Gary Schafer wrote:
Geoff,
I am ONLY talking about a linear amplifier here. The drive signal is
already modulated. If you are modulating the final stage these
efficiency rules discussed here do not apply. Unless you would be
talking about grid modulation and then they "do apply".
Yup - you're right ;-) I'm getting muddled, here, between
plate-modulating a grounded grid amp, and using the grounded grid amp,
after a low-level AM source.
Big difference. Sorry for the misunderstanding.
It does not matter what type of signal is modulating the signal when
the linear amp is involved. Whether it is a sine wave or speech or
anything else. The PEP rule still applies for 100% modulation! You can
not exceed 100% positive peak modulation if the carrier is set at any
level greater than 1/4 the PEP output capability of the amplifier.
Unless you want to operate it in a non linear mode.
I Disagree with that.
K4KYV said:
"Overmodulation" is defined not necessarily as exceeding 100%, but
exceeding the MODULATION CAPABILITY of the transmitter. As soon as
the final amplifier plate voltage goes past zero, you have exceeded
the modulation capability of the transmitter, whether there is still
rf output or not.
That being said, if I were to take a small rig, like a Ranger, or an
AF-67/8, modify it so that there was plenty of audio available to
faithfully reproduce my peaked voice, (natural asymmetry) the positive
peaks -will- reach beyond the 2:1 level, that one would observe when
looking at an AM carrier (on a scope) modulated to 100% with a sine-wave.
Here's an example...
http://w5omr.shacknet.nu:81/~w5omr/pictures/2005-08-08/000_0049.MOV
it's a short 1meg video file
The text spoken into the mic is "helloooo test, one, two threee, four."
Note that the unmodulate carrier occupies 2 squares (centimeters) on the
scope. The positive peaks are reaching 8 centimeters, pk-to-pk, while
the negative cycle comes right down to the carrier. That's a positive
to negative peak ratio of 4:1. Let's assume that's a 100w carrier. 4x
the carrier for PEP would mean 400w PEP output. That's with a standard
2:1 ratio. Since the voltage and current are both being doubled, how
much power, in PEP do you think a 4:1 ratio is?
(hint: it ain't 800w PEP)
With this in mind, if I attempt to use this 100w rig with a linear
amplifier and 8:1 Symmetry Ratio, I'd have to have at -least- an
amplifier with a pair of 4-1000's in it, to properly handle the maximum
PEP power out, with a nominal 100w of drive.
The whole purpose of this part of the discussion, Gary, is to 'bust' the
mythical "375w AM legal limit output carrier power" nonsense, and to
alert and advise people that while PEP = 4x Carrier leve for 100%
modulation is true for a sinewave, it ain't true for the human voice.
Especially the peakedness of the typical male-patterned voice.
I'm not saying that you shouldn't run your AM rig to 500w DC Input
(which, for a class C amp at 75% is around 375w) or at whatever power
level you feel comforable with(*), but just be aware of the PEP 'Federal
Law' that is place.
PEP = 4x the AM Carrier at 100% modulation is true, -ONLY- if you're
using a sine-wave to modlulate the carrier.
(Operating your AM Rig without a 'Scope, is like driving your car at
night without headlights!
-Don Chester/K4KYV
Ex. Editor: AM/Px)
--
73 = Best Regards,
-Geoff/W5OMR
(* = Remember in 1990, before the 1991 ruling that made this a federal
law, when the phrase "When AM kW's are outlawed, then only Outlaws will
be running AM kW's" was popular?)
