[AMPS] Automatic tuning of a VHF amp

[Dave Kirkby]

Geurts, Gerard wrote:

> In theory it is easy. If the triode is in grounded grid, the tube has
> a
> purely resistive load when the cathode and plate voltages are exactly
> 180 degrees out of phase. This takes care of output capacitance etc.
> as
> well. You need a way to detect the phase difference between the
> cathode
> and the plate voltages and use this to drive a servo controlling
> 'tune'.
>
> Problem is that you now measure one parameter (phase difference) and
> need to control two variables ('tune' and 'load'), so you will need a
> second parameter you can measure to control the 'load'. I guess output
>
> power (or RF voltage at the output) could be used. It might be
> possible
> to do something with grid current as well.
>
> This is all theoretical. I have never tried to implement this in real
> life...........
> I would go for the eeprom option.

> Gerard, MoAIU / PA3DQW / AA3ES

Thanks for your comments on automatic tuning. I assume a computer could
be programed to do it from just the RF output, grid current and anode
current - after all, thats all we use. However, I did not intend using
computer control. I did at one time consider using an old 386 computer
motherboard to look after things, but gave up the idea, for fear of RF
noise, computer crashes that might be expensive. One gets the feeling it
will be more complex than necessary, even though I have the computer and
necessary A/D cards sitting around doing nothing useful.

I had in mind a simple feedback loop, but perhaps that is not possible
in practice.
--
Dave Kirkby G8WRB
Department of Medical Physics,
University College London,
11-20 Capper Street,
London,
WC1E 6JA.

Tel: 0171 209 6406 (International +44 171 209 6406)
Fax: 0171 209 6269 (International +44 171 209 6269)



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