Why is there no need for an external matching capacitor at the output terminal of the active crystal oscillator frequency signal? | Xtalong
Active crystal oscillator is a complete oscillator after integrating. That is to say, there is already an oscillation circuit with IC and the best matching capacitor inside, and there is no need to add matching capacitors to the signal output. active crystal oscillators are characterized by high signal stability and precision. Before the signal is output, the sine wave has completed the shaping process from itself to the square wave, and the output signal of the active crystal oscillator is mainly a square wave (CMOS) that directly corresponds to the digital circuit signal.
Passive crystal oscillator circuits usually require additional matching capacitors, generally one capacitor of equivalent value to ground at each of the frequency output pin and the input pin. The role of these two capacitors is mainly to facilitate the start-up of the passive crystal oscillator, increase frequency stability and accuracy. The waveform of the passive crystal oscillator has not been shaped, so the output is a basic wave - a sine wave (Sine wave). A sine wave cannot be directly connected to a digital circuit. Generally, it needs to go through a shaping circuit and finally become a square wave signal.