Question
Which of the following ensures that a crystal oscillator operates on the frequency specified by the crystal manufacturer?
Answer Options
- A) Provide the crystal with a specified parallel inductance
- B) Provide the crystal with a specified parallel capacitance
- C) Bias the crystal at a specified voltage
- D) Bias the crystal at a specified current
Correct Answer: B
Explanation
A quartz crystal’s resonant frequency is determined by its physical cut and dimensions, but the precise operating frequency of the oscillator circuit it is used in can be slightly shifted (‘pulled’) by external components. Crystal manufacturers specify a load condition to guarantee the crystal oscillates at the marked frequency.
To ensure that a crystal oscillator operates on the frequency specified by the manufacturer (which is usually the parallel resonant frequency), you must provide the crystal with a specified parallel capacitance (the load capacitance, C_L). This external capacitor compensates for the inherent shunt capacitance of the crystal holder and the circuit components, forcing the crystal to oscillate at the exact frequency specified by the manufacturer’s calibration.
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