PLL Loop Filter Calculator

A Phase-Locked Loop (PLL) is a circuit that locks an output signal to a precise frequency — used in radios, synthesizers, and clocks — and its loop filter determines stability and noise performance. Enter your Reference Frequency, Output Frequency, VCO Gain, Charge Pump Current, Loop Bandwidth, Phase Margin, and Filter Order into the PLL Loop Filter Calculator to get the required component values. Results include the primary Series Resistor R2, along with Shunt Capacitor C1, Series Capacitor C2, and the Division Ratio N.

MHz

PLL reference input frequency

MHz

Desired VCO output frequency

MHz/V

Voltage controlled oscillator gain

mA

PLL charge pump output current

kHz

Desired loop filter bandwidth

°

Desired phase margin for stability

Loop filter complexity

Results

Series Resistor R2

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Shunt Capacitor C1

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Series Capacitor C2

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Division Ratio N

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Frequently Asked Questions

What is the purpose of a PLL loop filter?

A PLL loop filter removes high-frequency noise and spurious signals from the phase detector output, ensuring stable operation and reducing phase noise in the locked output signal.

How does loop bandwidth affect PLL performance?

Wider loop bandwidth provides faster lock time and better tracking of input frequency changes, but increases noise. Narrower bandwidth reduces noise but slows response time.

What is phase margin and why is it important?

Phase margin is a stability measure indicating how much additional phase shift would cause instability. A phase margin of 45-60 degrees provides good stability with reasonable settling time.

When should I use a 3rd order filter instead of 2nd order?

Use 3rd order filters when you need better spurious suppression and can tolerate increased complexity. 2nd order filters are simpler and adequate for most applications.

How do I choose the charge pump current value?

Higher charge pump current reduces lock time but may increase noise. Typical values range from 0.5mA to 5mA, with 1-2mA being common for most applications.

What happens if my VCO gain is too high?

Excessive VCO gain can cause instability and increased sensitivity to supply voltage variations. It may require narrower loop bandwidth to maintain stability.