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Old 28th Jul 2022, 5:08 am   #162
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

Q as we know it today, used to be called the Circuit Magnification Factor. Marconi Instruments' Q meter was called their Circuit Magnification Meter, making it sound rather grand.

What this means is that the circulating power in a tank circuit of reasonable Q is significantly larger than the input and output power.

In FM tuners, it's usual to use dual-gate MOSFETs and to match these circuit impedances are run high. This means that the signal voltage in the tank is much larger than the signal voltage coming from the antenna.

Varactors are very much voltage controlled devices. Voltage sets their capacitance, so the increased voltage from unwanted large signals is made even worse, modulating the tank's tuned frequency by a greater amount, dramatically worsening the strengths of intermod products.

To avoid overload, most people would think to narrow the input tank to try to filter them out, but this means higher Q and greater voltage magnification of unwanted signals close to the wanted channel.

Ever get the feeling you just can't win?

The root cause is that the varactor diode needs to operate n a low-signal-voltage environment, and you want to use diodes needing plenty of tuning volts to scale the problem down. So the trend to low voltage varactors so tuning voltages can come from 5v supplies is very undesirable.

Varactors can be embedded in low impedance tanks and can produce reasonable Q and not suffer magnified signal voltages.... they get magnified signal currents instead, but that's not so devastating.

An R&D engineer at HP in California, Bart McJunkin, invented what he called a 'cartwheel oscillator'. It used a printed inductor looking like a very short very fat piece of coax. say 50mm in diameter, 1.6mm long! So it was a 2 inch ring of track on each side of the PCB with hundreds of stitches connecting them. This formed the outer of the coax, and the inner was a lily-pad of copper in the middle of each side, also connected together with loads of stitches. His oscillator circuit amplifier lived on the lily pad.

So this needs a tuning capacitor... The cartwheel has spokes, each spoke being a back-to-back pair of varactors. So there is a dozen varactor pairs, all working in parallel. Our tank has only tiny inductance and very large capacitance. It is very low-Z. Signal voltages are very low, yet the varactors run with tens of volts of DC bias to tune them.

Coupling into and out of this monster tank was by coupling loops also printed in the PCB. A thin ring at half diameter distributed tuning voltage to the common cathode points of the varactor pairs.

It was patented as an oscillator, where it gave an advantage in phase noise, but I spotted that it was a route to low intermod varactor tuned agile filters. It would be superb for an FM tuner and would haul back some of the advantage that mechanical variable capacitors still have. It could be taken to extremes and beat the mechanical capacitors (unless you use butterfly variable capacitors, you wind up with a sliding contact in your tank - everyone forgets this!)

So, yes, there is still fun to be had in FM tuner design, and designers still have unused tricks up their sleeves. But FM radio seems to be in decline. The BBC doesn't broadcast my sort of music, and I don't see any commercial return from designing something a bit different. I could do it for the hell of it, but I might as well pick an area with some profit in it to spend the effort on.

Anyway, so now you know why mechanical variable capacitors currently have advantages in FM tuners.

David
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