[Milsurplus] Command set spurs
Miles Anderson
k2cby at optonline.net
Thu Jun 22 09:15:33 EDT 2006
Hey, let's not malign Class C output stages. Until SSB came along, virtually
all AM and CW transmitters used Class C RF amplification for the PA, IPA and
Frequency multipliers. Everything, in fact, except the audio stages and the
oscillator (if you wanted to keep the drift down to a minimum). It was up to
the final (and intermediate) tank circuits to kill off the harmonics below
30 MHz. An antenna coupler also helped. The problem was that you were
relying on the Q of a single tuned circuit. Unless the output impedance was
properly matched the mismatch dropped the Q and up went the harmonics. In
the case of a command set this meant proper adjustment of the series loading
coil and the rotating link inside the PA tank coil. This was simply the
state of the art until PI networks came along in the mid 1950s. The chief
advantage of the PI network was that, properly adjusted, it would couple
your Class C final (a la Heathkit DX-35, DX-100, etc.) to a wide range of
impedances without degrading the Q (and harmonic suppression) all that much.
Miles, K2CBY
Miles Anderson
16 Round Pond Ln
Sag Harbor, NY 11963
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