Hi,

For RF diode detectors to work, one needs a device that has a
non-linear V/I curve. In other words, the slope of the V/I curve
must change as a function of applied Voltage. The slope must be
steeper (or shallower) at higher voltages and shallower (or steeper)
at lower voltages than at the quiescent operating point.

If there's any truth to this, someone please enlighten me. I've always
been of the belief that an envelope detector diode would be most perfect
if the diode was a perfect switch, i.e. zero attoamps reverse current
and perfectly linear forward current (as though the diode was a wire
during the forward conduction period). I don't see how a changing slope
during forward conduction could do anything other than distort the
demodulated waveform, especially on tiny signals.

You believe correctly! A diode detector is nothing much more than a
rectifier and should be characterised accordingly. Curvature of the
forward characteristic has to be lived with and is not something generally
to be desired. There are exceptions to this where the law of the initial
curvature is employed in certain instruments but not usually in
demodulating an AM signal.

However, with very weak signals a small degree of forward biassing is
sometimes employed to obtain the greatest recovered audio. This comes
about because the positive peaks are driven onto the greatest slope while
the negative ones, which would normally distract from the charge on the
load capacitor, are placed on the flatter part below the bias point and so
not recovered so well.


Cheers - Joe