Question E4C03:
What is the term for the suppression in an FM receiver of one signal by another stronger signal on the same frequency?  

Choices:  
A: Desensitization  
B: Cross-modulation interference  
C: Capture effect  
D: Frequency discrimination    

Correct Answer is  C: Capture effect  

Correct Answer: C — Capture effect

1) Why C is correct
- In FM receivers, the limiter and discriminator favor the stronger of two co-channel FM signals. When two signals are on the same frequency, the stronger one “captures” the receiver, effectively suppressing the weaker one. This is called the capture effect.

2) Why the other choices are incorrect
- A: Desensitization — A strong off-frequency or nearby signal drives the receiver front end or AGC into a state that reduces sensitivity to other signals. It’s not specifically the suppression of one co-channel FM signal by another.
- B: Cross-modulation interference — A very strong signal modulates the gain of a nonlinear stage, causing its modulation to appear on another signal. Common with AM in overloaded stages; not the FM co-channel capture behavior.
- D: Frequency discrimination — This is the detection method for FM (converting frequency deviation to voltage), not the phenomenon of one signal suppressing another.

3) Useful formulas and rules of thumb
- Capture ratio (CR) is the minimum power difference needed for the stronger FM signal to dominate:
  $$\text{CR (dB)} = 10 \log_{10}\!\left(\frac{P_{\text{strong}}}{P_{\text{weak}}}\right)$$
- Rule of thumb: Many narrowband FM receivers have a capture ratio of about 3–6 dB (e.g., 6 dB ⇒ $P_{\text{strong}}/P_{\text{weak}} \approx 4:1$). A lower capture ratio is better (easier to capture).