Modulation and Digital Signals Flashcards
7 cards from real HAM practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Modulation and Digital Signals flashcards as text
What is the significance of Carson's rule in FM signal analysis?
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.
What is a major difference between Gaussian Minimum Shift Keying (GMSK) and standard MSK?
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.
What happens to an FM receiver's capture effect when two signals are present on the same frequency?
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.
In direct digital synthesis (DDS), what determines the output frequency of the synthesizer?
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.
What is the primary purpose of a raised cosine filter in a digital communications system?
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.
What causes spectral regrowth in a digitally modulated transmitter operated near amplifier saturation?
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.
Which of the following describes the term 'baud rate' in a digital communications context?
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.