A Pi filter consists of Inductors and Capacitors (3 in total) arranged in a **Π **configuration. There are four tools on this page:

- The first two calculate Cutoff frequencies for both High and Low Pass Pi filters when L and C component values are known.
- The last two calculate L and C component values for both LPF and HPF with specified cutoff frequencies.

Contents

## Cutoff Frequency for Pi Low Pass Filter

A LPF consists of one series inductor (L1) and two identical shunt capacitors (C1=C2) as shown in the picture below.

Enter **L** and **C** values below to find the cut off.

**Formula**

**f _{c}= 1/(π*√(LC))**

**Example Calculation**

For **L = 1 uH** and **C = 1 uF**, the cutoff frequency is **318 kHz**.

**Cutoff Frequency for Pi High Pass Filter**

A HPF consists of one series capacitor and two identical shunt inductors as shown in the picture below.

Enter **L** and **C** values below to find the cut off.

**Formula**

**f _{c}= 1/(4*π*√(LC))**

**Example Calculation**

For **L = 1 uH** and **C = 10 uF**, the cutoff frequency is **25 kHz**.

**Component Values (L, C) for Pi Low Pass Filter**

A LPF consists of one series inductor (L1) and two identical shunt capacitors (C1=C2) as shown in the picture below.

Enter cutoff frequency for the LPF and impedance to find the inductor and capacitor values for this pi network.

**Formula**

**L = Zo/(π*Fc)**

**C = 1/(Zo*π*Fc)**

**Example Calculation**

A low pass filter with **500 MHz** cutoff frequency requires **L = 31.8 nH** and **C = 12.73 pF**.

**Component Values (L, C) for** **Pi High Pass Filter**

A HPF consists of one series capacitor and two identical shunt inductors as shown in the picture below.

Enter cutoff frequency for the HPF and impedance to find the inductor and capacitor values for this pi network.

**Formula**

**L = Zo/(4π*Fc)**

**C = 1/(Zo*4π*Fc)**

**Example Calculation**

A high pass filter with 1 GHz cutoff frequency requires **L = 4 nH** and **C = 1.6 pF**.

## Why do we need filters in a power supply?

Pi filters are typically used at the input to switching supplies. They prevent high-frequency noise from getting out of the switcher and into the source and other parts of the electronic circuit. Filters also prevent high frequency noise from entering the switching supply.

The calculator on this page can be used if the input and output impedance are the same. If however, the impedances are unequal then this Power supply filter design tool can be used to design a Butterworth LPF.

**Related Calculators**

- Pi Attenuator
- Filter Bandwidth – compute the bandwidth and center frequency from the start and stop frequencies