Definition of Crystal Oscillator

A crystal oscillator, in electronic circuits, is divided into two types: active crystal oscillator and passive crystal oscillator. A passive crystal oscillator is a non-polarized device that requires a clock circuit to generate an oscillation signal; it cannot oscillate on its own. The signal level is determined by the oscillation circuit. Passive crystal oscillators can be used for various voltage and clock signal voltage requirements and are available in different packages, including PTH-2P, SMD-2P, and SMD-4P.

An active crystal oscillator is a complete oscillator comprising a quartz crystal, a transistor, and passive components. It offers high precision but has the drawback of a fixed signal level with little flexibility, requiring the selection of a suitable output level. Active crystal oscillators are typically available in SMD-4P packages.

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.