[AMRadio] Power Levels

[Dale Gagnon]

Dave,

I grabbed one of my old ARRL handbooks off the shelf to quote the answer for
you.  The handbook happened to be dated 1968, but I am sure the same info
has been in for decades.

Chapter 8 is on "Audio Amplifiers and Double-Sideband Phone".  On page 241
it talks about Plate Modulation.

"The audio-frequency power generated by the modulator is combined with the
D.C. power in the modulate-amplifier plate circuit by transfer through the
coupling transformer (mod xmfr).  For 100% modulation the audio-frequency
power output of the modulator and the turns ration of the coupling
transformer must be such that the voltage at the plate of the modulating
amplifier varies between zero and twice the d.c. operating plate voltage,
thus causing corresponding variations in the amplitude of the r.f. output."

The key finding in this paragraph is at the point of 100% modulation the RF
amp sees an instantaneous plate voltage of twice the d.c. operating voltage.

Later down the same page we are reminded about Class C amplifier operation.

"The power output of the r.f. amplifier must vary as the square of the
instantaneous plate voltage for the modulation to be linear.  This will be
the case when the amplifier operates under Class C conditions."

On page 67 of the same handbook Class C amplifiers are discussed in more
detail.

"With proper design and adjustment, a class C amplifier can be made to
operate in such a way that the power input and output are proportional to
the square of the applied plate voltage.  This is an important consideration
when the amplifier is to be plate-modulated for radiotelephony..."

So we have learned that in Class C if you increase the instantaneous voltage
on the plate of the amplifier by a factor of two, and that is what happens
when you modulate 100%,  you get two squared or four times the power out.

That's where the X4 comes from.

Dale
KW1I




From: Wt8r at aol.com <Wt8r at aol.com>
To: amradio at mailman.qth.net <amradio at mailman.qth.net>
Date: Saturday, June 29, 2002 7:35 PM
Subject: Re: [AMRadio] Power Levels


>In a message dated 6/29/02 7:20:44 PM Eastern Daylight Time,
>wwd at netheaven.com writes:
>
>
>>
>> > but
>> >where  did the "X4" come from to get the PEP figure?
>>
>> > WT8R
>>
>> Time was an 'extra-class' ticket holder would know where that x4 came
>> from, or know how to look it up.
>>
>> Elmer FUD
>>
>>
>
>Excuse me, but I think you for got to include yer callsign, if you have
one?
>GB
>
>73 de
>Dave in Dayton, WT8R
>
>
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