[AMRadio] AM power

Sun Jun 19 13:12:01 EDT 2011

Yes Geoff, I have read John's article and it is right on. No different than
what I am trying to tell you.

73
Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Geoff Edmonson
> Sent: Sunday, June 19, 2011 7:31 AM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] AM power
> 
> On 06/18/2011 10:29 PM, Gary Schafer wrote:
> > Sorry for the misspelled name Geoff.
> >
> > It sounds like you have your transmitter set up to never reach 100%
> > modulation on positive or negative peaks. That's ok too.
> >
> > But there are two different 100% areas to be concerned with. The
> positive
> > and the negative. If your transmitter output voltage doubles on
> positive
> > peaks, that is 100% positive modulation and is what you are concerned
> about
> > for measuring peak envelope power. It does not matter that your
> negative
> > peaks never hit 100% on the negative side. You still calculate PEP
> with
> > positive modulation  peaks.
> 
> Since no one apparently wants to go to the site to read for themselves,
> here's an excerpt from http://www.qsl.net/wa5bxo/asyam/aam3.html
> 
> ========================================================
> "The high-level modulation method is the application of the modulating
> voltage to the plate circuit of the class C final, causing the output
> amplitude to vary in accordance with the applied modulation. One hundred
> percent (100%) modulation was generally defined as the point where the
> maximum modulating voltage, during its negative half cycle, opposed the
> DC supply voltage sufficiently to reduce it to zero. If this voltage
> dipped below zero, over-modulation and splatter were the result."
> 
>    Most people agreed that the peak of the positive half cycle of the
> modulating audio voltage, added to the DC supply, could go as high as
> necessary to faithfully reproduce the audio as an image of the
> microphone output. Even if the positive peak was more than two times the
> amplitude of the negative peak, the modulation was not considered
> illegal unless it contained distortion products that caused splatter
> over an excessive bandwidth. Over-modulation was only considered to
> occur at the point where modulation characteristic became non-linear,
> producing distortion and splatter.
> 
>      The audio voltage from a microphone is often not symmetrical,
> unlike a sine wave from a signal generator. This asymmetry is a /natural
> quality/ of speech and other sounds. This article discusses the use of
> voice waveform asymmetry in AM systems.
> 
>      When an AM transmitter is 100% modulated by a pure sine wave, the
> PEP (Peak Envelope Power), is 4 times the un-modulated carrier power.
> This is because the Audio Voltage modulating the carrier doubles the RF
> voltage at the peak, since the load resistance is constant, the RF
> current doubles at the same instant as the RF voltage. Since P = E * I,
> then P at the instant of the positive peak must be 4 times greater than
> the power of the original carrier.
> 
>      If the transmitter modulation is increased until the peak RF
> voltage is 2.5 times the original carrier RF voltage, the peak RF
> current occurs at the same instant, and it is also 2.5 times as great as
> the original carrier RF current The result is a PEP of 2.5 * 2.5 = 6.25
> times the un-modulated carrier power.
> 
> Here's the math:
> PEP = ((peak-to-peak modulated RF voltage / un-modulated carrier
> voltage) squared) * un-modulated carrier power
> 
>      This large PEP can occur without negative over modulation, if the
> modulating audio is acquired by a voice from a microphone. Microphone
> audio is generally asymmetrical.
> 
>      To help me understand and explain the relationship between audio
> and purity of modulation, I've defined a function, which I call Symmetry
> Ratio (SR).
> 
> Symmetry Ratio (SR) defined:
> 
> SR = (Peak-to-Peak audio voltage) / (lesser of the two Peak Audio
> Voltages above or below the quiescent line)
> SR = 2 if the signal is a Pure Sine Wave
> SR cannot be less than 2
> 
> ______________________________
> In John's example, he shows a representation of an O'scope with his
> voice imprint on it.
> ------------------------------------------------------
> 
> While using the quiescent level as reference, note that the positive
> narrow peaks go twice as high as the negative wide peaks.
> 
> SR = (Peak-to-Peak audio voltage) / (lesser of the two Peak Audio
> Voltages above or below the quiescent line)
> 
> Peak-to-Peak audio voltage = near 3 units
> 
> Lesser of the two Peak voltages (negative in this case) = near 1 unit
> SR=3
> 
>      Figure 4 is the RF envelope view that would be produced by
> modulation with the audio as represented in fig 3. The RF final plate
> input power is 1000 watts, and the efficiency is 75%. This yields a
> carrier level of 750 watts as represented at the quiescent level on the
> chart.
> 
> (Figure 4 show's Peak Envelope Power at 6,750w)  There are more tables
> and pictures that greatly help explain.
> 
>      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.
> =============================================================
> 
> 
> Peak envelope power is the average power supplied to the antenna
> transmission line by a transmitter during one radio frequency cycle at
> the crest of the modulation envelope, under normal operating conditions.
> 
> 
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