Tunable inductors

I was looking at the 4 work-arounds for HF reception and wondered if there is a tunable inductor. And sure enough such things exist. Could they be incorporated into the LimeSDR? Because it would be a nicer fix then removing parts or replacing parts.

But maybe I misunderstand what “tunable” means in this context. For example it could mean that it is tuned when they make it to your specifications.

I am thinking along the lines of “tunable” as-in field tunable. i.e. software tunable.

Tunable inductors have a moveable core that can be screwed in or out, very slightly adjusting the inductance. They aren’t electronically adjustable, not really repeatable, and don’t offer enough adjustment range to be worth it in any application outside of ultra-high Q filters.

Interesting that they are mechanical. I wonder if an electronic analogue (not analog) is on someone’s drawing board.

Tunable inductors are probably a good idea but they’re too big to fit on the lime board. They also come with a whole load of different ranges so a different variable inductor would be used for a different frequency range. The answer would be to design, build and tune external filter modules or try and buy one ready made.

Please check this solution ->

My local first aid HF solution, is from local OM Adam 9A4QV
and his LNA4HF with LPF cutting on 30 MHz
http://lna4hf.blogspot.hr/ /waiting for parcel :wink:


Usually this is done with a switched network if inductors, where the inductance increases 2x per step. So, if for instance, you have 8 inductors from 1uH to 128uH, that will give you all values from 1uH to 256uH in 1uH steps.

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this is also an interesting solution: HPSDR - Alex

73 de Sabine, OE1YVW

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How is this adjusted? With a knob?

Usually with a controller of some form, like a microprocessor, or maybe some logic arrray.
If I was building such a thing, I would use a microcontroller, like one of the Atmel devices, mainly because that’s what I am comfortable with.
The sort of network I am describing is often used in antenna auto-tuners, along with a switched bank of capacitors, and usually in an L-network configuration.
For your purposes, which I believe is configurable low or band-pass filtering, I think switched banks of filters, as described by some of the other posters might be a better idea.
There are a number of ways you could do this…

  1. roll your own.
  2. use one of the many switched BPF and LPF kits available.
  3. use a subsystem pulled from a transceiver. eBay is full of these.

My main reason for asking is for understanding the trade-offs and what’s possible and what’s not, etc. From a design standpoint.

I am only at the start of exploring RF hardware. The LimeSDR will keep me busy for a while as an excellent learning tool.

I will no doubt refer back to this thread in the future.