I'm answering the following not to be correcting you Brian, but
instead to make sure we don't mislead some the new comers that might be
"reading the mail". (To coin a phrase that's really true here on email,
HIH)
We must be careful about taking things out of context. Gee, I
hope I can write this properly or it will be the pot calling the kettle
black. (There is another old coined phrase. I hope I don't get in
trouble with using it here.) Anyway here goes!
*************************
Gary's statement is true if we are talking about a high level
plate modulated class C rig. If you or hitting near 100 percent
modulation with the voice type narrow peaks going up and the round
troughs are just at the envelope pinch off, and then just flip a switch
to invert the modulation you will certainly pinch the envelope with
those narrow peaks far before the round side of the audio wave hits
anywhere near the 100 percent positive level of 2 X carrier voltage. In
order to have the same audio level injected from the modulator with out
pinching the envelope you would need to increase the carrier level.
This could be done by increasing the plate voltage on the class C final
if it has a separate power supply than the modulator or by increasing
the load to draw more current. The voltage increase is the more
desirable way because the final would not change in resistance load to
the modulator and the final should maintain its same plate efficiency.
Where as, increasing the load would cause the modulator to see a lower
load resistance and therefore alter the modulation power that it puts
out.
However, if you're talking about a rig where the modulation is
developed by a product modulator that has been unbalance to produce
carrier then it would develop more sideband/carrier power ratio than the
1:2 ratio that is norm for AM and it could be called DSB-reduce carrier.
When this happens then there would be some receiver generated distortion
in an envelope detector however a product detector would only produce
more output with out distortion. Of course it would need to be a
synchronous product detector for AM
In the OLD DAYS!
There were some "product modulators/class C finals" of this
nature produced to work directly into the antenna with out a linear
amplifier to achieve power in the 1KW Range. However if the quiescent
level of power to the final was 1KW and modulation average power
exceeded 500 watts then there would be an increase in total plate
current of the RF output tubes and the input power as measurement
defined by 1/4 sec. damped meters would increase over the legal input
level. The key words above are "average power". What we had been
talking about, in the inverted audio discussion, is instantaneous peak
voltages of audio and not average power.
-----Original Message-----
On 2 Oct 2002 at 18:38, Gary Schafer wrote:
Flopping the envelope over so that the negative peaks are higher than
the positive doesn't help either. In that case you will over modulate
before your average modulation level gets very high.
Only happens if something causes the signal to square off and flat-top.
You can
have "over modulation" perfectly legaly.
It is called "double-sideband-reduced carrier."
