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
On 6/13/2010 8:10 AM, Rob Atkinson wrote:
> Hi Jerry,
>
> Kindly expand upon your statement re "DC offset." You lost me with that one.
>
> I think what you are writing below has to do with vacuum tube plate
> modulated rigs? A number of asymmetric peak limiter processors for AM
> have come to market since the early 1970s. Yes, some of the older
> broadcast rigs had a hard time with them.
>
> Merle: Too bad Ten Tec assumes everyone wants to be limited to 100%
> positive and negative. The power is in the audio.
>
> 73
>
> Rob
> K5UJ
>
>
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