[AMPS] Q of twelve

Mon, 11 Aug 1997 12:28:51 +0000

> From:          Peter Chadwick <Peter.Chadwick@gpsemi.com>
> Date:          Mon, 11 Aug 97 10:01:07 +0000

> Tom says
> 
> >Linear PA's, with their longer conduction angles, generate much less 
> >harmonic energy than the old class C PA's. Class AB PA's make
> >very poor harmonic generators, that's why doublers and triplers are 
> >all operated in low conduction angle modes (like class C). 
> 
> Sorry Tom, I don't agree. In a class B stage, (and the same pretty well
> applies to a Class AB stage) the second harmonic component of plate
> current is about 6dB down on the fundamental - see Pappenfus et al.,
> p164 et seq.  In a Class C stage, the second harmonic is slightly lower,
> while (assuming a 120 degree conduction angle) the third harmonic is
> rather higher than at 180 degree conduction - see Termans 'Radio

Hi Peter, 

While the figures you gave seem reasonable, I'm sure you know most 
linear PA's aren't operated at 180 degree conduction angles, just as 
most multipliers or class C PA's aren't operated at 120 degrees. As a 
matter of fact a 120 degree conduction angle often produces very 
poor AM modulation linearity.

Speaking in general terms, the amount of harmonic energy increases as 
conduction angles are shortened.

There is absolutely no reason to pick an arbitrary rule of Q having 
to be 12. Everything improves as Q is reduced EXCEPT harmonic 
suppression. Once the PA has enough harmonic suppression, additional 
Q wastes efficiency and needlessly stresses the components.

I suspect the Q=12 rule came from having the old grid driven class C 
tubes meeting acceptable harmonic standards, since virtually EVERY 
linear cathode driven PA grounded grid PA will meet FCC with Q's of 
much less than 12.  As a matter of fact doubling the Q often provides 
almost no improvement in second harmonic suppression in many cases. 

My point was that the Q=12 "suggestion" has become a "rule" only  
because people have no idea WHY a certain Q is selected.

People now seem to think Q=12 is perfect, and lower Q will cause poor 
performance and low efficiency. 

In reality maximum efficiency and lowest tank component stress occurs 
with the lowest practical operating Q. The only change by increasing 
Q from some minimal value to 12 is losses-- and tank heating--- 
increase while harmonic suppression goes up a few dB out of 40 or 50 
dB.

Every automobile and truck tire in the world should have exactly 32 
PSI air pressure, and every tank a Q of twelve. Each rule makes as 
much sense.

73 Tom

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