Q&A: Quartz Crystals, Load Cap & more

Q: I am choosing a crystal for a clock oscillator. Does the load capacitance really affect timing accuracy?

A: Yes. The capacitance of the circuit defines the resonant frequency of the crystal. If the capacitance in the circuit changes, the crystal frequency changes too.

During production, a crystal is calibrated for a specified load capacitance. When the circuit has the correct load capacitance, the crystal will oscillate within its stated tolerance. If that capacitance changes, the frequency shifts. In the worst case, the crystal can oscillate outside its tolerance.

For any frequency control design using a quartz crystal, load capacitance is therefore a key part of achieving the required timing accuracy.

Read more about Load Capacitance here

See our Load Capacitance Calculator here

Q: What happens if the circuit load capacitance does not match the crystal specification?

A: The frequency will shift. How much it shifts depends on both the specified load capacitance and the difference between the capacitance in the circuit and the crystal’s specified load.

The impact can be dramatic. If the shift is large enough, the crystal may operate outside specification and may not work correctly in the application. This is why load capacitance should be reviewed carefully during the design process, whether you are designing with a quartz crystal or evaluating the frequency requirements of a crystal clock oscillator.

Q: How can I estimate how sensitive a frequency product is to a change in load capacitance?

A: Use the ‘trim sensitivity’ (aka Pullability). It shows how many parts per million (ppm) the crystal frequency changes when the load capacitance changes by 1 pF.

Trim sensitivity can range from 1 ppm/pF to 100 ppm/pF. It depends on several factors:

• Size of the crystal blank
• Size and shape of the electrode
• Frequency
• Load capacitance

The higher the trim sensitivity, the more the frequency shifts for a given capacitance change.

Q: Which crystal characteristics have the greatest effect on trim sensitivity?

A: Two points are especially important:

• Package size: In general, a larger package has a higher trim sensitivity. A 1.6 × 1.2 mm crystal will therefore have a lower trim sensitivity than a 7.0 × 5.0 mm crystal.
• Specified load capacitance: A lower load capacitance results in higher trim sensitivity. For example, a crystal specified at 10 pF will have a higher trim sensitivity than one specified at 20 pF.

Trim Sensitivity (or Pullability) is not a linear relationship , please refer to the information in our application note.

Round up

When timing accuracy is critical, do not treat load capacitance as a minor detail. Confirm that the circuit load matches the crystal specification and, if needed, ask for the trim sensitivity of the specific frequency product under consideration.

Design takeaway: A few picoFarads can shift crystal frequency enough to affect system timing. Check the load capacitance early in the design process to keep your clock oscillator within tolerance.