Dick and Adele Bingham wrote:
Don't try to
> use one of the narrow extra-deep notch resonant filters that
> have second harmonic traps and the like. In the first place
> you probably won't need it, in the second place voltages and
> currents will be too high, and in the third place you might
> not terminate the PA stage with the proper load impedance on
> harmonics to prevent high voltages from damaging the FET's.
You normally want an inductor input filter so that the FET can
clip if it wants to, but no current will be drawn at harmonic
frequencies. For a given peak to peak voltage, a square wave
has 16/( pi squared) = 62% more power in the fundamental that
a sine wave does, using Fourier series analysis. This gives
you a lot more headroom. This seems to be a little known
fact, since I often see capacitor input filters used.
Rick N6RK
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