# Pi Filter Cutoff Frequency Calculator (with Examples)

• Pi Low Pass Filter
• Pi High Pass Filter

A Pi filter consists of Inductors and Capacitors (3 in total) arranged in a Π configuration.

## Cutoff Frequency Calculator 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.

fc= 1/(π*√(LC))

### Example Calculation

For L = 10 uH and C = 1 pF, the low pass cutoff frequency is approximately 100 MHz.

## Cutoff Frequency Calculator 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

fc= 1/(4*π*√(LC))

### Example Calculation

For L = 10 uH and C = 1 pF, the high pass cutoff frequency is approximately 25 MHz.

## What is Cut off frequency?

In electrical engineering, the cutoff frequency (also known as corner frequency) is the point in a filter’s frequency response at which energy flowing through is reduced or attenuated rather than passing through.

The picture below shows a cutoff frequency of an ideal low pass filter. At frequencies above Fc, no signal passes through while signals less than Fc pass through with no loss.

In practice however, a filter doesn’t behave this way.

The 3 dB cutoff frequency is commonly used as a reference point because it represents the frequency at which the filter’s output power is reduced to half (-3 dB) of its maximum value. This point is considered significant in determining the effective range of frequencies that a filter can process. The picture below shows the cutoff for a low pass filter.

In this case if the cutoff frequency is 100 kHz for example, then this is the point where the signal power is attenuated by 3 dB. Frequencies lower than this are attenuated by a smaller amount. Typically the attenuation for a well-designed filter is less than 2 dB.

## Why use this calculator?

Many times analog designers have to understand a schematic with pi filters.

These filters are commonly used around power supplies, Phase-locked Loops (PLLs), etc. In this case, the configuration can easily be understood to be either Low or High pass depending on the inductor and capacitor configuration.

Then either of the two calculators can be used to determine the cutoff frequency. A first step in understanding the design intent.

## Related Calculators

• CLC Filter – provides component values for a desired frequency response
• Pi Filter – includes four different calculators for both low pass and hi pass filters
• Power supply pi filters – for both equal and unequal impedance
• Filter Bandwidth – compute the bandwidth and center frequency from the start and stop frequencies. Useful when you look at the data sheets for SAW filters that don’t provide this info up front.
• T Filter – use this when the input impedance is low
• Inductor Impedance