Crystal oscillators — dependence of frequency: For a quartz crystal used in oscillators, what primarily sets the oscillation frequency under normal operating conditions?

Introduction / Context:
Quartz crystals are precision frequency-determining elements. New learners sometimes assume that increasing the applied voltage changes frequency the way a motor speeds up with more voltage. In reality, a crystal’s frequency is set by its mechanical resonance and the surrounding network, while the drive level mainly affects amplitude and long-term aging if excessive.

Given Data / Assumptions:

• Pierce/Colpitts-style crystal oscillator or clock generator.
• Proper biasing so the crystal operates in its linear region (specified drive level).
• Stable temperature and supply conditions.

Concept / Approach:
A crystal behaves like a very high-Q resonator with motional L, C, and R. Its series and parallel resonant frequencies are determined by cut and geometry. The external load capacitance slightly “pulls” the parallel-resonant frequency. Drive voltage primarily influences oscillation amplitude and can cause nonideal effects (nonlinearities, aging) if excessive, but it does not intentionally tune frequency over a wide range.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets specify load capacitance for target frequency (e.g., 18 pF) and give tight tolerance and temperature coefficients. No “voltage-to-frequency” spec exists for normal operation beyond small, unintended shifts.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing VCOs (varactor-tuned) with crystals; ignoring the specified load capacitance; overdriving the crystal and causing reliability issues.

Final Answer:
The crystal’s mechanical resonance (cut, dimensions) and the external load capacitance, not the drive voltage amplitude