Crystal Oscillator Specifications

• Crystal oscillators are key components in many electronic devices, especially digital electronics that need an outside clock frequency to operate. A crystal oscillator is a precisely cut quartz crystal that resonates at a specific frequency when wired in combination with a few other passive components. If you're choosing an oscillator for your next project, it is important to understand the meaning of the different specifications for crystal oscillators.

Stability

• The stability of an oscillator is a measure of how well it stays on frequency given different conditions. Stability is normally given as the deviation in frequency in terms of parts per million, although a percentage may also be used. The stability of an oscillator generally depends on the cut and the quality of manufacture. AT-cut crystals are less expensive, but their stability varies predictably with temperature changes. SC-cut crystals are more difficult to produce and therefore more expensive, but are more stable under temperature variations. Stability is also measured with regards to time, as crystals degrade as they age. Stability is also tested under shock and vibration conditions, a measurement which is important for crystals that may be used in rough conditions.

Phase Noise

• Phase noise is error in the waveform produced by an oscillator. Phase noise is the inevitable result of characteristics of the crystal itself and the other components in the oscillator circuit. Phase noise is specified as a graph of the relative noise level in decibels versus the frequency offset from the oscillator's carrier frequency.

  The closer to the carrier frequency, the higher the noise level because noise similar to the carrier frequency is difficult to filter out. This is not problematic because noise similar to the carrier frequency won't affect the signal much. The farther from the carrier frequency, the lower the noise level should be.

Jitter

• Jitter is similar to phase noise, but is relevant for digital signals. Jitter is the measure of the maximum time variation from the ideal transition of a square wave. A digital square wave needs to transition between low and high states accurately and on schedule. Jitter is the measure of the error in the timing of the transition from one state to another. The faster a system needs to run, the more accurate it needs to be and the more important jitter becomes. High-speed digital processors need oscillators with very low jitter ratings to perform accurately.

Frequency

• The most basic specification of an oscillator -- frequency -- is the ideal carrier frequency of the wave produced by the oscillator. In practice, the frequency of the oscillator varies minutely according to the specifications described above. Choose the frequency of an oscillator based upon the application or use. Different processors, for example, are meant to run on different oscillator frequencies.