HAM Modulation and Digital Signals 5 — Questions and Answers
Question 1: What is the significance of Carson's rule in FM signal analysis?
- It defines the minimum number of sidebands needed to reconstruct the original audio
- It provides an approximation of the bandwidth occupied by an FM signal as 2(Δf + fm) (Correct answer)
- It states that FM and PM signals have identical spectra when the modulating frequency is held constant
- It establishes the maximum permissible frequency deviation allowed by FCC rules
Correct answer: It provides an approximation of the bandwidth occupied by an FM signal as 2(Δf + fm)
Carson's rule approximates FM bandwidth as twice the sum of the peak deviation and the highest modulating frequency, capturing about 98% of the signal power.
Question 2: What is a major difference between Gaussian Minimum Shift Keying (GMSK) and standard MSK?
- GMSK uses coherent detection while MSK uses non-coherent detection
- GMSK passes the data through a Gaussian filter before modulation, reducing spectral occupancy and sidelobes (Correct answer)
- GMSK changes the carrier frequency while MSK changes the carrier phase
- GMSK requires twice the bandwidth of MSK for the same bit rate
Correct answer: GMSK passes the data through a Gaussian filter before modulation, reducing spectral occupancy and sidelobes
GMSK applies a Gaussian low-pass filter to the data stream before MSK modulation, smoothing transitions and greatly reducing out-of-band spectral emissions.
Question 3: What happens to an FM receiver's capture effect when two signals are present on the same frequency?
- Both signals are demodulated simultaneously with equal weight
- The stronger signal suppresses the weaker one, with the receiver locking to the stronger signal (Correct answer)
- The weaker signal is amplified by the squelch circuit to equalize both signals
- Both signals produce an intermodulation product at the difference frequency
Correct answer: The stronger signal suppresses the weaker one, with the receiver locking to the stronger signal
FM's capture effect causes the receiver to lock onto the stronger of two co-channel signals and largely reject the weaker one, unlike AM where both signals mix audibly.
Question 4: In direct digital synthesis (DDS), what determines the output frequency of the synthesizer?
- The analog voltage applied to the VCO input
- The phase increment value loaded into the phase accumulator register (Correct answer)
- The crystal resonator temperature coefficient
- The divide ratio of the output prescaler
Correct answer: The phase increment value loaded into the phase accumulator register
A DDS generates a frequency by repeatedly adding a phase increment to a phase accumulator; larger increments produce higher frequencies from the same reference clock.
Question 5: What is the primary purpose of a raised cosine filter in a digital communications system?
- To amplify the signal uniformly across the occupied bandwidth
- To minimize intersymbol interference by shaping pulses so they have zero crossings at adjacent symbol intervals (Correct answer)
- To convert the analog signal to digital using oversampling
- To remove the carrier from a DSBSC signal before detection
Correct answer: To minimize intersymbol interference by shaping pulses so they have zero crossings at adjacent symbol intervals
A raised cosine filter shapes each transmitted symbol pulse so it reaches zero exactly at the sampling instants of adjacent symbols, preventing ISI without excess bandwidth.
Question 6: What causes spectral regrowth in a digitally modulated transmitter operated near amplifier saturation?
- The mixer's local oscillator leaks energy into adjacent sidebands
- Amplitude clipping due to amplifier nonlinearity regenerates out-of-band spectral components that were filtered before the amplifier (Correct answer)
- The FEC coding adds extra symbols that broaden the spectrum
- Phase noise in the DAC reference clock spreads the output spectrum
Correct answer: Amplitude clipping due to amplifier nonlinearity regenerates out-of-band spectral components that were filtered before the amplifier
When a non-linear power amplifier clips the peaks of a bandlimited signal, it recreates the spectral shoulders that were filtered out, causing out-of-band interference known as spectral regrowth.
Question 7: Which of the following describes the term 'baud rate' in a digital communications context?
- The number of bits transmitted per second
- The number of symbol state changes per second (Correct answer)
- The number of error-free bits delivered per second
- The number of carrier cycles per transmitted bit
Correct answer: The number of symbol state changes per second
Baud rate (or symbol rate) is the number of symbol transitions per second; it equals the bit rate only for binary (2-state) modulation, while multi-level modulation can carry multiple bits per baud.
HAM Modulation and Digital Signals 5
What is the significance of Carson's rule in FM signal analysis?