John Byrns wrote:

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.

As soon as my eyes see pages full of calculations, my mind goes into a fog....

I only like the test results of practical circuits.
I am only interested in what is proven to work, or not work.



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.

It should be *obvious* from the circuit!

A germanium diode once turned on with a bias current has a low variation in its
"on"
voltage, and is a far lower impedance rectifier than any tube rectifier which has a

varying plate resistance with Ia.
The Ge rectifier has the same "on voltage" during the charge peaks into the cap
or the RC time constant circuit.
Its possible to arrange a tube rectifier with a virtually constant current bias
from a suitable
CF driver tube, but why? a Ge diode is easier and better.

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.

My circuit is as simple as it gets.
Hve the cathode of the CF at +50v, and have a 1M R to drain 0.05mA
through the diode. Much more current could be used.

This method means that detection of weak signal lower than the forward voltage
of the Ge diode of 0.27v peak approx are not subject to the non linear turn on
of the diode, ie, there is no clipping by the diode.

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.

My methods have not been seen in 99% of old domestic tube radios because they
employ an extra tube or two, and two germanium diodes.
I would have been hanged by management in 1955 if I had insisted that
any extravagant use of tubes were to be employed.
The industry was dominated by lowest common denominator ideas.

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.

Transistor AM radios were a major step backwards for audio quality
in 99% of cases.
It went from bad to plain ****ehouse.

Quantity not quality was what dominated radios in old days.

If you wanted better sound, you bought a Quad AM tuner, which only
rich folks could afford.

95% of radio manufacturing was to produce lo-fi junk,
where 5% thd and 150 Hz to 2 kHz of bw was very very common, at a 1/2 watt of
output.
People just were not concerned about fidelity, it didn't help
the cricket or football scores, or make the news about the Suez Canal
crisis any better.


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%.

My biased SS diode has lowest thd at high levels of signal.

But if you have a normally gronded last IF coil feeding a diode to 100pF
with a typical 1M discharge R to ground, then with low signals on weak stations,
the thd is appalling, and I thought such issues would be obvious to anyone
familiar with diode operation. No need for me to copy out
the wave form analysis I did, build a detector like I have and you won't be
dissapointed!





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.

I try to stick with what works well in practice, and discard all BS.....
I am too busy to be intellectual about bloomin diode detectors.

Patrick Turner.



Regards,

John Byrns

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