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Mixed-Frequency Signals
31 questions By Tony R. Kuphaldt
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Question 7 of 31
What causes harmonics to form in the output of a transistor oscillator circuit such as a Colpitts or a Hartley, which is designed to produce a sinusoidal signal? Be as specific as you can in your answer.
Reveal answerAny feature (or fault) of the amplifier portion of the oscillator circuit causing imperfect signal reproduction will necessarily create harmonics, for it will turn a perfectly sinusoidal input signal (from the LC network) into a distorted (non-perfect-sinusoidal) output signal.
Challenge question: Colpitts oscillators tend to produce “purer” sine-wave outputs than Hartley oscillators, all other factors being equal. Explain why.
Notes:Discuss with your students the nature of harmonics: how multiple sinusoidal waveforms are necessarily contained in any periodic waveform that is not perfectly sinusoidal itself.
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Question 8 of 31
A clever way to produce sine waves is to pass the output of a square-wave oscillator through a low-pass filter circuit:
Explain how this principle works, based on your knowledge of Fourier’s theorem.
Reveal answerThe LP filter blocks all harmonics of the square wave except the fundamental (1st harmonic), resulting in a sinusoidal output.
Notes:Ask your students what they think about the rolloff requirement for this LP filter. Will any LP filter work, or do we need something special?
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Question 9 of 31
What causes harmonics to form in AC electric power systems?
Reveal answerNonlinear loads.
Notes:My answer to this question is intentionally vague. It is correct, but does not reveal anything about the real nature of the cause, or more importantly, why a “nonlinear” load would cause harmonics. Discuss with your students what a “nonlinear” device is, and what it does to a sinusoidal signal to generate harmonics.
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