Common Test Signals

Common test signals include the sine wave, the square wave, the triangle wave, the sawtooth wave, several types of noise waveforms, and multitone signals consisting of a superposition of sine waves.

The most common signal for audio testing is the sine wave. A single sine wave is often used to determine the amount of harmonic distortion introduced by a system. Multiple sine waves are widely used to measure the intermodulation distortion or to determine the frequency response. The following table lists the signals used for some typical measurements.

Measurement	Signal
Total harmonic distortion	Sine wave
Intermodulation distortion	Multitone (two sine waves)
Frequency response	Multitone (many sine waves, impulse, chirp), broadband noise
Interpolation	Sinc
Rise time, fall time, overshoot, undershoot	Pulse
Jitter	Square wave

These signals form the basis for many tests and are used to measure the response of a system to a particular stimulus. Some of the common test signals available in most signal generators are shown in the following figures.

The most useful way to view the common test signals is in terms of their frequency content. The common test signals have the following frequency content characteristics:

• Sine waves have a single frequency component.
• Square waves consist of the superposition of many sine waves at odd harmonics of the fundamental frequency. The amplitude of each harmonic is inversely proportional to its frequency.
• Triangle and sawtooth waves have harmonic components that are multiples of the fundamental frequency.
• An impulse contains all frequencies that can be represented for a given sampling rate and number of samples.
• Chirp signals are sinusoids swept from a start frequency to a stop frequency, thus generating energy across a given frequency range. Chirp patterns have discrete frequencies that lie within a certain range. The discrete frequencies of chirp patterns depend on the sampling rate, the start and end frequencies, and the number of samples.