What Is an Oscillator?
An oscillator is a circuit or ready module that produces a continuous, regular output signal (usually a square or sine wave) with no external input signal.
How Does It Work?
An oscillator contains an amplifier and positive feedback. Part of the output is fed back to the input; at a certain frequency the circuit feeds itself and sustains a stable oscillation.
Difference From a Crystal
A crystal is passive and needs an external circuit to work. An oscillator module combines the crystal, amplifier and capacitors in a single package; once powered, it directly produces a clock signal.
| Property | Crystal | Oscillator Module |
|---|---|---|
| Structure | Passive | Active (ready) |
| External circuit | Required | Not required |
| Output | Vibration | Ready clock signal |
Oscillator Types
There are crystal oscillators (most stable), RC/LC oscillators (simple but less stable) and oscillators with a ceramic resonator. They're chosen by stability and cost. For high temperature stability, TCXO/OCXO/VCXO are used, and for sharp band filtering at RF a SAW filter.
Where Is It Used?
Processor/MCU clocks, communication devices, clock generators, sound generation and test equipment.
The Barkhausen Criterion
For a circuit to sustain a stable oscillation, two conditions are required (the Barkhausen criterion): the feedback loop gain must be exactly 1 and the total phase shift must be 360° (i.e. 0°). If these aren't met the oscillation either dies out or grows uncontrollably; the essence of oscillator design is this balance.
Common Mistakes
- Ignoring stability: in a precise application an RC oscillator isn't accurate enough.
- Supply noise: noise increases the output jitter.
- Output load: excess load distorts the frequency and shape.
Frequently Asked Questions
Ready oscillator or crystal?+
What kind of signal does an oscillator produce?+
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