Question
Why is de-emphasis used in FM communications receivers?
Answer Options
- A) For compatibility with transmitters using phase modulation
- B) To reduce impulse noise reception
- C) For higher efficiency
- D) To remove third-order distortion products
Correct Answer: A
Explanation
De-emphasis is the reciprocal process of pre-emphasis, and it is performed in the receiver’s audio stage. Pre-emphasis circuits in FM and Phase Modulation (\text{PM}) transmitters deliberately boost high audio frequencies to improve the signal’s noise immunity over the transmission link.
De-emphasis is used in \text{FM} communications receivers for compatibility with transmitters using phase modulation (or \text{FM}). The de-emphasis network is an \text{RC} filter that attenuates the high audio frequencies by the exact amount they were boosted at the transmitter. This flattens the frequency response back to normal while simultaneously reducing the noise that was introduced in the receiver’s intermediate frequency stage, thus improving the overall signal-to-noise ratio.
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