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Re: LF: Key Clicks

To: [email protected]
Subject: Re: LF: Key Clicks
From: Rik Strobbe <[email protected]>
Date: Wed, 25 Aug 2004 12:28:57 +0200
In-reply-to: <[email protected]>
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Hello Hugh,

Just look at CW signal as an AM signal.
Assume you have very hard edged "on / off" keying  and key an long series of dots at a speed of 12 WPM :
12 WPM = 600 dotlengths per minute (according to the "PARIS" system 1 word = 50 dotlengths), so in fact you modulate an AM TX with a 5 Hz rectangular "audi signal".
The spectrum of this signal will be a carrier with a whole bunch of sidebands in 5 Hz intervals from the carrier. The first sidebands (carrier +/- 5Hz) is useful all the other (higher) sidebands are waste (wasted power + useless bandwidth) and and show up as keyclicks.
As the keying gets softer (longer rise- and falltime of the keying) the higher sideband will become weaker (less keyclicks). In order to have no higher sidebands at all you would have to get modulation (= keying) sinusoidal. In that case the modulation signal has no harmonics and thus keying will be clickless.
So far the theory, because the above assumes a long (a la limite endless) series of dots.
Our real-world CW instead is a rather random sequence of dots and dashes. But regardless how you do the keying the worst condition (largest bandwidth) is a series of dots. This means that the "useful bandwidth" of a CW signal is determined by speed and the maximum "audio component" of the keying can be determined as :

Fmax = WPM/2.4    where Fmax = the highest useful  "audio frequency" (in Hz) and WPM = keying speed (in words per minute)

So the ultimate way to get a clean CW signal is to filter they keying signal through a good LPF at Fmax.
But in most cases a simple RC filter will give the best "effort vs. result" ratio.

73, Rik ON7YD

At 10:32 25/08/2004 +0100, you wrote:
Good morning all,
I keyed my original LF TX with a relay - crude, but it got me on the air.

I know many designs just filter the keying waveform to "round off the
corners" of the envelope.
But I read that the best envelope shape is a "Gaussian waveform". This
produces the smallest sidebands.
[ and lots of other
stuff ]
RC and LC filter circuits produce exponential waveforms, although the
waveform diagrams that accompany these circuits in the texts often make then
look as though they produce a gaussian waveform.

My question is ...
Is there a circuit that will produce a nice "gaussian wavform" that I could
feed into an AM modulator to produce really clean CW - or is there no such
thing ?

Hugh M0WYE

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