What is the use of capacitors in crystal circuit?

 

In the crystal oscillator circuit, the capacitors (C341 and C351 in this case) serve several important functions:

1. Load Capacitance:

The capacitors provide the necessary load capacitance for the crystal to oscillate at its intended frequency. The crystal’s frequency depends on its physical properties and the load capacitance provided by external components. Typically, these capacitors form a parallel resonant circuit with the crystal and the input/output pins of the microcontroller or IC.

The effective load capacitance seen by the crystal, $C_{L}$, is given by:

$$C_{L} = \frac{C_1 \times C_2}{C_1 + C_2} + C_{stray}$$

Where $C_1$ and $C_2$ are the values of the capacitors , and $C_{stray}$ accounts for parasitic capacitance from the PCB traces and the input capacitance of the IC.

2. Stabilization of Oscillation:

The capacitors help stabilize the oscillation of the crystal by filtering out noise and reducing any parasitic inductance or capacitance effects in the circuit. This helps maintain a stable frequency over a range of temperatures and voltages.

3. Ensuring Correct Start-Up:

Proper capacitance values ensure that the crystal oscillator starts up reliably. Too much capacitance could slow down the start-up time, while too little capacitance could lead to an unstable or failed start-up.

4. Preventing High-Frequency Noise:

The capacitors also play a role in filtering out unwanted high-frequency signals or noise, which could otherwise interfere with the operation of the crystal oscillator. They help reduce spurious oscillations that might occur due to stray inductances or capacitances in the layout.

The values of these capacitors are usually specified in the crystal's datasheet and should be chosen carefully to ensure proper functioning of the oscillator.