John Byrns wrote:
In article , "craigm"
wrote:
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.
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.
With one exception, see below, the ripple should be more or less constant
with this scheme. At one time I was enamored with this scheme, but gave
it up for reasons I no longer remember. A reason that comes to mind now,
but was not the one I can't remember, is that while the ripple may be
relatively constant with modulation, the frequency at which tangential
clipping sets in will be dependent on the carrier amplitude, since the
slope of the discharge is independent of carrier level.
The "tangential clipping" also becomes more likely
if the audio modulation F is higher and the audio voltage is high.
That's because of the discharge rate of the cap charged by the diode.
Its worse with a detector which does not have a biased diode
and constant current source.
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'm not sure, but I would think that to a first approximation the variable
ripple voltage of the normal envelope detector doesn't add distortion or
nonlinearity, but with the current source approach, there is going to be
distortion on negative modulation peaks when the amplitude of the ripple
wave form is clamped by the diode before the next carrier cycle starts.
For example at 100% negative modulation the ripple voltage must go to zero
even with a current source, and this is a nonlinear effect, since the
ripple voltage is constant up to a certain modulation depth, at which
point it abruptly starts reducing towards zero on 100 % negative
modulation peaks.
In my detector, the output voltage cannot ever go to to zero, or 0V.
Its at 55 volts even with no RF input, and then when a carrier of 10v p-p appears,
you get a rise in the direct voltage voltage across the 270pF of 5 volts to +60v.
Then if there was 100% modulation, you get a varying voltage at the 270 pF between
+55v and +65v.
For low frequency audio, there is almost no change in the ripple voltage at any
part of the audio
wave form.
But at 20 kHz modulation F, the ripple voltage is less on the negative going
slopes of the sine wave.
But the recovered audio wave is very close to the shape of the envelope.
The process introduces a slight phase lag in HF audio modulation.
The discharge rate of the 270 pF in my detector and the 1M form a time constant
of 270 uS.
But when you draw the curve for a 270 uS cap discharge, the first 10v discharge is
a nearly a straight
line discharge rate of 55v per 135 uS, or 2.45 V/uS, or 5V/12.2 uS, and this
allows an undistorted
sine wave of 2.5v peak v and at 20.49 kHz.
Or 10 peak volts at 5 kHz.
The undistorted sine wave maximum rises as F reduces.
But the HF content of music reduces as F rises, so this is not a problem.
Its possible to increase the current discharge from the 270 pF cap by using 470k
instead of 1M but all this would do is raise the undistorted threshold of the sine
wave
by twice, when it isn't really needed if you arrange the detector to produce no
more than
around 3 vrms of audio at 100% modulation.
This sort of audio voltage would mean the IF amp isn't working too hard, and into
its high distortion
region.
I also believe it is a lot easier to do this in the solid state world than
in the tube world.
There are high voltages available in vacuum tube circuits which in
combination with a large resistor would make a pretty good approximation
to a current source.
If you use my detector circuit, you could easily establish a -200v voltage source,
and take a 4.7M resistor from the 270 pF, to give a closer approximation to a
CCS.
Patrick Turner.
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/