Crystal oscillator is the most common type of oscillator used and has applications in virtually every aspect of the electronics industry. The crystal oscillator is used to establish a reference frequency used for timing purpose.
Crystal oscillator typically consists of an amplifier and a feedback network that selects a part of the amplifier output and returns it to the amplifier input. A simplified block diagram of such a circuit is shown below in (Fig.1).
FFigure 1. Simplified Block Diagram of a crystal oscillator
The basic criteria for oscillation in an oscillator are:
1. The open loop gain must be greater than the losses around the oscillator loop and
2. The phase shift around the oscillator loop must be either 0 or 360 degrees.
An oscillator can be used to generate different type of waveforms. The most common types of waveforms produced by an oscillator are sinusoidal and square.
The main parameters used in specifying a clock oscillator are listed below.
Logic TTL, HCMOS
In general, an HCMOS oscillator will drive TTL circuitry (not vice versa). The industry is moving away from the TTL logic as IC manufacturers are discontinuing the supply of many common TTL ICs.
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