Normally in high level modulation the PA has to produce a peak power 4
times the carrier power, that's accomplished by doubling the supply
voltage in a plate modulator. That works IF the tubes can take it.
Asking for 200% positive is asking the PA to put out more power by
running the plate supply to four times the carrier value and expecting
the PA to put out 16 times the carrier power on the modulation peaks. No
dinky 1 KW AM tubes will peak at 16 times without distortion (or melting
the guts).
I'm sure of my recollection and the physics haven't changed. My QSTs
from that era are in boxes and I don't know which boxes right now. There
is a chance I may never find them but might go to a foot long shelf of
CD roms with every QST before 2000 intead of this wall of magazines I
have now. In theory the CD roms might be searchable but the sample I
have is not good for searching other than titles and then its easier to
look up the magazine's index than to depend on the title search alone.
Maybe the remaining aftermarket distributor has changed the software, I
would if it was me.
I didn't chase down all the details in part 97 but it refers to the FCC
standards on modulation when referring to AM. These days the limit is
peak power OUTPUT, not DC power input as it was in 1956 and a peak power
16 times a 100 watt carrier for 200% positive modulation exceeds the
power limit of 1500 watts pep. When the FCC changed from DC plate input
of 1 KW to 1.5 KW PEP output, AMers grumbled because they could only run
375 watts of carrier but with 1 KW input carrier they could with a
really good class C PA get at least 750 watts of carrier. And 3000 watts
PEP output. That's with 100% positive modulation.
73, Jerry, K0CQ
On 6/15/2010 10:18 AM, Rob Atkinson wrote:
Hi Jerry,
Thanks for your reply. I'm afraid I remain unconvinced. A 56 year
old QST article (there have been some technological developments since
then) and your recollection are not an adequate substitute for a cite
of a current FCC Part 97 portion explicitly prohibiting A3 positive
modulation over 100%. I understand that domestic medium wave AM
broadcasters are limited to 125% positive now. Such a limit for hams
would be good practice however, once again, if an operator is linear,
not clipping his carrier, i.e. meeting current spectral purity
requirements and is within the American 1.5 KW PEP limit, there is, as
far as I know, no reason why he cannot go asymmetric even to 150%
positive. As another fellow mentioned, the human voice (especially
the male voice) results in a natural asymmetry.
In your paraphrasing of the FCC's comments, (I imagine they employed a
more technically rigorous term than "splatter") from memory, you
mention that this ham had a problem with "splatter." That tells me he
probably clipped his carrier and indeed that would violate an FCC
spectral purity rule then and now; but I invite you to examine Part 97
for specifics regarding a positive modulation limit. I would like to
know about it because I regularly hit about 120% positive while
getting up to around 95% negative, because I have an asymmetric voice
that I limit accordingly. If you can find anything I will be
grateful and gladly comply with it however it would certainly be news
to me.
73
Rob
K5UJ
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
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