In article ,
"Frank Dresser" wrote:

"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.

Yes, that's exactly what I thought, given the analysis methodology he
seemed to be using on the web page, the distortion seemed way too low to
me. You have inspired me to take a closer look and see exactly what he
did, and where he went wrong, or if I have just misinterpreted his
methodology. I will report back in a few days time.

I do
remember the bigger point, that is, that operation in the square law region
is to be minimized for AM radio detectors.

Exactly, which is why I said that the apparent square law analysis given
on the web page was "complete nonsense" in the context of a High-Fidelity
AM receiver.

Although I do remember reading that any part of a diodes curve can be
characterized as part of a parabola.

Yes, I think that is essentially correct.

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.

Some people say there is no "knee" in the diode curve, which follows from
your observation "that any part of a diodes curve can be characterized as
part of a parabola."

There is one more thing that contributes to linear operation of a diode
detector, and that is the peak hold capacitor. As long as the capacitor
charges to the peak envelope voltage, the shape of the diode curve getting
there doesn't matter much, whereas if you take the output of the raw diode
and average it by putting it through a low pass filter, then the curvature
of the diode characteristic greatly affects the linearity of the output.
Of course even with the peak hold capacitor there are still problems at
very low signal levels, and also with high negative modulation, and the
peak hold capacitor does introduce problems of its own like tangential
clipping when the modulation at high frequencies is high.


Regards,

John Byrns


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