I need to dig out the results of some studies I made about a year ago
that clearly demonstrated that most of the keying artifacts that we
object to result from the manner by which the leading and trailing
edges of each keyed code element leave and attain their initial and
final values.
It is true that rise and fall times and keying rates are involved in
determining the basic keyed spectrum, but there is a source of
additional spurious output that is not commonly known or discussed.
And that is the manner in which the keyed character is actually
formed.
If the leading edge, for example, is an abrupt change from zero to
whatever the slope is and ends abruptly at the final value, then
regardless of keying speed or slope, there will be additional keying
artifacts present.
On the other hand, if the waveform presents a gentle transition from
off to the slope and a gentle transition to the maximum value, and
ditto for the trailing edge, then the artifacts are almost
imperceptible.
Any keyed waveform will have a non-zero spectrum width. That width is
determined largely by the slopes of the leading and trailing edges as
elementary Fourier analysis tells us. A secondary cause that can, and
apparently does in the cases I studied, generate artifacts at least as
bad if not much worse, is the transition from off to risetime to on
and back down again on the trailing edge.
This effect is capitalized upon with so-called "raised cosine" keying
waveforms which have very gentle "slow" transitions and are noted for
generating a minimum of keying artifacts.
As a general statement. after watching CW waveforms on a scope at all
times on the audio output of my Icom 756PRO for over two years now, I
am convinced that 99% of all key clicks and other artifacts that I
hear are actually present on the transmitted signal.
A clean signal on the scope sounds clean without clicks and thumps
even though I may be listening through a 50-Hz IF-DSP filter.
Our older crystal filters simply did not have the shape factor and
selectivity to isolate the higher-frequency artifacts from the "main
signal" so we did not notice the trash as much. I have made direct
comparisons between a 765 and the PRO on the same signal and could
clearly see and hear the artifacts with the PRO and scope that were
all smudged together by the crystal filters in the 765 so that I heard
only the lower-frequency thumps instead of the clicks, etc.
But, again, the culprit in my view lies in the detailed behavior of
the keyed waveform in the transitions from off to risetime to on to
falltime to off again. If those transitions are gentle (small second
derivatives), then even with relatively short rise and fall times, the
keying will sound clean. But, it the transitions are abrupt, even with
long rise and fall times, there will be clicks.
Take a look at your r-f output envelope while sending a string of dots
or dashes and focus upon the leading or trailing edge to see how the
transition is actually made. I have seen some rigs that start with a
gentle curved transition from off to the leading edge and then make an
abrupt change from the slope of the leading edge to the flat-top on
the "on" code element. Other rigs key differently, and in general
those with the abrupt waveform elements are those with the clicks.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489 Icom IC-765 #2349 Icom IC-756 PRO #2121
----- Original Message -----
From: "Barry N1EU" <n1eu@yahoo.com>
To: <tentec@contesting.com>
Sent: Tuesday, November 26, 2002 4:00 PM
Subject: Re: [TenTec] CW Key Clicks and Filters
> The rampant clicks I am referring to are +/- 500-800hz
> from the fundamental and clearly audible with a xtal
> filtered receiver. They are not the artifact of the
> receiver, they are present in the xmit signal (keying
> rise/fall time too fast) of some (not all) modern
> rigs. Just tune across 40M and you'll hear some
> signals that are completely clean and some signals
> that are horrendously clicky.
>
> 73,
> Barry N1EU
>
> --- Steve Ellington <N4LQ@igLou.com> wrote:
> > Now let's tune our RX
> > to within 200hz of a strong cw signal. You will hear
> > strong clicks. Why?
> > Because they are there. That's why. No matter how
> > clean his TX is, there's
> > still clicks when you tune that close. > -----
> Original Message -----
> > From: "Barry N1EU" <n1eu@yahoo.com>
> > To: <tentec@contesting.com>
> > Sent: Tuesday, November 26, 2002 2:57 PM
> > Subject: [TenTec] CW Key Clicks and Filters
> >
> >
> > > W8JI deserves a lot of credit for drawing
> > attention to
> > > this serious problem. Clicks are a function of
> > the
> > > rise and fall time of the trasmitted waveform, and
> > > should therefore be independent of keying speed.
> > >
> > > I have to take exception to W4BQF: "I seriously
> > doubt
> > > that there were very many contest stations
> > generating
> > > key clicks during the contest as most contesters
> > are
> > > pretty knowledgeable technically."
> > >
> > > I don't know which contest is being referred to,
> > but
> > > key clicks were rampant on the bands in CQ WW and
> > > they're caused by poor transmitter design, and not
> > > poor operators or poor receivers. Some rcvr
> > filters
> > > can emphasize the clicks, but if the clicks are
> > being
> > > transmitted you're going to receive them.
> > >
> > > 73,
> > >
> > > Barry N1EU
> > > www.albany.net/~bg
> > >
> > > __________________________________________________
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> > >
> >
> >
>
>
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