Definition of Crystal Oscillator
A crystal oscillator, in electronic circuits, is classified into two types: active crystal oscillators and passive crystal oscillators. A passive crystal oscillator is a non-polarized component that requires a clock circuit to generate an oscillation signal; it cannot oscillate independently. The signal level is determined by the oscillation circuit. Passive crystal oscillators can meet various voltage and clock signal voltage requirements and come in different packages, such as PTH-2P, SMD-2P, and SMD-4P.
An active crystal oscillator is a complete oscillator consisting of a quartz crystal, a transistor, and passive components. It offers high precision but has the limitation of a fixed signal level with minimal flexibility, requiring careful selection of the appropriate output level. Active crystal oscillators are typically available in SMD-4P packages.
The Difference between Passive and Active Crystal Oscillators
• Different Internal Structures
A passive crystal oscillator consists of a quartz crystal, electrodes, and a ceramic base.
On the other hand, an active crystal oscillator adds an IC (Integrated Circuit) to the passive crystal oscillator, functioning as an independent oscillation-starting chip.
• Different Crystal Oscillator Casings
If the casing of a crystal oscillator has a dot (·) in the lower-left corner, it indicates an active crystal oscillator. The dot (·) represents pin 1, which is often designated as the tri-state* pin in clock oscillators. Passive crystal oscillators do not have a dot (·) marking on their casings.
*Tri-state: The output can be either a normal high (“1”) or low (“0”) logic level, as seen 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. It typically has only input and output pins and relies on an external clock circuit (connected to the main IC’s internal oscillator circuit) to generate the oscillation signal, as it cannot oscillate independently. 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 left unconnected, the second connected to ground, the third serving as the output, and the fourth 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.
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