HAM Modulation and Digital Signals 2 — Questions and Answers
Question 1: What is the purpose of pre-emphasis in an FM transmitter?
- To increase the deviation of higher audio frequencies before transmission (Correct answer)
- To reduce the carrier frequency drift
- To compress the audio signal amplitude
- To filter out spurious emissions above 3 kHz
Correct answer: To increase the deviation of higher audio frequencies before transmission
Pre-emphasis boosts higher audio frequencies before transmission to improve the signal-to-noise ratio, and de-emphasis at the receiver restores flat frequency response.
Question 2: Which modulation method conveys information by varying the timing of pulses relative to a reference clock?
- Pulse amplitude modulation (PAM)
- Pulse position modulation (PPM) (Correct answer)
- Pulse width modulation (PWM)
- Pulse code modulation (PCM)
Correct answer: Pulse position modulation (PPM)
Pulse position modulation (PPM) encodes information in the time displacement of each pulse from a fixed reference position.
Question 3: In a phase-locked loop (PLL) frequency synthesizer, what does the voltage-controlled oscillator (VCO) output frequency depend on?
- The amplitude of the reference oscillator
- The DC control voltage from the loop filter (Correct answer)
- The duty cycle of the phase detector output
- The supply voltage to the prescaler
Correct answer: The DC control voltage from the loop filter
The VCO output frequency is controlled by a DC voltage produced by the loop filter, which integrates the phase detector error signal.
Question 4: What is the principal advantage of using OFDM (Orthogonal Frequency-Division Multiplexing) in digital HF communications?
- It requires no forward error correction
- It is highly resistant to multipath fading and selective fading (Correct answer)
- It uses less bandwidth than single-carrier PSK
- It eliminates the need for synchronization
Correct answer: It is highly resistant to multipath fading and selective fading
OFDM spreads data across many narrow subcarriers that are orthogonal to each other, making the signal robust against multipath and frequency-selective fading common on HF.
Question 5: What does the term 'frequency deviation' refer to in FM modulation?
- The difference between the carrier frequency and the crystal oscillator frequency
- The peak change in instantaneous carrier frequency from the unmodulated carrier frequency (Correct answer)
- The rate at which the modulating signal changes frequency
- The amount the receiver's discriminator is offset from center
Correct answer: The peak change in instantaneous carrier frequency from the unmodulated carrier frequency
Frequency deviation is the maximum instantaneous shift of the carrier frequency above or below the rest frequency, determined by the amplitude of the modulating signal.
Question 6: In single sideband (SSB) transmission, why is the carrier suppressed?
- To improve audio fidelity by removing DC components
- To conserve power since the carrier carries no information (Correct answer)
- To prevent adjacent channel interference from the carrier
- To allow two sidebands to be transmitted simultaneously
Correct answer: To conserve power since the carrier carries no information
The carrier in AM contains no information, so suppressing it allows all transmitter power to be concentrated in the information-bearing sideband.
Question 7: What is the function of a balanced modulator in a SSB transmitter?
- To amplify both sidebands equally before filtering
- To produce a double-sideband suppressed-carrier (DSBSC) signal (Correct answer)
- To select which sideband is transmitted by the filter
- To stabilize the carrier frequency using a reference oscillator
Correct answer: To produce a double-sideband suppressed-carrier (DSBSC) signal
A balanced modulator combines the carrier and audio in a way that cancels the carrier, producing a double-sideband suppressed-carrier signal that a sideband filter then reduces to one sideband.
HAM Modulation and Digital Signals 2
What is the purpose of pre-emphasis in an FM transmitter?