Amateur radio license exams typically have a question about the
bandwidths taken up by various modulation types. The concept behind the
question is pretty obvious - as guardians of the spectrum, operators
really should know how much space each emission type occupies. As a
result, the budding ham is left knowing that continuous wave (CW)
signals take up a mere 150 Hertz of precious bandwidth.

But is that really the case? And what does the bandwidth of a CW signal
even mean, anyway? To understand that, we turn to [Alan (W2AEW)] and
his in-depth look at CW bandwidth. But first, one needs to see that CW
signals are a bit special. To send Morse code, the transmitter is not
generating a tone for the dits and dahs and modulating a carrier wave,
rather, the "naked" carrier is just being turned on and off by the
operator using the transmitter's keyer. The audio tone you hear results
from mixing the carrier wave with the output of a separate oscillator
in the receiver to create a beat frequency in the audio range.

That seems to suggest that CW signals occupy zero bandwidth since no
information is modulated onto the carrier. But as [Alan] explains, the
action of keying the transmitter imposes a low-frequency square wave on
the carrier, so the occupied bandwidth of the signal depends on how
fast the operator is sending, as well as the RF rise and fall time. His
demonstration starts with a signal generator modulating a 14 MHz RF
signal with a simple square wave at a 50% duty cycle. By controlling
the keying frequency, he mimics different code speeds from 15 to 40
words per minute, and his fancy scope measures the occupied bandwidth
at each speed. He's also able to change the rise and fall time of the
square wave, which turns out to have a huge effect on bandwidth; the
faster the rise-fall, the larger the bandwidth.

It's a surprising result given the stock "150 Hertz" answer on the
license exam; in fact, none of the scenarios [Allen] tested came close
to that canonical figure. It's another great example of the subtle but
important details of radio that [Alan] specializes in explaining.

https://hackaday.com/2024/03/30/how-much-bandwidth-does-cw-really-occupy/
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