Contents
A CLC filter is comprised of a Capacitor-Inductor-Capacitor in a pi π configuration. This creates a low pass filter (LPF).
The first calculator on this page finds values for each of the L and C components based on cutoff frequency values. The second calculator finds the cutoff frequency when the L and C values are specified.
CLC Filter Design Calculator
Enter cut-off frequency for the LPF and desired impedance to find the Inductor (L) and capacitor (C) values for this Pi network.
Background
What is a Low-Pass Filter?
A low-pass filter (LPF) allows frequencies below a specific cut-off frequency to pass through while attenuating higher frequencies.
The picture below shows an ideal low pass filter. Signals with frequency less than the cut-off frequency Fc pass through while signals with higher frequencies are blocked or attenuated.
In practice a low pass filter doesn’t have the ideal brick wall characteristic shown in the picture above. Instead it has a cut off frequency which is where the signal is attenuated by 3 dB (also called insertion loss). After that it rolls off as shown in the picture below. Attenuation increases with frequency.
How to design a CLC Filter
A LPF consists of one series inductor (L1) and two identical shunt capacitors (C1=C2) as shown in the picture below.
The formula for the calculation is given below. Inductor and Capacitor values are calculated from the characteristic impedance and Fc.
Formula
L = Zo/(π*Fc)
C = 1/(Zo*π*Fc)
CLC Filter Design Example Calculation
A low pass filter with 500 MHz cutoff frequency requires L = 31.8 nH and C = 12.73 pF.
The filter is symmetric. This means that the attenuation does not change if the input and output are reversed.
💡 It’s important that the filter impedance match that of the source and load (usually 50 ohm and sometimes 75 ohm). In the event that there’s a mismatch, some of the power will be reflected back. Use this tool to calculate the forward and reflected power.
Applications of Low Pass Filters
It is commonly used to remove or reduce high-frequency noise or signals. In power supplies a LPF will be used prevent high frequency signals from entering other parts of the circuit or the main supply.
How to calculate the cutoff frequency of a CLC filter?
In some situations you might find an application note or chip data sheet that has a CLC filter design. To calculate the cut off frequency for the filter from the capacitor and inductor component values use the calculator below. Simply enter the L and C values and it will provide Fc.
Formula
fc= 1/(π*√(LC))
Example Calculation
For L = 1 uH and C = 1 uF, the cutoff frequency is 318 kHz.
Related Posts
- 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
- Pi Filter Cutoff Frequency – Use the component values from a schematic to find the cutoff frequencies for both low and high pass filters
- Inductor Impedance – calculate the reactance and impedance from the inductance