As a piece of test equipment, an audio oscillator has to be considered essential for anyone working in with hi-fi gear. Together with an audio millivoltmeter, and even better if you have access to an oscilloscope, you will be able to make proper measurements on everything from preamps, RIAA equalisation stages (for vinyl disks), tone controls, crossover networks, etc.
An oscillator is simply an amplifier whose positive feedback is greater than the negative feedback, resulting in a signal which is amplified over and over again (by the same amplifier) until the output can increase no further.
This generally results in a square wave if the frequency of oscillation is low enough relative to the amplifier's bandwidth. There are several things that must be done in order to create a usable audio oscillator:
- The frequency must be defined with a suitable filter, so the output will be at a known frequency
- The gain must be stabilised to exactly that value which will sustain oscillation, without dying away or becoming a square wave (or just distorting)
- The frequency response of the amplifier should be considerably greater than the highest frequency to be generated to ensure amplitude stability at all frequencies
- Output impedance must be low enough to ensure that there is no significant loading from the input circuitry of any expected load
- An output attenuator is needed so that a defined level can be preset, preferably without having to measure it before use
- Ideally, a square wave output should also be provided - this is only really useful if the user has access to an oscilloscope
The choice of filter circuit is discussed below, as is the stabilisation process...
This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright (c) 1999.
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