E class 75 meter AM transmitter

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If you are interested in making something, there is a web site someplace (if it hasn't been turned off) that showed how a low level exciter panel was removed from an RCA ampliphase 50kw transmitter and a 1kw AM transmitter was built.  If you are not familiar with Ampliphase  it was essentially 2 FM (Phase) amplifiers where the output was combined while the inputs were fed audio varying phased signals to each amp 135 degrees out of phase for 0% modulation and +/- 90 degrees for 100% modulation.  I am not really sure about this stuff since its been 45 years since I screwed with it.  The big advantage is there is no high power modulation amps or 2000 lb modulation reactors were needed and it produced very good audio.  

I can just see some CB 'billy getting one of the exciters and use a couple of their class c "linears" using 3cx3000 tubes which seem to be popular with the chicken banders.

bruce greenleaf:
Very good info, and schematics for a Class-E transmitter, Class-H and PWM modulators (all solid state)

http://www.classeradio.com

bruce greenleaf:
Meant to add the transmitters at the link above, you can achieve well over 100% positive modulation (150% easily)
And do it with NO 'splatter', by keeping the negative peaks below 100%.

HI-Fi, and lots of audio!

Michael S. Higgins:
Class E transmitter turn on and off  the  output signal by varying the pulse width supply voltage to the final at a much higher ratio than the audio waveform. If the extremes of the pulse width produce an analog composite waveform that is essentially a square wave you will still get splatter. A modulated square wave with a Class B, AB or E final will produce splatter due to the harmonic rich waveform making a hard transition from full power to no power. This transition rate, as it gets sharper, becomes richer in high frequency products.

In order to modulate the power supply in Class E operation the modulation must be stripped from the original RF feed exciter. This is then applied to the power supply to re-modulate the final stage. Most Class E transmitter can obtain efficiencies approaching 90%. In large broadcast facilities this is a huge reduction in operation cosat.

The flyback energy in the tank circuit fills in the signal/no signal periods and the signal averaging is composite RF signal. You cannot get more than 100 modulation. You can fill in the RF envelope but 100% modulation is the ratio between full output and no output. Beyond that point you have waveform distortion and then the spectrum fun begins.

Most Class E transmitters require switching speed at 100 times or more than the level of highest signal frequency. This is why they are popular for broadcast AM but have had little acceptance at frequencies much above 4 MHz. The power supply modulator would need to be at much higher frequencies and cost starts to out weigh any energy savings benefits.

bruce greenleaf:
"""Class E transmitter turn on and off the output signal by varying the pulse width supply voltage to the final at a much higher ratio than the audio waveform. """

Well, that's in a Pulse Width Modulator/powersupply.  Which may or may not be utilized to power a Class-E transmitter.

"""In order to modulate the power supply in Class E operation the modulation must be stripped from the original RF feed exciter. This is then applied to the power supply to re-modulate the final stage."""

Most transmitters would use an independent VFO, synthesizer for the RF drive. So there would be no modulation to "strip, and re-aply"

"""Most Class E transmitter can obtain efficiencies approaching 90%. In large broadcast facilities this is a huge reduction in operation cosat. """

Absolutely true. (though over 90% is very do-able)


"""The flyback energy in the tank circuit fills in the signal/no signal periods and the signal averaging is composite RF signal. You cannot get more than 100 modulation. You can fill in the RF envelope but 100% modulation is the ratio between full output and no output. Beyond that point you have waveform distortion and then the spectrum fun begins. """

Huh? Are you saying that you can't modulate a class-e transmitter at more than 100% positive, without splatter, and distortion??


"""Most Class E transmitters require switching speed at 100 times or more than the level of highest signal frequency."""

True, 160khz switching in a PWM is average...


"""This is why they are popular for broadcast AM but have had little acceptance at frequencies much above 4 MHz. The power supply modulator would need to be at much higher frequencies and cost starts to out weigh any energy savings benefits."""

Not so, The switching frequency of the modulator stays the same, it doesn't need to be higher, just because the operating frequency of the transmitter changes.

Class-E amateur transmitters are in use regularly on 40 meters, and have been built for 10meters.

Of course, Class-E is used up into the Ghz range, for cell-phones etc, granted, not in the AM mode.

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