Quartz crystal paired with circuit design
1. Load capacitance
Quartz resonators are generally used as inductive components to stabilize the frequency in oscillating circuits, while other components of the circuit can be equivalent to a load capacitor connected in series or parallel with the quartz resonator. Different load capacitors can affect frequency offset. Therefore, it is recommended to measure the actual load capacitance of the product during design to determine whether it meets the requirements of the matching circuit design. The calculation method for load capacitance is shown in the following figure.
Note: Capacity Stray refers to factors such as the stray capacitance inside the IC, the capacitance between layout wiring, and the parasitic capacitance between different layers of the PCB board.
2. Negative impedance
For the stability of the oscillation circuit, designers can refer to the circuit shown in the following figure to test the negative impedance on the board; Connect one side of Crystal to an adjustable resistor (VR). When the value of the adjustable resistor (VR) reaches a certain resistance value, the oscillator stops oscillating. This value is the negative impedance value (- R); The general negative impedance value (- R) is recommended to be at least 3 times the internal resistance of the oscillator to comply with the negative impedance specification design on the circuit.
