Bill Smith wrote:
Hi Geoff,
Been there, tried that.. have run increased carrier, and with an
asymmetrical limiter have limited positive peaks to comply with the 1500 W
PEP limit.
I've gone to the trouble for you (and anyone else that cares, for that
matter) of the 'heart' of the discussion that John/BXO, Bacon, Tim and
Don/KYV came up with...
*"The maximum audio modulator power requirement for 100% modulation of a
1000 watt high level modulated class C final is 500 watts, /but only if
the modulating signal is a sine wave/. The reason for this will become
apparent from the following example. *
* The following example will show that the required audio power
necessary to achieve 100% sine wave modulation is 50% of the value of
the DC Plate input power supplied to the final class C RF amplifier. * * *
* Assume that the final amplifier has 2000 volts applied to the plate
with a current of 500ma. This makes the power equal to 1000 watts. To
achieve 100% modulation, the peak modulation voltage of the sine wave
must be 2000 volts and because of the symmetry of the sine wave will
yield a 4000volt peak-to-peak swing (0 – 4000V when the quiescent DC is
at 2000V) thus providing the desired 100% modulation. To calculate
average power rather than instantaneous peak power we must convert the
peak voltage output from the modulator to a “Root Mean Square” voltage
(Peak voltage X square root of 0.5 = RMS Voltage). RMS voltage is equal
to 0.707 of the peak voltage. *
*Again if the plate voltage on final is 2000 and the current is 500ma
then in the above asymmetrical example the peak-to-peak swing of the
plate voltage would be (0 – 6000 volts with the quiescent plate voltage
at 2000 volts DC). (6000-2000 = 4000 volts peak) This is twice as much
peak audio voltage as was needed before. (4000 * 0.707 = 2828 V RMS) and
P = E*E/R or ((2828 * 2828) / 4000 ohms) = 2000 watts. Since we have
doubled the needed peak voltage, then we have quadrupled the power
required to produce it. This does not mean that you will be putting that
amount of power into the final continuously, but you will need that
capability in order to get the needed voltage swing on the peaks. *
* An RF ammeter using a thermocouple actually responds to average power
rather than RMS current, because it is heat that causes the indication.
This type of ammeter does not indicate true RMS current. Because it is
calibrated with an un-modulated sine wave signal, it will read
un-modulated signal currents correctly, but its reading will be elevated
when normal amplitude modulation is present. It will display a current
reading equivalent to the RMS current that would produce the same
average power as the modulated wave. In fact, the true RMS current of an
AM waveform does not change until modulation exceeds 100% in the
negative direction, unless there is nonlinearity or some sort of carrier
level shift action taking place. *
* With a 50 ohm load, a 750 watt carrier will cause the thermocouple RF
ammeter to read 3.87 amps. If this carrier is modulated 100% by a sine
wave, the average power will increase by 1.5 times (the 50% added power
comes from the sideband energy created by modulation). The total average
power will be 1125 watts with modulation at 100%. The RF ammeter will
show about 4.74 amps, which is the current that would be necessary to
produce an un-modulated signal of this power level. If a person’s voice
is used to modulate the rig, and the modulation envelope looks something
like the scope picture in figure 4, you will see approximately the same
increase in RF current, even though PEP with voice modulation is 6750
watts, and PEP with sine wave modulation is 3000 watts. This is because
the speech waveform is spiky, so its peaks need to be relatively high in
order for it to have the same RMS power as a sine wave. With all this in
mind, it boils down to the fact that in order to faithfully reproduce my
voice with the legal carrier level of the time, it was necessary to have
a modulator capable of 2000 watts. Let me say again that I was not
putting 2000 RMS watts of audio into a 1000 watt (input power) rig. But
if I had not had the 2000 watt modulator, then the peaks of my voice
would have been chopped off or clipped, resulting in distortion and
splatter. *
* Now with the 1500 PEP ceiling, I can only legally run 220 watts input
and 165 watts output if I want to properly reproduce my voice."
*
*Asymmetrical Audio.
*
*
The whole article can be seen at
http://www.qsl.net/wa5bxo/asyam/aam3.html
*
*--
73
*
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