Question E8C12:
What factors affect the bandwidth of a transmitted CW signal?  

Choices:  
A: IF bandwidth and Q  
B: Modulation index and output power  
C: Keying speed and shape factor (rise and fall time)  
D: All these choices are correct    

Correct Answer is  C: Keying speed and shape factor (rise and fall time)  

Correct answer: C — Keying speed and shape factor (rise and fall time)

Why C is correct:
- A CW signal is on-off keyed, so its spectrum is set by the time-domain keying envelope.
- Faster keying speed (higher WPM) means more transitions per second, which increases the occupied bandwidth.
- Sharper keying edges (short rise/fall times) create more high-frequency components (key clicks), widening the signal. Smoother, longer rise/fall times concentrate energy near the carrier and narrow the bandwidth.

Why the others are incorrect:
- A: IF bandwidth and Q
  - These are receiver selectivity parameters. They affect how you receive CW, not the bandwidth of what you transmit.
- B: Modulation index and output power
  - Modulation index applies to angle modulation (FM/PM), not on-off keyed CW. Output power does not set bandwidth (assuming linear operation).
- D: All these choices are correct
  - False because A and B do not control transmitted CW bandwidth.

Useful formulas and rules of thumb:
- Dot length vs speed:
  $$T_{dot} = \frac{1.2}{\text{WPM}} \text{ s}$$
- Edge-shaping bandwidth trend:
  - Bandwidth grows as rise/fall time $T_r$ gets shorter. A common approximation for the frequency extent set by the edge is:
    $$B_{\text{edge}} \approx \frac{0.35}{T_r}$$
    Example: $T_r = 5 \text{ ms} \Rightarrow B_{\text{edge}} \approx 70 \text{ Hz}$
- Practical guidance:
  - Use shaped keying with rise/fall times of about 4–8 ms (5 ms is common) to minimize key clicks.
  - For the same shaping, doubling WPM roughly doubles the occupied CW bandwidth.