What Is a PLL (Phase-Locked Loop)?
A PLL (Phase-Locked Loop) is a closed-loop control circuit that locks the output signal it produces to the phase and frequency of a reference signal.
How Does It Work?
There are three basic blocks: a phase detector (compares the output with the reference), a filter and a voltage-controlled oscillator (VCO). Through feedback the output is continuously corrected; in the end it "locks" to the reference.
Frequency Multiplication
If a divider (counter) is placed in the feedback path, the output is at a frequency that's a multiple of the reference. For example, producing 100 MHz from a 10 MHz reference is done this way; it's the basis of clock generators.
| Block | Job |
|---|---|
| Phase detector | Measures the difference |
| Filter | Produces the control voltage |
| VCO | Produces the output |
Locking
At startup the PLL searches for the reference and adjusts until the phase difference is zeroed. Once locked, the output stays in the same phase as the reference and at a stable frequency.
Where Is It Used?
Clock generation, frequency synthesis (radio/phone), clock recovery (data communication), FM demodulation and processor clock multiplication.
Fractional-N PLL
A classic PLL can only produce whole multiples of the reference. A fractional-N PLL can make the division ratio fractional, so it also produces intermediate frequencies in very fine steps. This is used in radio and communication to lock to exactly the desired frequency.
Common Mistakes
- Filter design: a wrong loop filter causes instability or slow locking.
- Jitter: a poor design increases the output jitter.
- Lock range: the reference must be within the PLL's capture range.
Frequently Asked Questions
What is a PLL used for?+
What is a VCO?+
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