Hi Gary:
The reverse carrier control is Bacon's idea but it is feasible.
It seems to me that most modern radios using low level modulation and
linear amplifier output have a ALC circuit built in and will reduce the
carrier level automatically if the peaks go to high or if the carrier is
set to high and not leaving enough head room for the modulation peaks.
I'm really not sure but I think this accomplishes the reverse carrier
control scenario?
Yes there is always a way to increase the average power.
However, I think that the upside down modulation where the sharp peaks
go toward envelope cutoff and the nice round troughs go to expand the
envelope to the 1500 PEP limit may be a quick way to get the quiescent
carrier level up in the 500 to 600 watt output level and may be more to
the liking of folks that have the extra power available but want to stay
legal. All they need to do is turn the audio over by reversing the
microphone or the modulator plate caps and then raise the carrier up
nearer where it used to be run. Juggle the carrier level by plate
voltage variance or by loading and then the audio level so that the 1500
PEP is right but enough carrier level to not pinch off the envelope with
those peaks. I realize in either case it is the same amount of average
power in the sidebands but most are still not using a product detector
or a synchronized product detector. I'm betting the envelope detector
is going to have less receiver generated distortion products that would
be due to QSB and selective QSB, than with the conventional "peaks high,
decreased carrier" modulation method. This is of course holding the
ceiling at 1500 PEP in both cases and no compression or clipping. I
wonder if those nasty noise limiters might be a little less annoying.
Interesting discussion don't you think!
Thanks fer the mail Gary!
73, John, WA5BXO
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Gary Schafer
Sent: Wednesday, October 02, 2002 5:39 PM
To: [email protected]
Subject: Re: [AMRadio] New link to Info on AM and legal power limits.
Nice write up John!
Hmm, AM with reverse carrier control. Sounds interesting. That would
look
something like an FM signal but with only one set of side bands.
Actually I have worked on some marine radios in the 70's that did do
some
reverse carrier control. They were SSB radios with AM capability. You
could set the AM carrier level just about any level and when you
modulated
it would adjust the carrier down to the proper level. It was hard to
make
the radio sound bad on AM.
I don't remember how the ALC was detected for the AM mode or exactly how
the carrier level was controlled.
I don't know if it would gain much but it would eliminate a little band
noise when there was no modulation.
ASYMMETRICAL AUDIO
The fact that most of our voices are not symmetrical can be an advantage
or disadvantage when modulating a radio. If the positive peaks are
higher
than the negative peaks then you will not over modulate in the negative
direction while having positive peaks that exceed 100%.
However if you are trying to run the legal limit power then it can be a
disadvantage. Your average modulation level must be held low in order
that
the positive peaks do not cause you to exceed the power limit. 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.
The solution may be to make the modulation "more symmetrical". There are
circuits consisting of several "all pass" filters that are supposed to
do
that. By having more symmetrical audio to modulate the transmitter with
you will be able to obtain higher average levels of modulation without
exceeding the power limit and not over modulating in the negative
direction. It can also greatly reduce the need for large headroom in the
modulator.
Now add a little peak clipping and compression to the audio! You can
increase the average audio even more while still keeping within the
power
limit and not over modulating in the negative direction. The
broadcasters
do it every day and most sound great. Of course it can't be overdone or
it
will not sound great anymore.
Average power is what does the work in the audio. Those high sharp peaks
are all but wasted. They add little to the sound of the signal.
Especially
on a crowded band. It is those high peaks that keep you from attaining
the
high average power that does most of the work.
In RADAR there is a very high peak power to average power ratio. The
reason being is that a certain pulse width and is needed to get the
resolution. With a given pulse width the only way to increase the
average
power is to increase the peak power. Keeping the peak power the same and
increasing the pulse width raises the average power which increases the
range of the radar. But as the pulse width is increased the resolution
gets poorer. (ability to distinguish between two targets)
The point here is that it is average power that is important.
73
Gary K4FMX
