Design LC Filters (V 4.0 June 15, 2018 )

This web based application allows the user to design simple radio frequency filters with inductors and capacitors. These filters are most effective between 50 kHz and 500 mHz. Below 50 kHz active filters are usually more cost effective and above 500 mHz strip lines are generally used.

Inductor sources:
CoilCraft
DigiKey

Wind your own:
Inductor Calculator for hams
DIY Audio inductor calculator
Multilayer air core inductor calculator
Inductor calculator for Windows
Use Google to find more.

Formulas and tables used in this program are from Electronic Filter Design Handbook by Arthur B. Williams. Open source code is here.

Design LC low pass and high pass Filters


Type
Poles
3 db cutoff frequency
I/O Impedance in ohms

The user selects the basic type (Lowpass or Highpass), number of poles, 3 DB cut off frequency and the I/O impedance. The application generates the parts values for three response shapes: Butterworth, Bessel and 0.1 DB ripple Chebyshev. Two different configurations with schematics are generated. The frequency response plots are approximations which assume perfect components and construction. Real world results with not be as good. The Bessel plots are only accurate to 2x the cutoff frequency (1/2 for highpass).


Design LC band pass and band stop Filters


Type
Poles
Center Frequency
3 db bandwidth
I/O Impedance in ohms

The user selects the basic type, Bandpass or Bandstop, number of poles, 3 DB bandwidth and the I/O impedance. The application generates the parts values for three response shapes: Butterworth, Bessel and 0.1 DB ripple Chebyshev. Two different configurations with schematics are also generated.

Note: These filters are sensitive to stray capacitance and inductance and the Q of the components. Keep in mind that capacitors have series inductance and inductors have parallel capacitance and series resistance which in some cases can be very significant.

I have received emails from people having issues with the program computing inductor or capacitor values that are zero or very low. This is caused by the user specifying parameters difficult or impossible to achieve with LC filters. You may have problems with center frequencies higher than 250 mhz or bandwidths less than 10% of the center frequency. High frequencies require small value capacitors and inductors. Stray inductance and capacitance in the circuit add to the component values and in extreme cases may even exceed them. Band pass filters with narrow bandwidths require components with high precision and Q. Bandwidths less than 10% of the center frequency can be difficult or even impossible to implement with discrete LC components. Your mileage may vary.

Bandpass filters with a bandwidth wider than one octave may not perform as expected. If you require bandwidths greater than one octave you need to cascade a lowpass and a highpass filter.

If an LC filter can't do what you need because the frequency is too high I suggest looking at an interdigital bandpass filter.

This program had some bug fixes done on June 15 2018 to correct bad plots for wide bandpass filters. Please report remaining bugs on my contact page.