Active and passive crystal oscillators are both integral components in electronic systems, providing precise timing signals. However, they have distinct differences in terms of construction and operation. Here are ways to distinguish between them:
Active Crystal Oscillator (OCXO, TCXO, VCXO):
- Circuit Complexity: Active oscillators, such as Oven Controlled Crystal Oscillators (OCXO), Temperature Compensated Crystal Oscillators (TCXO), or Voltage Controlled Crystal Oscillators (VCXO), have a built-in active circuitry within their package. This circuitry may include a built-in amplifier and other components for maintaining a stable oscillation over varying conditions.
- Power Supply: They require an external DC power supply to function.
- Output Signal: Active oscillators generate a clock signal directly; they can directly drive the load (i.e., they provide an output with sufficient current).
- Physical Size: They are usually larger than passive counterparts due to additional electronics.
- Stability and Precision: They often offer better temperature stability and precision than passive oscillators.
- Cost: Active oscillators are typically more expensive due to their enhanced performance and internal complexity.
Passive Crystal Oscillator:
- Circuit Simplicity: Passive crystal oscillators, simply referred to as crystal resonators, generally consist of just the quartz crystal itself in a package, without any active components. They must be connected to an external circuit, like an oscillator circuit built around a microcontroller or another IC, to produce oscillations.
- Power Supply: They do not require a direct power supply; they are driven by the oscillation circuit they are connected to.
- Output Signal: Passive crystals do not generate a signal on their own; they rely on the external circuit to start and maintain oscillation.
- Physical Size: They tend to be smaller than active oscillators as they consist of fewer components.
- Stability and Precision: They offer stability and precision that is typically sufficient for many applications, but less than that of an active oscillator, especially under variable environmental conditions.
- Cost: Passive crystals are generally less expensive than active oscillators.
Visually and Physically:
- Markings: Active oscillators will often have more information on the packaging, including input voltage and possibly control voltage specifications.
- Pin Count: Active crystal oscillators typically have more pins than passive ones because they need connections for power supply and possibly control voltages.
- Package: They may come in different styles of packages, with active oscillators often being in metal cans or larger plastic packages, while passive crystals might be found in small glass or ceramic packages.
Testing and Operation:
- In a circuit, if you remove the oscillator:
- For an active oscillator, the system will not oscillate as the entire oscillator unit has been removed.
- For a passive crystal, the system may still oscillate if the rest of the active components required for oscillation are part of the main circuit board.
To sum up, the easiest practical way to distinguish them would be through power supply requirements, package type, and complexity, output signal, and system behavior upon their removal. In a new design context, circuit schematics or datasheets are often used to determine the type of oscillator being used, as these documents provide a clear description of the component and its application.