What are "key clicks"? How much does the rise time or bandwidth of a CW signal need to be limited to prevent other QSOs on adjacent frequencies from being annoyed by "key clicks"? Is there a legal limit regarding this (in the U.S.)?
How do (non-DSP) HF transmitters typically limit the bandwidth or rise/fall time of the keying on and off of a Morse Code dit or dah? (Is there a particular circuit one should look for in the schematic of a simple CW transmitter?)
W8JI has a great writeup on this topic. I'll summarize the key points.
Key clicks are generally undesired "clicks" or "thumps" generated by a CW transmitter as the key is put down or let up.
They can be caused several things. The most obvious is that a fast transition from carrier on to carrier off requires a wide bandwidth. You can think of this by considering that CW is a special case of AM with 100% modulation, where the baseband signal is a square wave. This square wave, theoretically unlimited in frequency, when multiplied with the carrier, will generate infinite sidebands.
Less obvious reasons arise from practical issues in transceiver construction. For example, a transceiver typically uses the same VCO for receive and transmit. When operating QSK and split, the VCO must change frequency with each change of the key. If the VCO does not settle fast enough, you get key clicks.
Modulating the carrier will always require some sideband power, so key clicks can't entirely be eliminated, but only reduced to an acceptable level.
W8JI's article says "The ARRL recommends a 5 mS [sic] rise and 5 mS fall time for CW, based on data in section 2.202 of FCC rules and CCIR Radio regulations." I don't care to dig into the original sources, but it sounds reasonable.
However, this is just a simple guideline. Just specifying the rise and fall times doesn't say much about the shape of the rise and the fall, which can have as much effect in the frequency domain as the rise and fall times. Remembering again that CW is a special case of AM, we'd want the rise and fall to be a sinusoidal envelope, since this limits the bandwidth maximally. Anything that's not sinusoidal will contain harmonics, which is exactly what we don't want.
So, how can this be implemented? You can filter the envelope (the on-off signal coming from the key), or you can filter the output. Filtering the envelope can be viewed as pulse shaping, a common technique among digital modulations.
The simplest approach would be to put a simple RC filter on the envelope, which is a little better than nothing. However, with a single-pole filter, you can get at best a 3dB/octave reduction in the harmonics of the envelope. Remember, we want ideally a sinusoidal envelope. A single-pole filter gets us closer, but not all that much, because although the harmonics from the rectangular envelope are attenuated, such a simple filter can't attenuate them very much.
We can make a better filter with more poles, giving us a steeper frequency roll-off, better attenuation of the harmonics, and thus a better approximation of our sinusoidal envelope, but this comes with increased cost. However, all radios come with what should be a pretty good filter: the receive filter. As it turns out, most radios send the transmitted signal through the same filter as is used for receive, since this is the most practical approach.
W8JI elaborates in his article that many transmitters use the SSB filter for this purpose, which is great, except that it's a lot wider than it needs to be. SSB filters are around 3 kHz wide, while just around 200 Hz would give us the ARRL recommended rise and fall times of 5 ms. He argues (and I tend to agree) that radios should be transmitting through their CW filter, instead of the SSB filter.
What are "key clicks"?
Morse code can be considered a square wave, and CW is then that square wave added to the carrier frequency. The sharp edges at the start and stop of the carrier include a lot of frequencies that aren't necessary to send good code. These frequencies interfere with adjacent spectrum, well outside the 80-200Hz needed for CW.
Since they are short in duration, they can sound like clicking to those listening on adjacent frequencies.
How much does the rise time or bandwidth of a CW signal need to be limited to prevent other QSOs on adjacent frequencies from being annoyed by "key clicks"?
One can filter their bandwidth to 80-200Hz going out of their keyer, the same way one would filter CW reception to 500Hz. Some use pulse shaping techniques to attain a specific rise time, others use simple RC circuits to define the shape of the waveform before the carrier is applied.
However, "rise time" is not a great way to put metrics on it. It's one way to stay inside the desired bandwidth, but since good practices and legal regulations are centered around bandwidth considerations, one should verify their rig maintains the smallest bandwidth required for the communications desired.
Is there a legal limit regarding this (in the U.S.)?
The only legal limit is the requirement to use only as much bandwidth as the specific communication requires. For CW, this varies based largely on the sending speed. 20wpm Morse can be limited to 80Hz bandwidth. Faster sending requires more bandwidth for good signal quality.
It is unlikely that you'll get in trouble for using more bandwidth than needed, but your fellow operators will appreciate your efforts.
How do (non-DSP) HF transmitters typically limit the bandwidth or rise/fall time of the keying on and off of a Morse Code dit or dah? (Is there a particular circuit one should look for in the schematic of a simple CW transmitter?)
A simple CW sender with some passive filtering will probably have a simple resistor in series with the key signal and a capacitor to ground after the resistor. It has some drawbacks, but a simple circuit like this can provide basic key click filtering. There are some example circuits here.
