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Square Wave Response

A measurement used to evaluate a number of time-domain, amplitude and phase behaviors in audio components such as crossovers, filters and amplifiers. This is measured by injecting a “pure” square wave into the device and connecting the output to an oscilloscope or equivalent software. The appearance of the output waveform from the device gives much qualitative information – the accuracy of its reproduction of the square wave. A number of quantitative measurements can also be obtained from the square-wave response. Some of the most common measurements are:

Rise time: The time required for the signal amplitude to change from 10 to 90 percent of the total square-wave amplitude.

Overshoot: The amount by which the peak of the square wave exceeds its normal positive or negative amplitude. Large overshoots indicate peaking or excessive phase shifts in the high-frequency response of the device under test. Overshoot normally occurs only on the leading edges of a square wave.

Ringing: The tendency of band-limited square waves to oscillate on the peaks. This is largely a subjective measurement with the results stated as the severity of ringing. The greater the high-frequency peaking, the greater the ringing.

Tilt: A measure of low-frequency behavior. As low frequencies are filtered, phase shifts are introduced which cause the leading edge of the square wave to rise and the trailing edge to fall at more of an angle. This produces a tilt to the top and bottom of the square wave. The tilt is usually expressed as a percentage of the peak amplitude of the square wave.

Slew Rate: A measure of how fast a signal changes from one instantaneous value to another. The ideal square wave changes from one amplitude extreme to the other instantly. Practical devices cannot keep up with this transition and will often have a maximum speed with which they can change. This speed limitation will result in a tilted straight-line portion of the square wave edge.

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