CET Communication Systems & RF 2 — Questions and Answers
Question 1: What is the primary advantage of using FM over AM for voice communications?
- FM has greater range
- FM provides better noise immunity (Correct answer)
- FM uses less bandwidth
- FM requires simpler receivers
Correct answer: FM provides better noise immunity
FM is inherently more resistant to amplitude-based noise because information is encoded in frequency variations, not amplitude.
Question 2: Which modulation technique encodes data by shifting the carrier signal's phase between discrete values?
- ASK
- FSK
- PSK (Correct answer)
- QAM
Correct answer: PSK
Phase Shift Keying (PSK) encodes digital data by shifting the carrier phase to represent binary or multi-bit symbols.
Question 3: A transmitter operates at 150 MHz. What is the quarter-wave antenna length?
- 0.25 m
- 0.5 m (Correct answer)
- 1.0 m
- 2.0 m
Correct answer: 0.5 m
At 150 MHz, wavelength = 300/150 = 2 m; a quarter-wave element is 2/4 = 0.5 m.
Question 4: What does VSWR of 1:1 indicate in an RF transmission line system?
- Maximum power reflection
- Complete impedance mismatch
- Perfect impedance match with no reflected power (Correct answer)
- Signal attenuation of 50%
Correct answer: Perfect impedance match with no reflected power
A VSWR of 1:1 means the source, line, and load impedances are perfectly matched, resulting in zero reflected power.
Question 5: In a superheterodyne receiver, what is the purpose of the IF stage?
- To amplify the antenna signal directly
- To convert the signal to a fixed intermediate frequency for consistent filtering (Correct answer)
- To demodulate the carrier
- To suppress the local oscillator signal
Correct answer: To convert the signal to a fixed intermediate frequency for consistent filtering
The IF stage converts the incoming RF signal to a fixed intermediate frequency, allowing stable, high-selectivity filtering regardless of the tuned channel.
Question 6: What is the function of a bandpass filter in an RF communication system?
- Blocks all frequencies
- Passes a specific range of frequencies while attenuating others (Correct answer)
- Converts RF energy to audio
- Amplifies only low-frequency signals
Correct answer: Passes a specific range of frequencies while attenuating others
A bandpass filter passes signals within a defined frequency range (passband) and attenuates signals outside it.
Question 7: Which of the following describes the capture effect in FM receivers?
- The receiver locks onto the stronger of two co-channel signals (Correct answer)
- Both co-channel signals are received equally
- Weaker signals are amplified above the stronger
- FM receivers reject all signals above threshold
Correct answer: The receiver locks onto the stronger of two co-channel signals
The capture effect causes an FM receiver to suppress a weaker co-channel signal and lock onto the stronger signal when both are present.
CET Communication Systems & RF 2
What is the primary advantage of using FM over AM for voice communications?