Hi Jerry,
Thanks for your explanation. There are a variety of tight asymmetric
peak limiters available to hams on the used equipment market these
days, such as the CRL PMC line, one or two Inovonics boxes, older
Dorrough DAPs to name some of them. In some cases these allow the
user to go to 150% positive while limiting negative peaks to < 100%
but yes indeed as you say, the transmitter RF has to be able to
support this, along with the mod. iron if it is a plate modulated rig.
With most of those, you start getting on thin ice at 120% positive.
Yes, certainly "asymmetric modulation" (or any other modulation) can
only happen in the modulation stage, but the baseband audio can be
limited to prevent up and down excursions that would clip the carrier
or exceed the RF and audio limits of the rig. Most can't handle that
much asymmetry but solid state low level exciters, those with 200 w.
PAs can with a 20 w. carrier, as well as the class E s.s. rigs.
Getting back to the Orion the 100 w. PA (and that may have to be
further derated for 100% duty cycle modes) does place a pretty low
limit on things so in light of that and the fact that most hams do not
operate with oscilloscopes or commercially manufactured trapezoid
monitor scopes, their (in my opinion) over-control of AM may be
warranted. Another argument for limiting modulation to 100% is that
the older vintage rx with envelope detectors can't really handle high
positive modulation signals, in fact some go above 10% distortion when
the received signal gets above around 80% modulation. However, if a
ham want's to go above 100% positive, and has to equipment with which
to do it and insure a clean signal, I know of no FCC Part 97 rule that
specifically prohibits it.
73
Rob
K5UJ
When you average an audio signal amplified with an AC amplifier, e.g.
capacitor or transformer coupled there is no DC component. It probably
is symmetrical about that average value. So when it gets to the
modulator it has to have even excursions up and down from 100% carrier.
If you modulate negative more than 100% you introduce clipping
distortion and splatter. If you modulate positive more than 100% you
often introduce distortion from clipping when the output stage can't
prove the required peak power. If you try to introduce asymmetrical
modulation (more up than down) you are asking the transmitter to produce
more average power than the rated carrier power. That's DC offset. Audio
states are hardly ever DC coupled because you want to limit the low
frequency components that don't add to communications. Usually for
communications a low cut of 300 Hz is common. That minimizes the
modulation from trombones, tubas, cellos, and basses but communication
is not music.
In vacuum tube or solid state transmitters (the biggest I've worked on
ran 250KW carrier, 1 MW peak) the only place you can cause asymmetrical
modulation is at the modulation stage, in many cases the PA. AT 250KW we
had enough trouble keeping tubes without trying some upward modulation
scheme demanding even more RF power.
Symmetrical 100% modulation is clean and can be low distortion. Going
above 100% negative is always a source of splatter from cutting off the
carrier abruptly. Going above 100% positive modulation can be a source
of splatter from exceeding the peak power capability of the transmitter,
whether low level modulation with linear or high level modulation. These
make good reasons to stick with 100% maximum modulation, which is
supported by FCC rules on high quality signals.
Yes, the power is IN the audio, that's why SSB works so much better than
AM for communications with weak or strong signals. AM remains popular
for consumer applications because receivers are simple and receiver
tuning is not critical. AM on CB is a downer because the FCC limits on
modulation causes CB makers to seriously limit modulation which makes
the radios ineffective at communication. That's why amplified
microphones are popular in CB. I used to check the 50.4 calling
frequency on 6m when it was open, most of the time I'd find carriers on
50.4 not long after hearing many SSB signals between 50.125 and 50.2,
but I'd copy hundreds of SSB signals before the band got good enough to
copy modulation on 50.4 AM. It was vivid demonstration of the benefits
of SSB. SSB benefits by putting all the transmitter power in one side
band. SSB benefits by limiting the noise bandwidth of the receiver to
that one audio sideband. Those two give SSB a great advantage over AM,
at least 8 dB less transmitter power for the same S/N at the speaker.
73, Jerry, K0CQ
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