http://www.arrl.org/files/file/Technology/pdf/5610027.pdf And I recall
its author loosing his license by insisting ultra modulation met the FCC
rules, probably
57 or '58. My general was dated October 56 the month of that article
(QST Oct 56 pages 27-29).
As I recall, he claimed it couldn't splatter, the FCC said it splatters
so quit using it and he didn't.
73, Jerry, K0CQ
On 6/14/2010 7:03 AM, Rob Atkinson wrote:
> 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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