1. Basic Matching Information

Customer Requirement	The customer has sent samples and requires our company to conduct matching tests on the crystal oscillator circuit of the circuit board.
Supplementary Information	Original Circuit Parameters Original circuit crystal oscillator: 2016-40.000MHz-12PF, 10PF, +/-10PPM Matching capacitance: 12PF (Crystal oscillator PIN2 and PIN4 connected to GND) Power supply: DC 5V (Red and black wires) Circuit Board Diagram:

2. Processing Steps

2.1 Preliminary Test:

Original Crystal Oscillator Frequency Deviation Test:

Test data of the oscillation circuit for the original X1 crystal (2016-40.000MHz-12PF) is as follows:

• Oscillation circuit operating frequency: FL = +26.26 PPM 

• Oscillation circuit drive power: DL = 127.46 μW 

• Negative impedance characteristic: -R > 125 Ω

2.2 Abnormal Points:

It was found that the original PCBA of the customer has a large positive frequency deviation.

2.3 Root Cause:

It is determined that the original load capacitance of the customer is relatively large, and when combined with the stray capacitance on the board, it does not meet the load capacitance value required by the crystal oscillator.

2.3 Handling Actions:

Based on the actual situation on the board, it is necessary to reduce the matching capacitance to adjust the external environment to the load capacitance value required by the crystal oscillator. Therefore, the matching capacitance is adjusted from 22PF to 10PF.

Retest:

Test data of the oscillation circuit after replacing the X1 crystal with SJK-2016-40.000MHz-9PF is as follows:

• Oscillation circuit operating frequency: FL = -3.48 PPM 

• Oscillation circuit drive power: DL = 113.88 μW (Reference value: 300 μW Max) Result: OK 

• Negative impedance characteristic: -R > 130 Ω (Reference value: |-R| > 120 Ω) Result: OK

3. Final Summary

• When the matching capacitance increases, the frequency deviation shifts from positive to negative; when the matching capacitance decreases, the frequency deviation shifts from negative to positive. 

• With the same matching capacitance: if the load capacitance increases, the frequency deviation rises; if the load capacitance decreases, the frequency deviation drops.