Question E8D05:
What is the most common method of reducing key clicks?  

Choices:  
A: Increase keying waveform rise and fall times  
B: Insert low-pass filters at the transmitter output  
C: Reduce keying waveform rise and fall times  
D: Insert high-pass filters at the transmitter output    

Correct Answer is  A: Increase keying waveform rise and fall times  

To reduce key clicks, you smooth the CW envelope so the RF amplitude doesn’t change abruptly. That’s done by increasing the keying waveform rise and fall times (i.e., envelope shaping).

Why A is correct (Increase keying waveform rise and fall times):
- Key clicks are caused by very fast on/off transitions (sharp edges) in the CW envelope, which introduce wideband sidebands around the carrier.
- Slower, shaped transitions reduce high-frequency components in the keying spectrum, narrowing the occupied bandwidth and suppressing clicks.
- Modern rigs do this with “raised-cosine” or similar shaping on the CW envelope.

Why the other choices are incorrect:
- B: Insert low-pass filters at the transmitter output
  - Key clicks are close-in sidebands around the carrier, not high-frequency harmonics. An RF LPF passes these sidebands and won’t fix the clicks.
- C: Reduce keying waveform rise and fall times
  - Shorter (faster) rise/fall times mean sharper edges, which increases spectral splatter and makes key clicks worse.
- D: Insert high-pass filters at the transmitter output
  - An HPF would remove low frequencies and pass higher ones, which is the opposite of what you want. It does nothing to suppress close-in sidebands from fast keying.

Useful formulas and rules of thumb:
- Click bandwidth scales inversely with rise/fall time:
  $$BW \propto \frac{1}{t_r}$$
  where $t_r$ is the keying rise (or fall) time.
- A rough rule: doubling $t_r$ roughly halves the click bandwidth.
- Practical guideline: use rise/fall times of about 5–8 ms (10–90% envelope) on HF to minimize audible clicks while keeping CW “snappy” enough for good copy.