Introduction

The Pierce oscillator (most common case) implemented in microcontrollers is built up around a class A amplifier and a narrow band filter such as a crystal or a ceramic resonator as shown in the below figure.

This device has a high input impedance characteristic outside of the resonance frequency range and has a low input impedance characteristic at the oscillation frequency.

The high impedance characteristic degrades its immunity when an electrical field is applied in its vicinity.

Furthermore, in the latest technology and also in order to reduce the consumption, the oscillation level is restricted to within the range of 1V, again increasing the susceptibility.