect with saturable inductors where you use a DC bias winding to adjust the core permeativity using the non-linear behaviour near saturation. Having said that, I doubt it's easy at RF frequencies.

I also think you really are going to be better off with a pre-approved module (including antenna) as the cost of certifying your own module will probably be $50,000 to $100,000.

[waclawek.jan]

Posted on December 14, 2017 at 11:42

using the non-linear behaviour near saturation. Having said that, I doubt it's easy at RF frequencies.

Huh, easy or not, I wouldn't want to do that in RF, generating a slew of harmonics. which in turn need then to be suppressed somehow....

JW

[Oliver Sedlacek]

Posted on December 15, 2017 at 10:31

I offer it more out of academic interest than as a practical suggestion