Hey,

The accuracy of the frequency counter is strongly dependent on the stability of its time base. A timebase is very delicate like the hands of a watch, and can be changed by movement, interference or even drift due to age, meaning it might not tick correctly. This can make a frequency reading, when referenced to the time base, seem lower or higher than the actual value. Highly accurate circuits are used to generate timebases for instrumentation purpose, usually using a quartz crystal oscillator within a sealed temperature - controlled chamber, known as an oven controlled crystal oscillator or crystal oven.

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