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Signal Modulation
31 questions By Tony R. Kuphaldt
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Question 22 of 31
Determine the frequency spectrum for a high-frequency, sine wave “carrier” signal that is amplitude-modulated (AM) by an audio-frequency sine wave signal, as the following block diagram shows:
The spectra for these respective waveforms are shown individually:
Plot the modulated signal spectrum here:
Reveal answer
Follow-up question: if the modulating (audio) signal were increased in frequency, what would the sideband spectra do?
Notes:The purpose of this question is to get students to recognize where sidebands come from, and how they relate to the frequency spectrum of the amplitude-modulated carrier wave.
In case anyone happens to ask, the symmetrical positioning of the sidebands around the carrier on the answer spectrum implies a linear frequency scale.
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Question 23 of 31
Determine the frequency spectrum for a high-frequency, sine wave “carrier” signal that is amplitude-modulated (AM) by an audio-frequency signal with a spectrum of its own. The spectra for these respective waveforms are shown individually:
Plot the modulated signal spectrum here:
Reveal answer
Notes:The purpose of this question is to get students to recognize where sidebands come from, and to recognize their symmetry about the carrier peak.
In case anyone happens to ask, the equal scaling of the sidebands around the carrier on the answer spectrum implies a linear frequency scale.
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Question 24 of 31
An important measurement of pulse waveforms is duty cycle. Give a precise. mathematical definition for this term.
Also, write an equation solving for pulse width given duty cycle (D) and frequency (f).
Reveal answer“Duty cycle” is a measure of a pulse waveform’s on time versus its total time (period):
D = ton ttotalI’ll let you figure out how to write an equation solving for pulse width (ton) in terms of duty cycle and frequency.
Notes:Duty cycle is a very important concept, as analog information may be conveyed through the variable duty cycle of an otherwise digital pulse waveform. Discuss this application with your students, if time permits.
