HAM Modulation and Digital Signals 4 — Questions and Answers
Question 1: What is the Nyquist sampling theorem's requirement for digitizing an analog signal without aliasing?
- The sample rate must equal the highest frequency component in the signal
- The sample rate must be at least twice the highest frequency component in the signal (Correct answer)
- The sample rate must be at least 10 times the highest frequency component
- The sample rate must match the bit rate of the resulting digital data
Correct answer: The sample rate must be at least twice the highest frequency component in the signal
The Nyquist theorem states that a signal must be sampled at a rate of at least twice its highest frequency to reconstruct it accurately from the digital samples.
Question 2: What causes intermodulation distortion in a transmitter's RF power amplifier when handling digital signals?
- Insufficient supply voltage causing clipping on positive peaks only
- Non-linearity in the amplifier producing unwanted mixing products of the signal's spectral components (Correct answer)
- Excessive negative feedback reducing the gain unevenly across the passband
- Phase noise in the local oscillator mixing into the baseband signal
Correct answer: Non-linearity in the amplifier producing unwanted mixing products of the signal's spectral components
Amplifier non-linearity causes the instantaneous frequency components of a signal to mix with each other, generating intermodulation products that appear as spectral regrowth and adjacent channel interference.
Question 3: In Amateur Radio digital modes, what is the function of a 'waterfall' display?
- It shows received signal strength versus time as a scrolling bar graph
- It provides a time-frequency spectrogram that scrolls downward, showing signal activity across a range of frequencies (Correct answer)
- It displays decoded text output in a scrolling window
- It is a type of frequency sweep used to tune a bandpass filter
Correct answer: It provides a time-frequency spectrogram that scrolls downward, showing signal activity across a range of frequencies
A waterfall display is a real-time spectrogram where frequency is on the horizontal axis, time scrolls downward, and signal amplitude is shown by color, making it easy to spot digital signal activity.
Question 4: What is the purpose of differential encoding in digital PSK systems such as DPSK?
- To increase the number of bits per symbol above that of non-differential PSK
- To allow demodulation without a coherent phase reference by encoding information in phase transitions rather than absolute phase (Correct answer)
- To reduce the required SNR by pre-distorting the signal before transmission
- To align the data clock with the carrier frequency automatically
Correct answer: To allow demodulation without a coherent phase reference by encoding information in phase transitions rather than absolute phase
Differential encoding represents each bit as a change in phase from the previous symbol, so the receiver needs only to detect phase transitions rather than an absolute phase reference.
Question 5: Which of the following digital modes uses a combination of frequency shift and multiple tones to achieve a low-baud-rate, high-sensitivity signal suited for weak-signal HF work?
- RTTY
- SSTV
- JT65 (Correct answer)
- Packet AX.25
Correct answer: JT65
JT65 uses 65 orthogonal tones with very low symbol rate and strong FEC, enabling contacts at signal levels well below what SSB voice can achieve.
Question 6: What is the effect of excessive phase noise in a transmitter's local oscillator on a digitally modulated signal?
- It causes the signal's power to decrease proportionally with frequency offset
- It spreads energy into adjacent channels and degrades the error vector magnitude (EVM) of the constellation (Correct answer)
- It shifts the carrier frequency by an amount equal to the phase noise floor
- It introduces AM noise but has no effect on the phase of the signal
Correct answer: It spreads energy into adjacent channels and degrades the error vector magnitude (EVM) of the constellation
Phase noise from the LO smears the phase of each transmitted symbol, degrading EVM, causing constellation point spreading, and producing spectral skirts that interfere with adjacent channels.
Question 7: In WSPR (Weak Signal Propagation Reporter), what modulation scheme is used?
- 4-FSK with 4 tones separated by the symbol rate (Correct answer)
- 2-FSK (RTTY) at 45 baud
- Single-sideband AM with audio tones
- GMSK as used in GSM cellular systems
Correct answer: 4-FSK with 4 tones separated by the symbol rate
WSPR uses 4-FSK with four tones spaced approximately 1.46 Hz apart and a symbol rate of about 1.46 baud, fitting in under 6 Hz of bandwidth.
HAM Modulation and Digital Signals 4
What is the Nyquist sampling theorem's requirement for digitizing an analog signal without aliasing?