Issue 91, February 1998
Choosing the Right Crystal For Your Oscillator

Start •Why Quartz Crystals • Timing Budget & Accuracy • Frequency Tolerance • Frequency Stability • Aging • Load Capacitance • Series and Parallel Resonance • Frequency Tolerance and Load Capacitance • AT vs. BT Cut • Mode of Operation • Package Considerations • Crystal Placement • Crystal Clear • References

FREQUENCY TOLERANCE AND LOAD CAPACITANCE

When the oscillator circuit load capacitance doesn’t equal the crystal spec load capacitance, the oscillator’s operating frequency is different from the crystal’s frequency tolerance spec.

Equation 3, for Fp, shows that as the board attributed load capacitance increases, Fp decreases. The change in frequency as a result of mismatched load capacitance is:

where Fp1 is the spec parallel resonant frequency, Fp2 is the actual parallel resonant frequency, CLspec is the crystal spec load capacitance, and CLsystem is the system load capacitance.

Equation 4 is known as the pullability equation and gives the frequency error of mismatched load capacitances. Often, this error is insignificant. However, it does come into play when there is a cumulative effect. If the crystal is used in a timekeeping application, cumulative effects are important.

If you’re trying to tightly control accuracy, you must consider PCB stray capacitances. Routing to the crystal and socket effects, if used, also add to the load capacitance.

It may be necessary to use a trim cap to hit the target accuracy. If the circuit load capacitance is less than the target load capacitance, add a parallel trim cap to the circuit. Connect the cap between either the crystal pin or ground.

If the circuit load capacitance is greater than the target load capacitance, a series trim cap should be added to the circuit. The trim cap is connected to either crystal pin and the corresponding oscillator pin.