Diode oscillatorsAt ultra-high and microwave frequencies, certain types of diodes can be used as oscil-lators. These diodes, called Gunn, IMPATT, and tunnel diodes, were discussed inchapter 20.Audio waveformsThe above described oscillators work above the human hearing range. At audio fre-quencies (AF), oscillators can use RC or LC combinations to determine frequency. If LCcircuits are used, the inductances must be rather large, and ferromagnetic cores areusually necessary.All RF oscillators produce a sine-wave output. A pure sine wave represents en-ergy at one and only one frequency. Audio oscillators, by contrast, don’t necessarilyconcentrate all their energy at a single frequency. A pure AF sine wave, especially ifit is continuous and frequency-constant, causes ear/mind fatigue. Perhaps you’veexperienced it.The various musical instruments in a band or orchestra all sound different fromeach other, even when they play the same note (such as middle C). The reason for thisis that each instrument has its own unique waveform. A clarinet sounds different thana trumpet, which in turn sounds different than a cello or piano.Suppose you were to use an oscilloscope to look at the waveforms of musical in-struments. This can be done using a high-fidelity microphone, a low-distortion ampli-fier, and a scope. You’d see that each instrument has its own “signature.” Eachinstrument’s unique sound qualities can be reproduced using AF oscillators whosewaveform outputs match those of the instrument.The art of electronic music is a subject to which whole books have been devoted.All electronic music synthesizers use audio oscillators to generate the tones you hear.Audio waveforms46725-8Block diagram of a PLL frequency synthesizer.