Basic Features of Crystal Oscillators

The crystal oscillators are implemented using the piezo-electric effect. This is actually realized when a voltage source is applied to a small thin piece of crystal quartz. The quartz crystal begins to change shape. This piezo-electric effect is the property of a crystal by which an electrical charge produces a mechanical force by changing the shape of the crystal. In reverse sense, a mechanical force applied to the crystal produces an electrical charge. This piezo-electric effect produces mechanical vibrations or oscillations which are used to replace the LC tank circuit and can be seen in several types of crystal substances with the most important of these for electronic circuits being the quartz minerals because of their superior mechanical strength.

The quartz crystal is a miniature, thin piece or wafer of cut quartz with the two parallel surfaces metalized to make the electrical connections in a crystal oscillator. The physical size and thickness of a piece of quartz crystal is tightly controlled since it affects the final frequency of oscillations and is called the characteristic frequency of the crystal. Once cut and shaped, the crystal cannot be used at any other frequency. The crystal’s characteristics or resonant frequency is inversely proportional to its physical thickness between the two metalized surfaces. A mechanical vibrating crystal can be represented by an equivalent electrical circuit consisting of low resistance, large inductance and small capacitance.

Just like an electrically tuned tank circuit, a quartz crystal has resonant frequency with very high Q factor due to low resistance. The frequencies of quartz crystals range from 4 kHz to 10 MHz; the cut of the crystal also determines how it will vibrate and behave as some crystals will vibrate at more than one frequency. Additionally, if the crystal is of varying thickness, it has two or more resonant frequencies having both a fundamental frequency and harmonics such as second or third harmonics. However, normally the fundamental frequency is more pronounced than the others and this is the one employed. The equivalent circuit above consists of three reactive components and there are two resonant frequencies, the lowest is a series-type frequency and the highest a parallel-type resonant frequency.

Related: The Fundamentals of Oscillators in Communication Systems

In a crystal oscillator circuit, the oscillator will oscillate at the crystal’s fundamental series resonant frequency as the crystal always intends to oscillate when a voltage source is applied to it. It is also possible to tune a crystal oscillator to any even harmonic of the fundamental frequencies (2^nd, 4^th, 8^th, and so on) and these are known generally as harmonic oscillators while overtone oscillators vibrate at odd multiples of the fundamental frequencies (3^rd, 5^th, 11^th, etc.). Crystal oscillators that operate at overtone frequencies do so using their series resonant frequency.

How to Maintain Frequency Stability in Crystal Oscillators

Frequency stability is the most important feature of an oscillator. Frequency stability is the ability to provide a constant frequency output under varying load conditions. Frequency stability of the output signal can be improved by proper selection of the components used for the resonant feedback circuit including the amplifier but there is a limit to the stability that can be obtained from normal LC and RC tank circuits.

Also read: The Basic Principle of Operation of an Oscillator

Some of the factors that affect the frequency stability of an oscillator include temperature, variations in the load and changes in the power supply. For very high stability, a quartz crystal is generally used as the frequency determining device to produce a typical type of oscillator circuit known as crystal oscillator that has been described above.