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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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