PURPOSE: To demonstrate the effect on a wave form of frequency modulation using the wave to modulate itself.

DESCRIPTION: One of the components of an FM synthesizer is a voltage-controlled oscillator with feedback: the oscillator output is used as the FM modulator input. This is accomplished readily using one of the Wavetek voltage-controlled oscillators.

This technique in fact produces a wave with a different shape and thus a different spectrum. The setup shows the wave shapes and spectra obtained by various modulation levels. For example, modulation of a sine wave by itself leads to the wave shape and spectrum shown in the photograph above. Using this technique a single oscillator with feedback can create a variety of wave shapes depending upon the level of feedback.

SUGGESTIONS: Start by setting the output at -10dB and change the fine control. Also try a square wave.

REFERENCES: (PIRA unknown.) See Yamaha synthesizer manual (Demonstration H4-55) for further information regarding FM synthesis.

EQUIPMENT: PC486 with R3 spectrum analyzer and dual-trace digital scope and Wavetek voltage-controlled oscillator.

SETUP TIME: 5 min.

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