[1000mp] Key Clicks -- My MP Experience

[George, W5YR]

Ian, not to sound too far out in left field, but this does not require a lot
of high-powered math. It is simple transform calculus and spectral
measurement. I have made spectral plots of the keying of my IC-756PRO2 and
IC-746PRO using one radio as the transmitter and the other as the monitor
receiver, and examining its audio output with a pro-audio spectrum analysis
program (not Spectrogram). It remains to predict the spectrum based upon the
properties of the waveform.

Experimentally, we know that a raised cosine, for example, is virtually
without artifacts though considered too soft for practical application.
Incidentally, the CWID keying tacked on the end with the PSK31 programs is
usually of that ilk and you can hear how soft it is.

I have put off going through the exercise with MathCAD or Mathematica out of
saturation with other commitments. But, maybe in a few weeks I'll take a
look at it.

But, to my mind there are four main causes for bandwidth expansion of a
keyed waveform, not in rank order: rise time, fall time, transitions, and
rate or keying speed.

A straightforward analysis should give the rank order and point toward
improved keying waveforms. But experimentally, I can see wider spectra for
those transmitters with sharp transitions, other aspects being the same,
than for those with smoother transitions.

It is good to see the amateur community taking serious notice of this
problem, although Yaesu seems to be catching the brunt of the abuse -
probably well deserved, however.

Thanks for your note and interest, Ian.

73, George W5YR
w5yr@att.net


----- Original Message ----- 
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
To: <1000mp@mailman.qth.net>
Sent: Thursday, February 05, 2004 2:44 AM
Subject: Re: [1000mp] Key Clicks -- My MP Experience


> George,  W5YR wrote:
> >Harry, unfortunately it is not as simple as that. One of the key
> >factors in determining click bandwidth is the manner in which the
> >keying waveform is accomplished. Specifically, all transitions from off
> >to leading edge onset to leading ledge completion to full power on, and
> >the trailing edge counterpart, all tend to contribute added bandwidth
> >unless the transitions are "smooth." Sharp on/off transitions generate
> >probably as much excessive bandwidth as does an overly short rise/fall
time.
> >
>
> ARRL's "target" figure of 5ms should give plenty of time for a clickless
> transition... but only if the *shape* of the transition is right too.
>
> What we need to identify are *exactly* which features of the waveshape
> are causing the wideband clicks. Is it the average rise time, or the
> sharpness of the corners... or both? Which features are important, and
> which don't matter much? Unfortunately that kind of insight needs some
> high-powered mathematical input.
>
> By coincidence, a ham in Europe who is a DSP expert and a high-powered
> mathematician has recently started to get interested in this problem.
> When there's news, I'll report back.
>
>
> -- 
> 73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
>                             Editor, 'The VHF/UHF DX Book'
> http://www.ifwtech.co.uk/g3sek
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