PLL
Understanding PLLs
The PLL is one of the most important building blocks in RF systems. Every receiver, transmitter, and test instrument contains at least one PLL for frequency generation. Modern fractional-N PLLs provide sub-Hz frequency resolution with state-of-the-art phase noise performance.
PLL Components
- Phase detector (PD): Compares reference and feedback frequencies, producing an error signal.
- Loop filter: Integrates the PD output to produce the VCO control voltage. Sets loop bandwidth and stability.
- VCO: Voltage-controlled oscillator that generates the output frequency.
- Divider (N): Divides the VCO frequency by N for comparison with the reference.
- Reference oscillator: Crystal or OCXO providing the stable reference.
PLL Types
- Integer-N: Step size = reference frequency. Simple but coarse steps.
- Fractional-N: Step size = reference/modulus. Fine steps. Better in-band phase noise.
- All-digital PLL: Digitized loop filter. Enables fast calibration and adaptation.
f_out = N x f_ref (integer-N)
f_out = (N + F/M) x f_ref (fractional-N)
Phase noise (in-band):
L_out = L_ref + 20 log(N) (reference noise amplified by N)
Phase noise (out-of-band):
L_out = L_VCO (VCO noise dominates)
Frequently Asked Questions
What is a PLL?
A PLL locks a VCO to a reference oscillator by comparing their phases via a feedback loop. The VCO output frequency = N x reference frequency, synthesizing precise frequencies from a single reference. PLLs are the core of all modern frequency synthesizers.
What determines PLL phase noise?
Inside the loop bandwidth: the reference oscillator noise amplified by 20log(N). Outside the loop bandwidth: the VCO free-running phase noise. The loop bandwidth crossover determines which source dominates at each offset frequency.
What is fractional-N synthesis?
Fractional-N PLLs allow the division ratio to include a fractional part, enabling much finer frequency steps than integer-N (where step = f_ref). This allows using a higher reference frequency for better in-band phase noise while maintaining fine resolution.