>----------
>From: Dave Kirkby[SMTP:davek@medphys.ucl.ac.uk]
>Sent: 10 November 1997 11:07
>To: amps
>Subject: [AMPS] Automatic tuning of a VHF amp
>
>Hi all,
> my latest VHF triode amp is going to use stepper motors to control
>the anode tuning and possibly loading, for two main reasons:
>
>1) That way I can get the controls in the best position on the front
>panel.
>2) It will allow remote (but manual) tuning of a noisy amp.
>
>I'm contemplating making the tuning automatic. Does anyone have any
>ideas of how it can be done ? I faintly remember seeing something about
>adjusting the tuning until the anode impedance is completely real (i.e.
>at resonance), but have no idea how it could be done.
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
>
>--
>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)
>
--
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