Technical Article: The Relationship Between External Load Capacitors and Crystal Oscillator Load

In electronic circuits, crystal oscillators are one of the most common clock sources. To ensure proper oscillation and stable operation of a crystal oscillator, the external load capacitors in the circuit must be appropriately designed based on its load capacitance (CL). Improper capacitor matching can lead to difficulties in oscillation, increased frequency deviation, and even system instability.

This article provides a detailed explanation of the meaning of crystal oscillator load capacitance, the calculation method for external capacitors, and practical considerations in circuit design.

Ⅰ.What is Load Capacitance (CL) of a Crystal Oscillator?
Crystal manufacturers typically specify a nominal load capacitance value, CL, such as 18pF, 20pF, or 12pF, in the datasheet. This indicates that the crystal is calibrated to achieve its nominal oscillation frequency under this capacitive load.
In other words, the crystal oscillator will output an accurate frequency only when the equivalent load capacitance equals CL.

Ⅱ.How Do External Capacitors Affect Load Capacitance?
A typical crystal oscillator circuit is configured as follows:

▪ C1 and C2 are external capacitors connected to ground (usually surface-mount capacitors).

▪ The two terminals of the crystal (XTAL) are connected to the two pins of the oscillator.

▪ The equivalent load capacitance, CL, is derived from the series combination of C1 and C2, taking into account the parasitic capacitance (Cp) introduced by the PCB and packaging.

Ⅲ.Calculation Formula: How to Determine External Capacitor Values Based on CL
The formula for equivalent load capacitance (CL) is as follows:

Where:

▪ C1, C2: External capacitors connected to ground.

▪ Cp: Parasitic capacitance introduced by PCB traces, packaging, etc. (typically 2pF to 5pF, estimated via simulation or experience).

▪ CL: Nominal load capacitance specified in the crystal datasheet.

【Example】
Assume a crystal specification of CL = 18pF and a PCB parasitic capacitance Cp ≈ 5pF. To determine suitable values for C1 and C2:

1.Substitute into the formula:

2.Assume C1 = C2 = C. The formula simplifies to:

3.Solve for C:

✅ Therefore, it is recommended to use external capacitors C1 = C2 = 27pF (standard value).

Ⅳ.Why Does Mismatched External Capacitance Cause Issues?

1.Frequency Deviation: If the external capacitors are too small or too large, the crystal oscillator will deviate from its nominal frequency, causing system clock inaccuracies.

2.Oscillation Difficulties or Instability: Excessive load reduces gain, potentially preventing oscillation or causing frequency jitter.

3.Increased Power Consumption: Improper loading can lead to higher energy consumption in the oscillator circuit.

4.Increased EMI: Unstable oscillation may generate spurious frequencies, affecting electromagnetic compatibility.

Ⅴ.Design Recommendations and Engineering Practices

1.Carefully review the crystal oscillator datasheet:

▪ Confirm the CL value.

▪ Note the recommended circuit topology.

▪ Check if a range of external capacitor values is suggested.

2.Consider the actual board environment:

▪ Estimate Cp using PCB simulation or empirical knowledge.

▪For high-speed or precision clocks (e.g., communication systems, MCU master frequency >100MHz), precise calculation or testing is recommended.

3.Use symmetrical capacitors:

▪ Generally, C1 = C2 is recommended to center the frequency and balance noise.

▪ Asymmetric design may be used for specific matching requirements.

4.Use surface-mount capacitor arrays for debugging:

▪ Test with standard capacitor values (e.g., 10pF, 15pF, 22pF, 27pF).

▪ Use a spectrum analyzer or oscilloscope to verify output stability and frequency accuracy.

Ⅵ.Frequently Asked Questions (FAQ)

Conclusion
The design of external load capacitors for crystal oscillators is a small yet critical detail that directly impacts the stability and accuracy of the clock system. Understanding the calculation relationship between CL and C1/C2, and adjusting parameters based on actual PCB parasitic capacitance, is essential for achieving stable oscillation and precise frequency.

In high-speed digital systems, communication equipment, or systems with stringent clock requirements, the proper selection and matching of crystal oscillator load capacitors is an engineering task that must be taken seriously.

Hangjing has a professional laboratory with multiple application engineers with over 20 years of experience, offering free specialized matching test services for customers. If you require such assistance, please feel free to contact our sales or technical team.