[AMRadio] AM linear operation

[D. Chester]

> From: "Jim Tonne" <Tonne at Comcast.Net>
> Subject: > To: "Discussion of AM Radio in the Amateur Service"

> One is that I have referred to screen-modulated amplifiers
> as using the "efficiency modulation" system.   This is right
> in line with what you just wrote.
>
> Secondly, I worked for a bunch of years at Continental
> Electronics in Dallas, Texas.   In their earlier years they
> made rather high quality screen-modulated AM broadcast
> transmitters.   (Let's not at this point talk about their awful
> efficiency!)   I remember one of their salesmen talking
> to me about them.   He said that "The way to keep those
> rigs cools was to either modulate heck out of them or else
> to turn them off!"
>
> - Jim W4ENE

Yes, efficiency modulation includes about anything except plate (anode) 
modulation, which is referred to a "high level" modulation. It could even be 
described as power supply modulation, since that is what you are 
modulating - the power supply that feeds the plate.

Screen grid modulation is a form of grid modulation, so it is efficiency 
modulation.  So is control grid, suppressor grid and  control grid 
modulation.  AM linear amplification is also a form of efficiency 
modulation.

AM linear is very close to the same thing as control grid modulation.  With 
control grid modulation, the rf drive is held steady while the control grid 
bias is varied with modulation.  With AM linear, the control grid bias is 
held steady while the rf drive is varied with modulation.

As far as efficiency is concerned, low-level efficiency modulation is not as 
"awful" as it may seem.  The "inefficiency" lies in the 30-odd percent plate 
efficiency of the final with the 100% duty cycle carrier.  The linear amp is 
just as inefficient with SSB at that power level, which is much of the time, 
since a SSB signal's average power is much lower than peak power (just as an 
AM signal's carrier power is much  lower than its peak power).  With a 
typical voice, without heavy processing, the average level of a SSB signal 
is also somewhere around 30%, or maybe even less.  Ever observe the VU meter 
in a broadcast studio? The power advantage of SSB  lies in its 
less-than-100% duty cycle.

The overall efficiency of a grid modulated transmitter or AM linear is just 
slightly worse than that of a high  level plate modulated transmitter.  With 
plate modulation, a lot of extra power is burnt up in the modulator tube 
filaments, the plate dissipation in the modulator tubes (the high level 
modulator is nothing but a linear amplifier running at audio frequencies, 
with all the same inefficiencies as any other linear amplifier), plus the 
additional power it takes to run the audio driver stage for the high-level 
modulator.

When you consider the ratio of a.c. power drawn from the mains to rf power 
delivered to the antenna, there isn't a whole lot of difference between "low 
level" and "high level" modulation.  That's the point Continental 
Electronics used to make when asked why they never produced plate modulated 
broadcast transmitters.

With amateur operation, since we don't operate on a fixed frequency like a 
broadcast station does, plate modulation has an advantage in ease of 
tune-up.  About the only thing that is really critical is the adjustment of 
audio level to approach 100% modulation without overmodulation or 
flat-topping.  The circuit is very forgiving as far as grid drive and plate 
loading are concerned.  Get roughly within the ballpark, and the transmitter 
will work OK.  With low-level modulation (grid modulation or linear), the rf 
drive to the grid is extremely critical, along with the antenna loading and 
the audio level.  All 3 factors must be exactly right in order to achieve 
100% modulation with good modulation linearity, while avoiding spurious 
distortion products.  Therefore, it takes more technical skill to properly 
tune up an AM linear or grid modulated transmitter than it does to tune up a 
plate modulated rig.

The above discussion primarily covers conventional tube type circuits 
running low and high level modulation.  Cathode modulation (actually a 
combination of grid and plate modulation), the high-efficiency Doherty 
linear amplifier  (and similar Terman-Woodward grid bias modulation system), 
class-E amplifier and pulse-duration modulator might each be a different 
story, meriting a discussion of its own.

Don k4kyv

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