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Old 15th May 2022, 2:21 am   #35
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: 6-gang FM stereo tuner heads

The high impedance inputs and outputs of a dual gate MOSFET have a strong influence on the sorts of resonator Q's you feel steered into using with them, and the antenna connection is the only low impedance around. So for a front end with 4 tuned coils and variable/varactor capacitors to tune them, although you can allocate the coils before and after an RF amplifier as wanted, the pattern of the bandwidths tends to be foisted on you.

More recently, with dual gate MOSFETs rapidly becoming obsolete and NRND, I was looking at alternatives for an aircraft transceiver I'd designed. This meant that the RF amplifier would need to become an MMIC low impedance sort. With two coils before it and two coils after it, a low-Z mixer and a low-Z antenna, then all four coils would want tappings. This then doesn't sit well with a management edict to use off-the peg VHF inductors. A pseudo-tap can be arranged by having a 1t inductor mounted at the earthy end of a longer one, with the common connection becoming the tap. Instead of messing with tiny coupling capacitors (or potted down ones) then just spacing the second resonator parallel to and a controlled distance away in the same screening compartment will effect field coupling.

It's all a set of nested compromises. There is no single 'right' solution. Optimisation, whether mathematical or empirical (Kanban) gets stuck on local optima and can't be relied upon to find the best one.

You don't value the convenient impedances of the MOSFET until you lose them!

David
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