[CW] CORRECTION - Speed vs Bandwidth

[David J. Ring, Jr.]

George,

I think the major factor in CW bandwidth is the waveshape.  I believe we are
in agreement that you can take a 5 wpm signal and make it wider by changing
the waveform to have less rise and fall time.

But I think that the actual pulse rate also influences the overall
bandwidth, and I believe you don't agree with that.  I believe it is lesser
factor, but it exists.  I believe the reason it exists is that the pulse
rate (and the frequency is the inverse of the pulse rate) is directly
proportional to the increase in bandwidth.

If when you spoke the pulse rate of the modulation were increased, I believe
the bandwidth would also increase due to this (don't they call something
like this the "slew rate" of an amplifier?) but compared with the bandwidth
needed to pass the high frequencies this increase would be a tiny component.
I believe if you took a 20 ms pulse (say from a recording of the human voice
with it's two tones) and then somehow changed the duration of the pulse -
leaving both frequency components of the voice unchanged, the bandwidth
would change in the upper direction.  This would be due to the change in the
repetition rate of the pulses (PRR).

Jeffrey Herman, KH6O found this for me from from the ARRL Extra Class
License Manual (1991):

"The bandwidth of a CW signal is determined by two factors: the
speed of the CW being sent and the shape of the keying envelope.
The usual equation for the calculation is Bw = B x K, where
B = the speed of the transmission in bauds, and
K = a factor relating to the shape of the keying envelope..."

Jeff adds:  Their derivation of K is a bit vague: "K is a measure of keying
shape. As CW rise and fall times get shorter (more abrupt, harder
keying), K gets larger... On paths with good strong signals, soft
keying can be used, and K will be around 3. On fading paths,
harder keying is necessary, resulting in a K of 5...."

Kai Siwiak, KE4PT who wrote , "Radiowave Propagation and Antennas for
Personal Comminications", [Secons Edition, Artech: Norwood MA, 1998.] sent
me earlier this information:

Lets look at a 'DIT' sequence at 25 WPM. That will produce an ON-OFF
sequence of pulses at a 20 Hz rate - the occupied BW will be a sin(x)/x with
20 Hz between the first nulls.

I don't understand what (x) is in his equation!  So I can't work with the
equation!

Does it help any for you?

73

David Ring, N1EA

----- Original Message -----
From: "George, W5YR" <w5yr@att.net>
To: <cw@mailman.qth.net>; "David J. Ring, Jr." <n1ea@arrl.net>
Sent: Monday, March 15, 2004 12:51 PM
Subject: Re: [CW] CORRECTION - Speed vs Bandwidth


> Some controversy exists over what is responsible for keying artifacts, but
> it seems fair to say that rise and fall times and the actual keyed
waveshape
> are the principal causes.
>
> In my view, keying speed enters into the picture only if the rise and fall
> times are decreased at the higher keying rates in order to maintain
> readability and prevent "soft" keying which tends to run the code elements
> together. Sending faster, like talking faster on SSB, does not tend to
> require a wider bandwidth if the keyed waveform is not changed.
>
> The theory of bandwidth vs rise/fall time is clearly understood and has
been
> around since the late 19th century, so nothing much new there. Any text on
> transform calculus applies.
>
> The remaining element to the picture, I believe, is the actual keyed
> waveform or shape. The critical points of the waveform which can account
for
> bandwidth increases not predicted solely by rise and fall time are the
> transitions in the waveform from off to leading edge start; leading edge
> finish to on; on to start of trailing edge; and end of trailing edge to
off.
> If these transitions are abrupt, transform calculus tells us that
> high-frequency transients will be produced that can be either prevented or
> minimized by making the transitions "gentle" or "rounded" instead of
square
> and abrupt.
>
> An example is the raised cosine keying waveform which at slow speeds is
> virtually without transients but which is difficult to read at higher
speeds
> or in QRN.
>
> It is encouraging to see that some manufacturers are paying attention to
> this aspect of keyed waveform design. In a DSP transmitter, it is almost
> entirely a matter of firmware and there is little excuse of having a CW
> signal with clicks or any distortion of dot/space timing at any speed.
>
> 73, George W5YR
> Fairview, TX
> w5yr@att.net
> http://www.w5yr.com
>
>
> ----- Original Message -----
> From: "David J. Ring, Jr." <n1ea@arrl.net>
> To: "George, W5YR" <w5yr@att.net>; <cw@mailman.qth.net>
> Sent: Sunday, March 14, 2004 11:50 PM
> Subject: Re: [CW] CORRECTION - Speed vs Bandwidth
>
>
> > I can't find that.  I know these figures aren't for square waves.
> >
> > Probably they are adjusted for softness or hardness depending on the
> > circuit.
> >
> > You need a harder waveform for a bad circuit.  The harder waveform makes
> > more harmonics and a wider signal.
> >
> > That would put the waveshape at 5 ms for soft (non-fading circuits) and
3
> ms
> > for hard (fading) - which I think is about right range, but 5 ms is fine
> for
> >
> > I have seen 20 ms waveshape which of course made the circuit ring like a
> > bell - beautiful copy for local work though - fun to copy when signals
are
> > right and code speed is below about 25 wpm.
> >
> > On the other hand if the message has to get there, and conditions are
> > terrible, I would adjust the waveshape to 3 ms or so - there would be
much
> > more clicking - but that helps reception.  Something I'd recommend to
> > QRPers!
> >
> > Do you have any sources on this?  I can't find the definative answer!
> >
> > 73
> >
> > David Ring
> > N1EA
> >
> >
> > ----- Original Message -----
> > From: "George, W5YR" <w5yr@att.net>
> > To: "David J. Ring, Jr." <n1ea@arrl.net>; <cw@mailman.qth.net>
> > Sent: Monday, March 15, 2004 12:16 AM
> > Subject: Re: [CW] CORRECTION - Speed vs Bandwidth
> >
> >
> > > David, what is the assumed waveshape?
>
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