Design a Low-Pass π (Pi) Filter
Use this calculator to estimate component values for a constant-k low-pass pi filter (C-L-C) using cutoff frequency and characteristic impedance.
What is a pi filter?
A pi filter is a three-element network shaped like the Greek letter π: a shunt capacitor at the input, a series inductor in the middle, and another shunt capacitor at the output. In low-pass form, it helps pass lower-frequency content while reducing higher-frequency noise and harmonics.
You will see pi filters in RF design, power conditioning stages, and EMI/noise suppression. The exact behavior depends on real-world loading, source impedance, and component quality factors, but a first-pass calculator gets you close to practical values quickly.
Formulas used in this calculator
This page uses the classic constant-k low-pass pi-section relationships:
L = Z0 / (π fc)
Ctotal = 1 / (π fc Z0)
C1 = C2 = Ctotal / 2
Where:
- fc = cutoff frequency in Hz
- Z0 = intended source/load impedance in ohms
- L = series inductor in henries
- C1, C2 = shunt capacitors in farads
How to use this pi filter calculator
1) Set your cutoff frequency
Choose the frequency where your response should begin rolling off. Lower cutoff frequency generally means larger inductors and capacitors.
2) Enter target impedance
Use the impedance your filter is intended to work with (for example, 50 Ω in many RF systems). A mismatch between actual source/load and design impedance shifts performance.
3) Review practical component choices
After calculation, select the nearest standard values and verify with simulation or bench measurements. Real components include ESR, DCR, parasitics, and tolerance drift.
Example design
If you choose fc = 10 kHz and Z0 = 50 Ω, you get approximately:
- L ≈ 1.59 mH
- C1 = C2 ≈ 318 nF each
That is a solid starting point for a low-pass section, then you can tune based on your real load and desired attenuation profile.
Practical engineering notes
- Inductor current rating: Make sure it does not saturate at your peak current.
- Capacitor voltage rating: Keep margin above operating voltage.
- ESR and DCR: These losses affect insertion loss and heating.
- Self-resonance: Above SRF, components stop behaving ideally.
- Damping: Sometimes a small resistor is added to control peaking/ringing.
FAQ
Is this calculator for power supplies or RF?
It is based on a generic constant-k low-pass pi model, often used for RF/filter prototyping. It can also serve as a starting point for power filtering, but power-supply ripple filtering usually needs extra context (rectifier frequency, load variation, ripple current limits).
Why are my measured results different?
Common reasons include impedance mismatch, parasitic PCB effects, component tolerances, and non-ideal source/load behavior.
Can I cascade sections?
Yes. Cascading can steepen attenuation, but it also increases insertion loss and sensitivity to tolerances. Simulate before hardware whenever possible.