Question E8B09:
What is deviation ratio?  

Choices:  
A: The ratio of the audio modulating frequency to the center carrier frequency  
B: The ratio of the maximum carrier frequency deviation to the highest audio modulating frequency  
C: The ratio of the carrier center frequency to the audio modulating frequency  
D: The ratio of the highest audio modulating frequency to the average audio modulating frequency    

Correct Answer is  B: The ratio of the maximum carrier frequency deviation to the highest audio modulating frequency  

To understand deviation ratio in FM:

- Definition:  
  $$\text{Deviation Ratio} = \frac{\Delta f_{\max}}{f_{m(\max)}}$$  
  where:
  - $\Delta f_{\max}$ = maximum carrier frequency deviation (Hz)
  - $f_{m(\max)}$ = highest audio/modulating frequency present (Hz)

Why B is correct:
- It matches the definition above. Deviation ratio is essentially the maximum FM modulation index, $\beta_{\max}$, using the system’s specified limits.

Why the others are incorrect:
- A: “The ratio of the audio modulating frequency to the center carrier frequency”  
  This compares unrelated quantities; center carrier frequency does not define FM deviation behavior.
- C: “The ratio of the carrier center frequency to the audio modulating frequency”  
  Produces a huge, meaningless number; not related to FM modulation depth.
- D: “The ratio of the highest audio modulating frequency to the average audio modulating frequency”  
  Has nothing to do with carrier deviation and is not an FM parameter.

Useful formulas and rules:
- FM modulation index for a single tone:  
  $$\beta = \frac{\Delta f}{f_m}$$  
  With maximum permitted values, $\beta_{\max} = \text{Deviation Ratio}$.
- Carson’s Rule for approximate occupied bandwidth:  
  $$B \approx 2\left(\Delta f_{\max} + f_{m(\max)}\right) = 2\,f_{m(\max)}\left(\text{Deviation Ratio} + 1\right)$$
- Example (broadcast FM): $\Delta f_{\max} = 75\,\text{kHz}$, $f_{m(\max)} = 15\,\text{kHz}$ ⇒ Deviation Ratio $= 5$.