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LF: Re: Tuning LF inductors and inexpensive 10000pF variable capacitors

To: [email protected]
Subject: LF: Re: Tuning LF inductors and inexpensive 10000pF variable capacitors
From: "James Moritz" <[email protected]>
Date: Sat, 29 Nov 2003 13:18:40 -0000
References: <[email protected]>
Reply-to: [email protected]
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Dear Jan-Martin, LF Group,
>somewhat puzzled that it hasn't been any discussions about the technique
>mentioned by G3VA Pat in last Radcom (11/2003).
The idea of using the drain-gate capacitance of biased-off power MOSFETs as high power varactors is interesting. I attended the conference at University of Bath where the paper mentioned in Technical Topics was presented, and WA1WLW's talk was very good. (There seemed to be a lot of amateurs about - a number of other subscribers to this reflector were there; I presented a paper about LF antenna measurements). The Q of these MOSFET-varactors was about 30-100 at HF. In the paper, they were used as tunable elements in a HF PA matching network with a fairly low loaded Q, so a high Q tuning element was not essential. For LF transmitting antenna matching, a rather higher Q would be desirable.  For a small-signal application (like a preselector or a VCO), it should be possible to use lower voltage power MOSFETs. As an experiment, I did some measurements with a back-back pair of STW34NB20 MOSFETs (200V, 34A). With a bias voltage of 5 - 200V, capacitance range was 750p - 140p, with most of the variation at the lower voltage end, as you would expect with varicaps. Rough measurements of Q (using a 3.6mH pot core inductor over a frequency range of 97kHz - 230kHz) gave values between about 500 and 1500, with higher values for higher capacitances. So this is obviously a useable technique for LF - the high tuning voltage is a bit awkward to produce, but would result in good linearity. Looking at the data sheets, similar capacitance variation can be achieved with lower tuning voltages by using MOSFETs with lower breakdown Vdss. I guess higher BVdss MOSFETs could be used for LF antenna tuning, especially if used as a "fine tuning" device making up a fraction of the total capacitance.
The idea of applying a magnetic bias to a ferrite core to vary the incremental permeability and so produce a tunable inductor has been around for quite a while - apart from the previous articles cited in Technical Topics. In the old RSGB "LF experimenter's sourcebook" there is an article about a tunable LF converter using mechanically moveable permanent magnets to bias a toroid core, which is apparently a re-print of an article from "Ham Radio" by OH2KT from 1974. I guess the problem with such a device is finding a ferrite material that gives at the same time a reasonably high Q, and a usefully large swing of inductance. To get a large inductance swing requires a ferrite with a high initial permeability - such ferrites unfortunately tend also to have high losses . Lower permeability ferrite has higher Q, but would need higher DC control current to get the same bias magnetic flux, and also would give less change in inductance. An air-gapped high permeability core like a pot core wouldn't really work because the reluctance is mostly determined by the air gap rather than the ferrite. The Technical Topics article is apparently aimed at MF/HF, but I think the idea would work better at VLF/LF, because high-permeability ferrites have lower loss in that range.
Cheers, Jim Moritz
73 de M0BMU
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