[CW] Speed vs Bandwidth

[N2EY@aol.com]

In a message dated 3/13/04 8:54:08 AM Eastern Standard Time, 
buck03@towncorp.net writes:


> For some unknown reason, I have been bugged lately about the theory I
> learned 25 years ago that the faster a cw signal is sent, the broader
> the bandwidth of the signal.  

That's not exactly correct. See below.

I didn't get my license by learning the
> 
> questions and answers.  While I am not a CET or equivilent, I have
> studied electronics and had to get my license by learning theory.  (This
> isn't a put-down to those who used the q/a method, its my background so
> readers get a feel for my understanding of electronics).  
> 
> When I studied it, I had no choice but to accept the theory that the
> signal width (band-width) is actually greater, the higher the speed of
> the cw. 
> 

What really happens is this: The bandwidth of any on-off keyed signal is 
dependent upon the shape of the rise and fall times when it is keyed. If the rise 
and fall shapes are very fast and very straight, approximating a square wave, 
you get a very wide signal, with sidebands at all of the odd harmonics of the 
keying waveshape. OTOH if the rise and fall times are smoothed and slow, the 
signal can be much narrower. 

The limiting factor is that the process has to stop someplace. If you're 
sending 6 wpm Morse, (equivalent to sending the word "PARIS" once every 10 
seconds), a dit's on-time is exactly 200 milliseconds long. So you could have 20 
millisecond rise and fall times and a very narrow signal.

But if you increase the speed to 60 wpm, a dit is only 20 milliseconds long, 
and the rise and fall times must be shortenend - which makes the signal wider.

In practice, what is usually done with *well-designed* CW/Morse transmitters 
is that the rise and fall times are set for the highest speed likely to be 
encountered. The shape is something else again! But the end result is usually a 
pretty good one (there are notable exceptions, and the cost of a rig is not a 
guarantee that the design of the keying was done right). 

What this means is that the bandwidth of the transmitted signal from such 
rigs is the same regardless of the code speed. I don't know of any rigs that 
readjust the shaping when the code speed changes.

> In practice, I see that the more power I receive from a station, ie the
> greater the signal strength, the greater the tuning band-width that I
> can receive some noise of interference, as it may be.  However, in cw,
> whether its a carrier or someone sending 60+ wpm, at s-5 or so, I can't
> tell the difference between the width of either station.  

A lot of that has more to do with your receiver than the transmitted signal. 
Particularly if you have the AGC on.


> 
> Has anyone actually tested this theory on a spectrum analyzer?  Is this
> a theory we just have to accept or is there any real basis for the
> statement.  You folk might have to forgive me, but I can't immagine that
> there is actually a cw speed that could consume the entire 80 meter ham
> band.  I can imagine a cw speed that is so fast that on 80 meters it
> can't be copied reliably but, and maybe I understand it wrongly, I can't
> think of anything that will key on and off so fast that my rig will
> actually transmit two or three times the signal width in it's specs.  

It's all in the rise and fall times. 


> 
> The world won't come to an end if I never understand this, but if
> someone can explain, I would appreciate it.
> 
> 

Hope this helped.

Another factoid:

If you try to copy a high-speed Morse signal through a filter that is too 
narrow, the result is the same as trying to use rise and fall times that are too 
slow.

73 de Jim, N2EY


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