A crystal oscillator is a component in a circuit that provides a highly stable clock signal. Typically, a system shares one crystal oscillator, which helps keep all parts synchronized to work together efficiently. For example, in commonly used computer systems, the crystal oscillator can be compared to the “heartbeat” generator for each board. If the main board’s “heartbeat” has issues, it will definitely cause malfunctions in other circuits. Just like the human heartbeat depends on blood, the crystal oscillator depends on electric current.
Simply put, the driving force for a crystal oscillator to start oscillating is electricity. We need to place a crystal oscillator in a complete circuit, supply power to the circuit to generate loop current, and from then on the crystal oscillator starts to “beat” rhythmically and steadily.
Of course, this oscillation loop has its intricacies (an active crystal oscillator has a built-in oscillation circuit, while a passive one requires an external circuit), and it needs to follow the Barkhausen Stability Criterion.
The crystal oscillator provides the clock signal in a traffic light system.
What is the Barkhausen Stability Criterion?
The Barkhausen Stability Criterion, proposed by German physicist Heinrich Georg Barkhausen in 1921, states that for an electronic oscillator system to start oscillating, the total phase shift from input to output and back to input must be 360°, and the loop gain must be greater than or equal to 1. These are the necessary conditions for oscillation.
To generate periodic oscillations, a simple oscillator usually outputs in the form of voltage. While continuously outputting, it must incorporate an amplifier to provide sustained feedback to the input. At high frequencies, the amplifier’s output may shift phase enough to turn the feedback positive, resulting in oscillation.
When the loop gain is ≥1, it means the input signal, after traveling through the loop, returns to the input end with a greater amplitude than before. A phase of 360° means that after a full loop in the circuit, the signal’s phase matches the original input signal exactly, thus perfectly reinforcing it.
Combining both, the signal keeps amplifying through repeated reinforcement. When the amplitude in the loop increases to a certain level, the non-linearity of the active components (such as inverters in the oscillator circuit) limits further amplitude increase, causing the oscillator output to stabilize. In simple terms, the oscillation amplitude cannot exceed the supply voltage.
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