*Question E8D04:
What is the primary effect of extremely short rise or fall time on a CW signal?*  

*Correct Answer is  C: The generation of key clicks*  

### AI Response: (may contain errors)  

Extremely short rise or fall times in a CW (Continuous Wave) signal can lead to the generation of key clicks. Here's why:

1. **Correct Answer: The generation of key clicks (C)**  
   - **Explanation**: Key clicks are unwanted sidebands that occur when a CW signal is turned on or off too abruptly. This is due to the fact that a rapid change in amplitude (rise or fall time) results in a broad frequency spectrum, which can cause interference on nearby frequencies. The abrupt transitions create sharp edges in the time domain, which translates to a wide frequency spread in the frequency domain, leading to key clicks.

2. **Why the other choices are incorrect**:
   - **A: More difficult to copy**  
     - While key clicks can cause interference, they do not inherently make the CW signal itself more difficult to copy. The main issue is the interference they cause to other signals.
   - **B: The generation of RF harmonics**  
     - RF harmonics are typically related to non-linearities in the transmitter, not directly to the rise or fall time of a CW signal. Key clicks are more about the bandwidth of the signal rather than harmonics.
   - **D: More difficult to tune**  
     - Tuning a CW signal is generally related to its frequency stability and not the rise or fall time. Key clicks do not affect the ability to tune into the signal itself.

3. **Useful Concepts**:
   - **Fourier Transform**: A signal with sharp transitions in the time domain (like those with very short rise or fall times) will have a wide frequency spectrum. This is a fundamental concept in signal processing that explains why key clicks occur.

By ensuring that the rise and fall times are not too short, you can minimize the generation of key clicks and reduce interference with other signals.