There are five timing budget parameters which influence the total system timing error of a quartz crystal and oscillator combination:
- Frequency tolerance, also known as calibration accuracy, is the amount of frequency deviation from a specified centerfrequency at ambient temperature (referenced at 25°C).
- Frequency Stability is the amount of frequency deviation from the ambient temperature frequency over the operatingtemperature range. This deviation is associated with a set of operating conditions including: Operating Temperature Range, LoadCapacitance, and Drive Level.
- Aging is the systematic change in frequency with time due to internal changes in the crystal which is related to the crystalcontamination and drive level. Over time, particles drop off or fall onto the quartz surface, hence slightly changing the resonantfrequency.
- Load capacitance (CL) is the fourth parameter to consider. A crystal can be characterized for either series or parallel loadresonant mode of operation. Both modes are physically the same; they are just tuned to operate in a different area of the crystalreactance curve.
- Oscillator accuracy is the fifth parameter to consider. Many times, this parameter is ignored, but process shifts in the silicon,temperature and voltage can have an effect on the center frequency. This variation is dominated by the process shift parametersand can be minimized by the amount of internal load capacitance in the oscillator and the trim sensitivity of the quartz crystal.