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PLL Loop Filter Calculator
Estimate starting values for a second-order passive charge-pump PLL loop filter using reference frequency, output frequency, charge pump current, VCO gain, loop bandwidth, and damping factor.
Input Parameters
Results
Treat the result as an initial design point, not a final PLL stability guarantee.
Equations Used
Divide Ratio:
N = Fout / Fref
Charge Pump Gain:
Kpd = Icp / (2π)
VCO Gain:
Kvco is converted from Hz/V to rad/s/V
Loop Capacitor Estimate:
C1 ≈ Kpd × Kvco / (N × ωn²)
Zero Resistor Estimate:
R2 ≈ 2ζ / (ωn × C1)
High-Frequency Pole:
C2 is estimated as C1 / 10 for a starting pole capacitor.
Frequently Asked Questions (FAQ)
Q1: What does this PLL loop filter calculator do?
It estimates initial R and C values for a passive charge-pump PLL loop filter.
Q2: Is this a full PLL design tool?
No. It is a starting-point calculator. Real PLL design requires device-specific noise, stability, and loop modeling.
Q3: Why is loop bandwidth important?
Loop bandwidth affects lock time, reference noise filtering, VCO noise suppression, and stability.
Q4: What damping factor should I use?
A damping factor around 0.7 is a common starting point for stable second-order response.
Q5: Why does VCO gain affect capacitor size?
Higher VCO gain increases loop gain and changes the required loop filter values.
Q6: Should I use these values directly in production?
No. Validate with the PLL manufacturer tool, phase-noise simulation, and lab measurement.
