"Telamon" wrote in message
...

[snip]


Some detector designs would use a DC bias on the diode to put it on the
edge of its liner region to improve its small signal sensitivity. The
optimum bias voltage will depend on the diode characteristics.


There's a linear region in the usual model of a semiconductor diode (a fixed
voltage drop with a series resistance), but that model is only an
approximation. The other model, the square law model, is also just an
approximation, although it's supposed to be close enough over small parts of
the curve.

However, the diode doesn't have to be linear in order to have a fairly
linear diode detector circuit. Imagine we have a diode whose forward
resistance drops in a square law with the voltage. At 0.1V the forward
resistance is 1 meg. At 0.2V the forward resistance is 1K. At .0.3V the
forward resistance is 32 ohms. At 0.4V the resistance is 5.6V, and so on.

Now, let's put this nonlinear diode in series with a linear load resistance
and decide that the circuit is pretty much linear once the diode resistance
drops to 10% of the load resistance. Well, it's obvious that diode detector
circuits which work into higher resistance loads will linearize themselves
at lower voltages than diode detectors which work into lower resistance
loads.

Below a certain voltage, the diode's non linear characteristics will
dominate the detector. Low voltage signals will have much more of their
waveform in this funky reigion than high voltage signals, even at the same
modulation index.

So, as I see it, there's alot more to know about a diode detector's audio
distortion than only the modulation index. There's the actual
characteristics of the diode, the resistance of the load and the signal
voltage the detector is operating at.

There's also the RF filtering, which will tend to "sawtooth" the audio a
bit, much as the rectifier and capacitor do in a power supply. There's also
some resistances/capacitances in the AVC line.

But I could be wrong. If so, let me know!

Frank Dresser