Crystal oscillators are usually divided into passive crystal oscillators and active crystal oscillators. Passive crystal oscillators are generally called crystals, while active crystal oscillators are called oscillators.
The active crystal oscillator is a complete resonant oscillator. It uses the piezoelectric effect of the quartz crystal to start the oscillation, so the active crystal oscillator needs power supply. After we complete the active crystal oscillator circuit, it can be The oscillation frequency is actively generated, and a high-precision frequency reference can be provided, and the signal quality is better than that of passive signals.
The passive crystal oscillator cannot oscillate by itself. It needs the oscillator circuit inside the chip to work together to oscillate.
The Difference between Passive Crystal Oscillator and Active Crystal Oscillator
• Different Internal Structures
A passive crystal oscillator consists of a quartz crystal, electrodes, and a ceramic base.
An active crystal oscillator, on the other hand, adds an IC (Integrated Circuit) on top of the passive crystal oscillator, which is an independent oscillation starting chip.
• Different Crystal Oscillator Casings
If there is a dot (·) in the lower-left corner of the crystal oscillator casing, it is an active crystal oscillator. The dot (·) represents the position of pin 1, which is often defined as the tri-state* in clock oscillators. Passive crystal oscillators do not have a dot (·) marking on their casings.
*Tri-state: The output can be either the normal high ("1") or low ("0") logic levels found in typical binary logic circuits, or it can remain in a high-impedance state (Hi-Z).
• Different Pin Configurations
A passive crystal oscillator does not require a power supply connection and can operate with various voltages. Therefore, it typically has only input and output pins. It relies on an external clock circuit (connected to the internal oscillator circuit of the main IC) to generate the oscillation signal as it cannot oscillate on its own. The following diagram shows the pin assignment for a passive crystal oscillator.
An active crystal oscillator requires a dedicated power supply and typically has four pins: one pin left unconnected, the second pin connected to ground, the third pin serving as the output, and the fourth pin connected to voltage. It can operate without the need for external matching capacitors. The following diagram shows the pin assignment for an active crystal oscillator.
