CBT - Certified Broadcast Technologist Broadcast Transmission Systems Questions and Answers — Questions and Answers
Question 1: In a broadcast transmission system, what is the primary function of an isolator?
- To combine two different frequency bands onto a single antenna feed line.
- To protect the transmitter's power amplifier from reflected power. (Correct answer)
- To modulate the video and audio signals onto the RF carrier.
- To measure the Voltage Standing Wave Ratio (VSWR) of the antenna system.
Correct answer: To protect the transmitter's power amplifier from reflected power.
An RF isolator is a two-port passive device that allows RF signals to pass in one direction with very low loss, while absorbing and dissipating any power flowing in the reverse direction. This protects the transmitter's final power amplifier from damage or performance degradation caused by high levels of reflected power from the antenna or transmission line.
Question 2: A television station needs to transmit both its main VHF channel and a new UHF channel from the same antenna. Which device is required to combine these two signals onto a single transmission line without interference?
- A duplexer
- An isolator
- A circulator
- A diplexer (Correct answer)
Correct answer: A diplexer
A diplexer is a three-port device that combines or separates signals based on their frequency. It uses filters (typically a low-pass and a high-pass) to allow signals from two different frequency bands, such as VHF and UHF, to share a common transmission line or antenna.
Question 3: Which modulation method is the standard for over-the-air terrestrial broadcasting in the ATSC 1.0 digital television system?
- QAM (Quadrature Amplitude Modulation)
- OFDM (Orthogonal Frequency-Division Multiplexing)
- 8-VSB (8-level Vestigial Sideband) (Correct answer)
- COFDM (Coded Orthogonal Frequency-Division Multiplexing)
Correct answer: 8-VSB (8-level Vestigial Sideband)
The ATSC 1.0 standard, used for digital television broadcasting in North America, specifies 8-VSB as the modulation scheme for terrestrial broadcasts. This method uses a single carrier and modulates it to one of eight amplitude levels to encode the digital data.
Question 4: A transmitter has an output power of 1,000 watts. The transmission line has a loss of 3 dB, and the antenna has a gain of 10 dBi. What is the Effective Isotropic Radiated Power (EIRP)?
- 1,000 watts
- 2,500 watts
- 5,000 watts (Correct answer)
- 10,000 watts
Correct answer: 5,000 watts
To calculate EIRP, first convert the transmitter power to dBW: 10 * log10(1000W / 1W) = 30 dBW. Then, apply the formula: EIRP (dBW) = Transmitter Power (dBW) - Transmission Line Loss (dB) + Antenna Gain (dBi). So, EIRP = 30 dBW - 3 dB + 10 dBi = 37 dBW. Finally, convert 37 dBW back to watts: 10^(37/10) = 5011.87 watts, which is approximately 5,000 watts.
Question 5: Which of the following would result in a Voltage Standing Wave Ratio (VSWR) of 1:1 on a transmission line?
- The load impedance is an open circuit.
- The load impedance is a short circuit.
- The load impedance perfectly matches the characteristic impedance of the line. (Correct answer)
- The forward power is exactly half of the reflected power.
Correct answer: The load impedance perfectly matches the characteristic impedance of the line.
A VSWR of 1:1 represents a perfect impedance match, where the load impedance is equal to the characteristic impedance of the transmission line. In this ideal case, all power is transferred to the load, and there is no reflected power. An open or short circuit would result in an infinite VSWR.
Question 6: The ATSC 3.0 (NextGen TV) standard offers significant improvements over ATSC 1.0, including a more robust and flexible physical layer. What is the primary modulation scheme used in ATSC 3.0?
- 8-VSB
- QPSK
- OFDM (Correct answer)
- 16-QAM
Correct answer: OFDM
ATSC 3.0 replaces the single-carrier 8-VSB modulation of ATSC 1.0 with Orthogonal Frequency-Division Multiplexing (OFDM). OFDM divides the data stream across thousands of closely spaced, slower-moving subcarriers, which makes the signal much more resilient to multipath interference and allows for greater flexibility in data rates and robustness.
CBT - Certified Broadcast Technologist Broadcast Transmission Systems Questions and Answers
In a broadcast transmission system, what is the primary function of an isolator?