Key clicks and linears
Thu, 13 Apr 2000 14:59:26 -0400
> 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