What is the use of a resistor in crystal circuit?

 

In the crystal oscillator circuit, the resistor $R1$ serves a key role, typically known as a series damping resistor or feedback resistor. Its functions include:

1. Limiting Oscillation Gain:

The resistor helps to limit the overall gain of the oscillator circuit, ensuring the oscillator operates in a linear region rather than being overdriven. If the feedback gain is too high, the circuit can become unstable, causing distorted waveforms or spurious oscillations.

2. Controlling Start-Up Time:

By limiting the current, the resistor controls the start-up time of the oscillator. Without it, the start-up can sometimes be too fast, potentially leading to incorrect or noisy oscillation. The resistor helps the crystal reach its steady-state oscillation smoothly.

3. Preventing Overdriving the Crystal:

Crystals are fragile devices, and overdriving them with too much current can cause them to age prematurely or even fail. The resistor limits the current through the crystal to protect it from being damaged due to excessive drive levels.

4. Stabilizing Oscillations:

The resistor helps stabilize the oscillations by providing damping, reducing the chances of parasitic oscillations. It acts as a buffer to prevent feedback signals from becoming too strong and ensures that oscillations are sustained within the designed parameters.

5. Temperature Stability:

By limiting excessive gain and reducing power dissipation, the resistor can also contribute to better temperature stability of the oscillator. Excessive current can lead to temperature-induced frequency shifts in the crystal, and the resistor minimizes this effect.

6. Reducing Phase Noise:

Excessive gain in the feedback loop can increase phase noise. The resistor, by damping the gain, can help in reducing phase noise, which is critical in applications requiring low jitter or precise timing.

The value of this resistor (often in the range of tens to hundreds of ohms) is typically chosen to balance these factors, ensuring stable and reliable operation without excessive power dissipation or distortion in the oscillation signal.

$R1$ is placed in series with the crystal between the XOUT pin of the ADAR7251 chip and the crystal. This position allows it to limit feedback and control the current through the crystal, providing the benefits mentioned above.