PLL Loop Filter Calculator

Calculate 2nd-order passive loop filter values (R, C1, C2) for charge-pump PLLs.

What is a PLL?

A Phase-Locked Loop (PLL) is like a musician listening to a metronome and constantly adjusting their tempo to stay perfectly in sync. It locks an oscillator (VCO) to a precise reference frequency, then multiplies it up to generate any frequency you need.

Why it matters: Every radio, phone, and GPS receiver uses PLLs to generate the exact frequencies needed for communication. The loop filter (the R, C1, C2 values this calculator provides) controls how quickly the PLL locks and how clean the output signal is.

PLL Parameters
Enter charge pump, VCO, and loop characteristics.

Stable clock the PLL locks to (e.g. a crystal).

The output frequency you want to generate.

Current output of the PLL IC's charge pump. From the datasheet.

VCO tuning sensitivity — how much frequency changes per volt.

How fast the PLL reacts. Wider = faster lock, more noise.

Stability margin. 45°–55° is recommended for stable operation.

N = 10

fVCO = 100.000000 MHz

Loop Filter Component Values
2nd-order passive filter · N=10 · BW=10 kHz · φ_m=45°

CP ──┬── R ──┬── VCO

│ │

C2 C1

│ │

GND GND

Resistor R0.02243 Ω (std: 0.0220 Ω)
Capacitor C1709.4 μF (std: 680 μF)
Capacitor C270.94 μF (std: 68.0 μF)
Time constant T115.915 μs
Time constant T215.915 μs
N divider ratio10 (exact: 10.0000)
Loop Bandwidth10.000 kHz
Phase Margin45°
Lock Time (est.)0.50 ms
Reference spurious rejection-20.0 dBc
Standard E12 Values Summary

R

0.0220 Ω

C1

680 μF

C2

68.0 μF

Passive 2nd-order filter (R-C1 in series, C2 across charge pump output). Phase margin of 45°–55° is recommended for stable operation. Loop bandwidth should be 1/10th to 1/20th of the reference frequency. Use IC manufacturer simulation tools (e.g., ADIsimPLL, TICSPro) for final verification.