GROL Receivers and Transmitters 3 — Questions and Answers
Question 1: What is the function of a buffer amplifier stage in a transmitter?
- To increase the output power to the antenna
- To isolate the master oscillator from load variations that could cause frequency pulling (Correct answer)
- To modulate the carrier with audio
- To convert the oscillator output to a lower frequency
Correct answer: To isolate the master oscillator from load variations that could cause frequency pulling
A buffer amplifier presents a stable, high-impedance load to the oscillator so that changes in subsequent stages do not pull the oscillator off frequency.
Question 2: At 100% amplitude modulation, what percentage of total transmitted power is contained in the carrier?
- 100%
- 66.7% (Correct answer)
- 50%
- 33.3%
Correct answer: 66.7%
At 100% modulation, each sideband carries 25% of the carrier power, so total power = 1.5 × Pc; the carrier fraction is 1/1.5 ≈ 66.7%.
Question 3: A frequency multiplier stage in an FM transmitter:
- Increases output power while keeping frequency constant
- Multiplies both the carrier frequency and the frequency deviation by the same integer factor (Correct answer)
- Converts FM deviation to AM modulation
- Reduces the bandwidth of the transmitted signal
Correct answer: Multiplies both the carrier frequency and the frequency deviation by the same integer factor
Frequency multipliers multiply every frequency component, so both the center frequency and the peak deviation increase by the same factor.
Question 4: What is the primary purpose of the low-pass filter at the output of a transmitter?
- To reduce the audio frequency response
- To suppress harmonic and spurious emissions before the signal reaches the antenna (Correct answer)
- To match the transmitter output to 50 ohms
- To prevent receiver desensitization during transmit
Correct answer: To suppress harmonic and spurious emissions before the signal reaches the antenna
The low-pass filter passes the fundamental transmit frequency but attenuates harmonics and other spurious products that could interfere with other services.
Question 5: A final RF amplifier has a DC input of 100 W and an RF output of 60 W. What is its efficiency?
- 40%
- 60% (Correct answer)
- 167%
- 6%
Correct answer: 60%
Efficiency = (RF output / DC input) × 100% = (60 W / 100 W) × 100% = 60%.
Question 6: In an SSB transmitter, the carrier is suppressed using:
- A low-pass filter after the audio stage
- A balanced modulator that cancels the carrier through circuit symmetry (Correct answer)
- A limiter stage following the IF amplifier
- A notch filter tuned to the carrier frequency
Correct answer: A balanced modulator that cancels the carrier through circuit symmetry
A balanced modulator produces a double-sideband suppressed-carrier (DSBSC) output by exploiting circuit balance to cancel the carrier while passing both sidebands.
Question 7: Automatic Level Control (ALC) in an SSB transmitter is used to:
- Maintain constant audio frequency response
- Prevent overdrive and excessive peak power by reducing gain when signal peaks exceed a threshold (Correct answer)
- Automatically select upper or lower sideband
- Stabilize the carrier insertion oscillator
Correct answer: Prevent overdrive and excessive peak power by reducing gain when signal peaks exceed a threshold
ALC reduces the drive to the final amplifier on voice peaks, preventing flat-topping, IMD, and splatter into adjacent channels.
GROL Receivers and Transmitters 3
What is the function of a buffer amplifier stage in a transmitter?