One of the most delicate tests for dynamic linearity is the use of two
identical diode detectors on the input and output of the amplifier. They
need feeding from something like a directional coupler with attenuators
to get identical input levels, and their output is applied to the x and
y channels of an oscilloscope. Any deviation from a straight line
indicates non linearity. This technique, and the equations for determing
3rd and 5th order distortion product levels from the line are detailed
in Pappenfus et al., Single Sideband Fundamentals and Circuits, McGraw
Hill, 1964.
I suspect that varying the ratio of screen volts to grid volts on the
G2DAF may well get the 3/2 power curve due to each grid down to a square
law for the overall transfer characteristic, although I admit I haven't
tried doing a mathematical analysis yet.
The original idea of the G2DAF was to simplify matters by getting rid of
the screen and bias supplies. However, better linearity might be
available if some bias is available. The G2MA linear (which predated the
G2DAF) may offer more flexibility in this respect. This amplifier used
the old fashioned clamp tube on the screen, and varied the grid bias
applied to the clamp tube by rectification of the envelope.
Of course, one can always look at using Polar Loop or even Cartesian
Loop to get Class C efficiencies with linearity far better than any ham
radio exciter on the market today.................
73
Peter G3RZP
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