Wave filter circuits are networks that contain reactive components (typically L and C) that accept or reject frequencies above or below stated cut-off frequency limits which are calculated from the values of the filter components.

Output amplitude and phase vary considerably as the signal frequency approaches a cut-off frequency and the calculations that are involved are beyond the scope of this book. The use of computer simulation, is advisable when designing such circuits.

Much more easily predictable responses can be obtained, for audio frequencies at least, by using active filters . Quartz crystals are used extensively in filter circuits to provide very sharp cut-off points, and in these applications the frequency–temperature characteristic of the devices is the most important parameter.

This leads to the use of AT-cut crystals as the preferred type, providing very good frequency stability over a wide temperature range.

Ceramic resonators, using materials such as lead zirconate titanate (PZT) are extensively used in filter circuits, and in microprocessor timing applications. These materials are piezoelectric, and can resonate in several modes depending on their resonance frequency.

Their precision of oscillation is lower than that of quartz crystals, but very much better than a discrete LC circuit, with a temperature coefficient of around 10−5/◦C in a temperature range of, typically, −10◦C to +80◦C.

They are considerably lighter and smaller than quartz crystals, and relatively immune to alterations on loading or in power supply voltage. Ceramic resonators in SM format often have load capacitors built in; other configurations may require load capacitors to be added.

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