An interesting yet simple waveform generator is the "white noise" generator circuit. When summed over a unit of time, this is a waveform that encompasses a whole range of randomly produced frequencies, each with identical mean energy. White noise could be used to test audio and radio frequency amplifiers, and it's also commonly employed to obscure background noise to facilitate sleep.
The concept for a basic, workable white-noise generator is shown in the figure below. It works on a theory which assumes that all reverse-biased Zener diodes can produce white noise by default.
R2 and 131 are interconnected in a negative-feedback loop between the collector and base of the common-emitter amplifier Q1 as shown in the above figure. The generator's DC operating thresholds are stabilized by negative feedback.
Capacitor C1 is used to disconnect the circuit from ac voltage. In series with the base of transistor Q1, the Zener diode works as a white-noise generator.
The Zener noise is amplified to a theoretical level of roughly 1 volt peak-to-peak by the transistor. This design would work with any Zener diode (D1) specified with a value between 5.6 V to 12 V.
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