"John Byrns" wrote in message
...

The analysis on this web page is complete nonsense, at least for the type
of diode detector we are discussing here. We are concerned with
High-Fidelity envelope detectors, while the web page analyzes a perfect
square law detector. It starts the analysis of by giving the complete
diode equation, but then quickly says we will forget that and consider the
diode to be a perfect square law device, and not only that, but that it
will be used in some sort of unspecified circuit that maintains the
perfect square law response for the complete detector. I didn't check all
the math after the perfect square law assumption was made, but I will
assume he got it all correct. This type of analysis may have some
application to crystal set design, but not to the type of detectors we are
discussing.

He did compare his results with the results from a test circuit, but I
could find no indication of the signal level he made the measurements at,
perhaps I missed that. Even though the test circuit did include an RC
network type load as used in a peak detector, if he made the measurements
at low levels in the square law region of the diode, the capacitor would
not cause the diode to act as a linear peak detector.

The whole analysis on this web page is too simplistic and is irrelevant to
the subject at hand.


Regards,

John Byrns

He came up with actual numbers, which is more than most do. Anyway, I also
noticed that there was no mention of the actual voltages the detector was
being driven at.

As far as the square law stuff goes, Terman says a the distortion of a true
square law detector will be m/4. So 80% modulaton will result in 20%
distortion. He might have derived that number, I don't remember. I do
remember the bigger point, that is, that operation in the square law region
is to be minimized for AM radio detectors.

Although I do remember reading that any part of a diodes curve can be
characterized as part of a parabola. I don't know if that's really true or
not, or if I'm actually remembering it correctly. But such an assumption
works fine with the usual rules of diode detectors. Run the diode at a
reasonably high voltage to minimize operation below the knee area of the
curve. Run the diode into a reasonably high resistance to minimize the
effects of the variation in the straighter part of the curve. Keep the DC
and AC resistances in balance.

Frank Dresser