On 7/27/2014 10:27 PM, Jim Brown wrote:
Off list.
Well, I meant to send it off list, but it was late. :)
I've been thinking about this.
Overnight, I thought some more.
The keying waveform is, fundamentally, modulation of a continuous
carrier by a waveform that approximates a square-wave -- hence the name
CW for continuous wave. ANY square wave has an infinite number of
harmonics, the relative strength of which depend primarily on the shape
of the switching waveform from on to off and off to on. The faster the
rise time, the more harmonics, and those harmonics will be stronger.
Those harmonics show up as sidebands, and the higher the order (harmonic
number), the farther they are from the carrier. In a rig without IMD,
that would be the end of the story, but real amplifiers have IMD, so
those harmonics produce IMD, which results in more sidebands.
Rigs with cleaner keying reflect the skill of the designer to minimize
the harmonic content of the keying waveform, the level of TX IMD, and TX
phase noise.
Close to the carrier, the key click spectra plots pretty much tell the
whole story of what the TX is doing, because they include the effects of
the keying waveform, phase noise and IMD. As we listen farther off
frequency from the transmitter, all that's left is phase noise.
73, Jim K9YC
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