[AMPS] G2DAF linears

[Peter Chadwick]

G3SEK says:

>There is an interesting circuit for a pulsed two-tone audio tester
>(which is essentially the same thing as a three-tone source) in the
1994
>RSGB Radio Communication Handbook. It's a normal two-tone audio source,
>with the combined outputs going through a MC3340 voltage-controlled
>attenuator. 

The only problem with this is that in order to get repeatable
measurements, you would have to use gated spectrum analyser techniques.
However, a lot can be determined from the screen supply time constants.
Al though the variations in voice are, as Ian says, at very low
frequencies, the screen voltage in the G2DAF is intended to follow these
variations. The intent was not to rectify the grid drive to produce a DC
screen voltage, but rather a unipolar voltage following the input
modulation envelope, such that the larger the instanataneous input
voltage, the larger the screen voltage and thus the larger the plate
current. The linearity of the amplifier is dependent upon the transfer
funsction of the screen grid and the transfer function of the control
grid: it is possible that suitable adjustment could get to a square law
which would be very good for linearity, although it would be depedent
upon the stability or otherwise of the tube characteristics.

A similar idea using a switching regulator to vary Vcc was used in a
Collins solid state HF tx back in the late 60' - early 70's.

73

Peter G3RZP

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