In article , Patrick Turner
wrote:


OK, I have taken a closer look at the analysis on the web page at this URL:
http://www.amwindow.org/tech/htm/diodedistortion.htm
and it is more screwed up than I thought.

snip a vastly complex and incomprehensible disputation of the
largely incomprehensible text and formulae at
http://www.amwindow.org/tech/htm/diodedistortion.htm

About all we want is low distortion detection, and it matters noe that we
cannot follow all this mathematical analysis.

Indeed, my original point was simply that the analysis on that web page,
which had been mentioned in this thread as being somehow relevant, was
actually totally irrelevant because it dealt with a square law detector,
not a linear diode peak envelope detector as is commonly used in
High-Fidelity AM receivers. It was then pointed out in this thread that
the conclusion of the web page did not agree with Treman's calculations
for the square law detector. My "incomprehensible disputation" was simply
to tie up the loose ends and show where the web page went wrong on its
square law detector analysis, which would still have been irrelevant to
High-Fidelity designs even if it had been done correctly.

There is no mention of the output voltages measured with respect to
the % of modulation.

I pointed out that very fact in my first post about this web page, that no
details were given of the operational under which the experimental results
were measured.

With respect to the square law detector analysis, the voltage level
doesn't matter, square law is square law irrespective of the carrier
level, so the distortion doesn't change with signal level in an ideal
square law detector, it only changes with the modulation percentage.

From the test circuit shown, there is no bias current flow in the diode
to keep it
turned on even
without an RF signal to demodulate.
This would also reduce thd.

You have still haven't enlightened us with some concrete information about
how much, if at all, your biased diode detector really helps reduce the
distortion of the diode peak envelope detector. I haven't looked at
biased diodes as AM detectors myself, although I am given to understand
that the proper bias can reduce the distortion of a diode peak envelope
detector, but I am also given to understand that the proper bias is
dependent on signal level, which requires a complex circuit to cause the
bias to maintain the proper relationship to the signal level. Although I
haven't seen it mentioned, I would assume that a very tight AGC circuit
would also serve to allow a fixed bias to be applied to the diode. I
would think that if a simple bias scheme such as yours really
significantly helped lower the detector distortion, we would have seen
more implementations of this idea in high quality receivers over the
years. There have certainly been plenty of expensive AM receivers built
over the years, that didn't skimp on the parts count, where an extra
resistor or two, to bias the diode wouldn't break the bank. That is not
to say that I haven't seen cheap transistor radios that had biased
detectors, but it never seemed to be actively pursued in the better AM
receivers of the tube era.

You could better make your point if you posted a couple of graphs for
distortion vs. signal level for a diode detector, with and without bias,
and for several modulation levels, maybe 80% and 100%.

Nobody needs to know math involved with diode detectors
to get much lower thd than is realised in most old fashioned and
attrocious tube
detector stages in
conventional AM radios.

Well you are probably right about that, but for a completely different
reason than you have in mind.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/