Varactor is to a capacitor like {insert unobtanium component} is to an inductor.

[cdev]

What is it I want? Something that changes inductance in response to some control signal or modifiable condition.

I want to make a tracking analog RF filter that can be maintained in optimum tune programmatically.

[ruffy91]

Transductor:
An inductor you drive into saturation with a dc current in a second coil so that the inductivity changes.

Edit: Also known as magnetic amplifier or mag amp when combined with a transformator.

[AF6LJ]

What is it I want? Something that changes inductance in response to some control signal or modifiable condition.

I want to make a tracking analog RF filter that can be maintained in optimum tune programmatically.

A slug tuned coil.

Such arrangements have been built, however they are mechanically linked and can become complex.

[AF6LJ]

Transductor:
An inductor you drive into saturation with a dc current in a second coil so that the inductivity changes.

Edit: Also known as magnetic amplifier or mag amp when combined with a transformator.

I thought of this but never saw anything like this implemented higher than power frequencies or analogue TV sweep frequencies.

[T3sl4co1l]

They were around for a fairly short time, I think.

Frequency range on a transductor is fairly modest, simply because ferrites and powdered irons don't go all that high: maybe into the low 100s of MHz.  And it's hard to saturate powdered iron for that frequency range, because its initial mu is so low to begin with.

The method works just fine, though.

A lot of classic Sony power supplies used resonant converters, where the primary side is a self-excited oscillator, tuned with a saturable reactor and therefore controlling output power.  (No controller chip needed whatsoever, just a TL431 on the secondary side.)


http://seventransistorlabs.com/Monitor/

Tim

[babysitter]

Also, Rohde & Schwarz used that method to sweep oscillators in some famous old boat anchors. Like SWOB 2.

RF coil with ferrite core, and a additional coil driven by mains hum (literally!). Core saturated more or less, inductance changed, oscillator was tuned up and down.

(Only the UHF oscillator was not sweeped this way. It had a little loudspeaker-like device to tune the cavity.)

I used this technique about once in my career, for a RIP simulator (belts that are worn over chest and abdomen to measure breathing movements which change their inductance depending on chest/abdomen diameter.)

[chris_leyson]

I've seen VCO designs whereby you add an additional DC control winding to the tuning inductor. Increasing the DC "control" current pushes the inductors operating point along its B/H loop causing the permeability to drop and the frequency to increase. A quick search didn't turn up much but I found this, http://www.techlib.com/files/crvco.pdf, which is a different take on the controlled permeability oscillator.

[PA0PBZ]

Gyrator, simulated inductance using an opamp:



https://en.wikipedia.org/wiki/Gyrator

Not sure how practical that would be with RF though...

[voltz]

+1 Gyrator assuming you can get the frequency response you need high enough. Some interesting info here including a tuned circuit:

http://sound.westhost.com/articles/gyrator-filters.htm

These tend to be used for high inductance simulation in the region of Henries. But could be modified with an RF transistor rather than op-amp to get what you need. And changing the fixed cap for a DC biased varactor diode would give you DC control. Might take a little bit of work but should be possible.

[retrolefty]

What is it I want? Something that changes inductance in response to some control signal or modifiable condition.

I want to make a tracking analog RF filter that can be maintained in optimum tune programmatically.

A slug tuned coil.

Such arrangements have been built, however they are mechanically linked and can become complex.

 Ah yes, Drake made some outstanding permeability tuned VFOs in their day. Costly today in size and mechanical costs, but wow what a great human interface to careful tuning. 

[AF6LJ]

What is it I want? Something that changes inductance in response to some control signal or modifiable condition.

I want to make a tracking analog RF filter that can be maintained in optimum tune programmatically.

A slug tuned coil.

Such arrangements have been built, however they are mechanically linked and can become complex.

 Ah yes, Drake made some outstanding permeability tuned VFOs in their day. Costly today in size and mechanical costs, but wow what a great human interface to careful tuning. 

Don't forget the passband tuners in the 2-B and R-4 series receivers, not to mention the permeability tuned preselectors in those and many of the Japanese radios.

[retrolefty]

What is it I want? Something that changes inductance in response to some control signal or modifiable condition.

I want to make a tracking analog RF filter that can be maintained in optimum tune programmatically.

A slug tuned coil.

Such arrangements have been built, however they are mechanically linked and can become complex.

 Ah yes, Drake made some outstanding permeability tuned VFOs in their day. Costly today in size and mechanical costs, but wow what a great human interface to careful tuning. 

Don't forget the passband tuners in the 2-B and R-4 series receivers, not to mention the permeability tuned preselectors in those and many of the Japanese radios.

 FB. I got a R4A hiding in closet somewhere. Because it's all LC tuned as you said it just sounds clearer and natural to me then crystal filtered rigs (which I think they went to with the R4C). Color me nostalgic. 

[rfeecs]

For higher frequency tracking filters, YIGs were often used:

http://www.microlambdawireless.com/uploads/files//pdfs/ytfdefinitions2.pdf

But not really a home made proposition.

[N2IXK]

Many older RF sweep generators used a device called an "Increductor" which was a variable inductance adjusted by varying a DC current through a control winding. See US patent #2837648 for an example.

[T3sl4co1l]

Gyrator, simulated inductance using an opamp:

https://en.wikipedia.org/wiki/Gyrator

Not sure how practical that would be with RF though...

Back in the day, the term was "reactance tube":



(ARRL Handbook 1971 I think)

Supposedly, the modulation range is quite narrow, a few percent at most.

The tube acts like gain controlled Miller effect, so it can be used wherever a variable reactance (most often C I think, but L could be made as well, in which case the effect would most likely be as a gyrator) is needed.  Attached to the oscillator tank, this allows NBFM; to a later tuned circuit, a little PM (you'd need several cascaded stages to get enough variable phase shift for PM of sufficient angle).

And obviously, this all works even better with transistors, there are just better ways of skinning that cat..

Tim

[PA0PBZ]

Supposedly, the modulation range is quite narrow, a few percent at most.

And there is your problem if you want to use it for tuning purposes. Also it is nice if you want to 'shake the carrier around a bit" but I don't think it can be used to have a fixed offset.

[cdev]

I'm kind of curious about Yaesu's pre-selectors, which use slug tuned, loosely coupled inductors (and a gain stage) I have nasty interference on HF, probably from some rusty item in my immediate proximity (a big fence, for example) or something like that, and its driving me batty. I live near some high powered broadcasting towers.

[AF6LJ]

If the noise is actually coming from the fence (diode action) Nothing like this will help short of tearing down the fence. The Yaesu preselectors are good for strong adjacent channel signals that may be overloading your receiver. 

I would explore the possibility your receiver itself is being overloaded by the strong BC signals. It takes a lot of signal to cause a fence to rectify RF. A BC band reject filter in your receive path may solve the problem and would be a lot cheaper (and less sexy)  than the Yaesu fpreselectors.

[babysitter]

I once had a RFT Teltow 215C which suffered a lot being located less than 6 km from the Radio station in Heusweiler. After removal of a pair of diodes (clipping rx input voltage) it was not completely gone, but at least much better.

A tuned notch filter, as i only had a single frequency to fight, helped even more.

BR

Hendi