craigm wrote:

"John Byrns" wrote in message
...
In article , Patrick Turner
wrote:

John Byrns wrote:

Now this seems like a reasonable concern about the effect of the diode
detector on the IF amplifier, unlike your concern about the current
pulses
into the diode which are rendered irrelevant by the filtering/flywheel
action of the IFT.

The load varies during thre audio cycle

Yes, that is correct, the reason is that the diode isn't "perfect".

if the ripple voltage changes.

No, that is wrong, even with a "perfect" diode which would present the
driving circuit with a constant load over the complete audio cycle if such
a diode existed, the ripple voltage will vary, being very low at the
negative modulation peaks, and high at the positive modulation peaks, the
amplitude of the ripple tracks the envelope, it is just the natural way a
diode peak detector works, there is nothing to be done about it that I
know of.

You have made conflicting statements about the 455 kHz ripple voltage out
of your detectors, first saying that your detector had a constant ripple
voltage over the audio cycle, then saying that the ripple voltage varied
over the audio cycle, and most recently you seem to be back to your
detector somehow having constant ripple voltage. This doesn't add up and
there is something fishy here.


John,

If I follow what you are saying, you are saying that the sawtooth decay of
the capacitor changes and differs significantly between negative modulation
peaks and positive modulation peaks.

If so, then that ripple is caused by the RC circuit connected to the diode
where the resistor is discharging the capacitor. If you replace the resistor
with a constant current source (to discharge the capacitor), the ripple will
be much more uniform. Or instead of tying the resistor to ground, use a much
larger resistor connected to a negative supply.

In my detector with Ge diode after a CF the 1M resistor with 55 volts across it
gives discharge current
of 0.055 mA from the 270 pF used for the charge C off the diode, and this
current stays fairly constant,
regardless of signal level.
But were the R grounded so that the current was negligible at low IF sig levels,

then huge variations in ripple voltage occur, and well as audio distortions
at high audio voltages, especially if the audio F is 10 khz.

Try building a detector or two, and use your CRO to see what happens.
Make sure the probe capoacitance does not affect what you are viewing.



If the discharge current is constant (does not change due to the voltage on

the capacitor), then this approach leads to ripple that does not vary with
the voltage on the capacitor.

I would think that this improves the linearity of the detector and this
improve distortion, however, I don't know how much of an improvement it will
make.

I also believe it is a lot easier to do this in the solid state world than
in the tube world.

No it isn't, and the circuit using two CF tubes and a couple of Ge diodes etc
prove this, and the dynamic range of my detector circuit is way above most SS
circuits.

Its no use only discussing this for 100 years to understand;
you must away from the PC, and build and measure something.
Observe the waveforms.

Patrick Turner.



craigm