In article , Patrick Turner
wrote:

John Byrns wrote:

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!

It's not, at least not to those among us, such as myself, who are not so clever.

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.

Is what you are saying is that the diode in your circuit is always "turned on"?

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.

I have one of those "rich folks" Quads right here next to my computer, and
I guess those "rich folks" got taken as the Quad uses a common vacuum
diode detector, like a common kitchen radio, nothing special, the only
special care taken in the detector design seems to be that the audio
output is tapped way down on the diode load resistor to minimize negative
peak clipping.

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.

One would expect you to be an advocate of your on circuit, but how low is
the thd at high modulation levels?

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!

Actually your wave form analysis would be very interesting to see indeed.
The impedance of the IFT is quite low at the harmonic frequencies in the
diode current wave form so the diode sees a low impedance drive at those
frequencies even without a cathode follower, and the filtering effect of
the IFT prevents the harmonics in the current pulses from reaching the
plate of the IF amplifier. See the discussion on page 1077 of the RDH4
relative to loading of the IFT secondary by the diode,although much more
comprehensive discussions can be found elsewhere.


Regards,

John Byrns


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