AM modulation

Tue Feb 16 18:08:20 CST 2010

As several have said, high level modulated Class C AM transmitters have a greater overall efficiency than linear amplifier transmitters.  Straight linear amplifiers transmitters are actually usually run in class AB2 and have an average plate efficiency in the final of about 30% or so. The efficiency off the power line is even worse because they usually operate at lower gain than Class C transmitters and have more stages, more parts to stock, etc.  RCA's Ampliphase improved on this. Western electric also had a high efficiency linear amplifier they marketed successfully based on the Doherty amplifier.  This, like the Terman Woodyard, is a carrier tube/ peak tube arrangement with the main difference being the Doherty is an amplifier and the Terman Woodyard is a modulated stage. W-E transmitters were modulated at very low level, about two watts, and used a series of Doherty amplifiers to achieve powers up to 50kW.  The transmitter arm of W-E became Continental Electronics and they continued to make and sell Doherty amplifier transmitters.  They developed a variant where the Doherty amplifier was high level screen modulated which achieved total efficiencies greater than Class C amplifiers. The Continental 317C used only 9 tubes, and drew only 86 kW off the power main to make a 50,000 watt carrier. The two final tubes, 4CX35000s were screen modulated by a pair of 3CX3000s.  Both final tubes were biased to operate in Class C, giving them good RF efficiency and modulation linearity. The two audio tubes, were driven in parallel, the plate of one being connected to one final tube, and the cathode of the other to the second tube. The transmitter could modulate flat to 30 kHz on a dummy load and into the subsonic region.  The distortion on a dummy load was in the region of a couple of tenths of a percent. 
   Many of these are still in service as are the RCA Ampliphase transmitters.  The tendency, though, is toward PW modulated transmitters with even greater efficiencies.

One other note.  When analyzing AM modulation it is incorrect to say that the 'carrier goes away,' or that 'the carrier goes to zero.'  In a correctly modulated AM transmitter of any design, with the exception of clamp tube modulated transmitters, the carrier level is invariant.  The FCC Rules in fact, require that the carrier amplitude not vary by more than 4% under any condition of modulation.  I realize this conflicts with high school physics books, and popular technical magazines, but checking a few references of the physics at a high level will bear this out.  Collins' book on SSB is a good place to start.  Even though we have all seen the AM carrier 'go to zero' on oscilloscopes, the fact is, it doesn't happen at all.  The normal presentation on oscilloscope is the resultant of the carrier and sidebands --which are out of phase -- adding (and canceling) in the scope.  Observing AM modulation on a spectrum analyzer will reveal that the carrier just sits there doing nothing while the sidebands come and go with the modulation.

Bob Bruner
W9TAJ

ransmitters were almost always plate-modulated
> why didn't they AM the signal when it was "small"
 and easy to do and then just use a linear final
 so all the muscle went into the signal instead
 of heating the modulation transformer?
There was a reason.  A linear amplifier is most efficient at maximum power, and 
he efficiency decreases for lower power, reaching
ero at zero output.  
In the old days, there were a few other alternatives to plate modulation that 
id get reasonably good efficiency.  One was called
mpliPhase, it belonged to RCA and it combined two oppositely phase-modulated 
utput stages.  Another was the Terman-Woodyard
mplifier, 
  Bob, WA3WDR

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