CET Communication Systems & RF 3 — Questions and Answers
Question 1: What is the impedance of a standard coaxial cable commonly used in RF systems?
- 25 ohms
- 50 ohms (Correct answer)
- 100 ohms
- 300 ohms
Correct answer: 50 ohms
50-ohm coaxial cable is the standard for RF and microwave systems, balancing power handling and signal loss.
Question 2: What type of antenna radiates equally in all horizontal directions?
- Yagi antenna
- Parabolic dish
- Omnidirectional antenna (Correct answer)
- Log-periodic antenna
Correct answer: Omnidirectional antenna
An omnidirectional antenna radiates and receives signals equally in all horizontal directions, making it ideal for general coverage.
Question 3: What does the decibel (dB) measure in RF systems?
- Absolute power in watts
- A logarithmic ratio between two power levels (Correct answer)
- Signal frequency in Hz
- Antenna gain in milliwatts
Correct answer: A logarithmic ratio between two power levels
The decibel expresses the ratio between two power levels on a logarithmic scale, making it easier to describe large gain or loss values.
Question 4: In a PLL (Phase-Locked Loop) frequency synthesizer, what component compares the reference and feedback frequencies?
- VCO
- Low-pass filter
- Phase detector (Correct answer)
- Frequency divider
Correct answer: Phase detector
The phase detector compares the reference oscillator frequency with the divided VCO output to generate an error signal for frequency correction.
Question 5: Which multiplexing method assigns each user a dedicated time slot in a shared channel?
- FDM
- CDMA
- TDMA (Correct answer)
- OFDM
Correct answer: TDMA
Time Division Multiple Access (TDMA) assigns each user a specific time slot within a repeating frame, allowing multiple users to share the same frequency.
Question 6: What is the primary cause of multipath fading in RF communication systems?
- Thermal noise in the receiver
- Multiple signal reflections arriving at the receiver with different delays (Correct answer)
- Transmitter power fluctuations
- Antenna polarization mismatch
Correct answer: Multiple signal reflections arriving at the receiver with different delays
Multipath fading occurs when reflected copies of a signal arrive at the receiver at slightly different times, causing constructive or destructive interference.
Question 7: What is the purpose of an RF circulator in a communication system?
- To amplify signals in all directions
- To route RF energy from one port to the next in a single direction (Correct answer)
- To convert RF to DC power
- To divide a signal equally among multiple outputs
Correct answer: To route RF energy from one port to the next in a single direction
An RF circulator routes signals from port 1 to port 2, port 2 to port 3, and port 3 to port 1, allowing a single antenna to be shared by a transmitter and receiver.
CET Communication Systems & RF 3
What is the impedance of a standard coaxial cable commonly used in RF systems?