Hi All,
Thought I'd try a bit of calm consideration in the key click
debate,and thus offer the following discourse:
Key clicks are broad-band radiation produced as a result of the rate
of rise and decay of the amplitude of the keyed carrier. The faster
the rise and fall, the closer to a square wave is the envelope and
the richer in harmonics is the resulting signal. Note that these are
not harmonics of the signal frequency, but of a very low frequency
related to the characteristics of the carrier's rise and fall. These
mix with the carrier frequency to produce a broad signal centred on
the carrier frequency. No amount of filtering of the clicky signal
can remove the clicks, although they are of course attenuated at
frequencies removed from the carrier frequency by any selectivity
between their source and the antenna.
The only way to prevent key clicks is to ensure that the rise and
fall times of the keyed carrier are long enough to avoid significant
harmonic generation.( Although too long rise and falls results in a
"mushy" sounding signal that can be difficult to copy).
Once a clean keyed signal has been produced, it has to be preserved
through the remainder of the transmitter amplifier chain. A truly
linear amplifier will reproduce the input signal perfectly, and the
output will be click free. But note that maintaining linearity when
dealing with a switched carrier places severe demands on the
amplifier - a favourite problem is with sagging supply voltages.A
'scope may show things a meter can't!
Any form of "switching" mode amplifier (ie class C and above)is
inherently non - linear and relies on the flywheel effect of its
tuned circuits to produce a sine wave, and to filter harmonics
from its output. This results in the steepening of the rise time of
the signal, and can cause the appearance of clicks on a signal that
was clean when generated. The effect can be counteracted by
increasing the rise time of the keyed signal to allow for subsequent
"sharpening" of the waveform during amplification.
In conclusion, any form of amplifier or exciter can be a satisfactory
solution provided it is designed and operated to be so - and this
includes digital techniques,provided that the appropriate filtering
and signal conditioning is used.
Thats all from my soap-box for now, thanks to all those carrying out
any work on the VLF bands, whether wheel reinventing or not - what
matters to me is that there will be people to talk to when I
eventually get my transmitter (clicky or otherwise) on the air!
73 de Paul G4MD
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