LC Filter Design

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Introduction

This is an interactive design package for designing analogue (i.e. hardware) filters made of inductors and capacitors (Ls and Cs).

Fill in the form and press the ``Submit'' button, and a filter will be designed for you.

If you've come here by mistake and are looking for a digital filter designer, look here instead.

I've been advised to say that, although I believe the package to be bug-free, you use the results at your own peril, and you're advised to check them for correctness before using them. Don't blame me if your aircraft falls out of the sky!

The graphs are produced with the help of the excellent gd GIF manipulation library, originally from Quest Protein Database Center.

When you press ``Submit'', you will be shown a circuit (schematic) diagram of the filter, with component values, as in the example on the right. The round circle is a voltage source and represents the input to the filter. The resistor in series with the input (R1 in the example) represents the filter input impedance, and the resistor at the output (R2) represents the filter output impedance. The circle with an M represents a voltmeter measuring the filter output.

The design procedure is given in the superb book ``Radio-Frequency Electronics'' by Jon B. Hagen.

An email message, saying what you like and don't like about this package, will be appreciated.


Specify Your Filter:

Select filter type:   (Note that Bandstop is not yet implemented.)

Enter the filter order. For lowpass and highpass, this is the number of poles. For bandpass and bandstop, the number of poles is twice the order.
Order:

Enter the corner frequency/ies, in Hz. For lowpass and highpass, one corner frequency is required: enter this in the first slot and leave the second one blank. For bandpass and bandstop, two corner frequencies are required. The corner frequency is defined as the -3dB point for Butterworth designs, as the highest frequency at which the response is equal to the ripple for Chebyshev lowpass designs, and in an obviously corresponding way for other Chebyshev designs.
Corner frequency 1: Hz
Corner frequency 2: Hz

Enter the filter characteristic impedance. In all cases, unless you specify a matching network (see below), this is equal to the filter input (source) impedance. For Butterworth and odd-order Chebyshev filters, this is also the filter output (termination) impedance. For odd-order Chebyshev filters, the output impedance is slightly less than the input impedance. You will be told the exact value of the output impedance.
Characteristic impedance: ohms

Filters come in two dual configurations. Do you want the filter to start with a series branch or a shunt branch?


Optional Matching Network:

If you are getting unreasonable component values, perhaps you have specified an inappropriate characteristic impedance for the filter. Multiplying the characteristic impedance by x multiples the inductor values by x and divides the capacitor values by x, so if you want smaller inductors you should reduce the characteristic impedance.

If you change the characteristic impedance, you can ask the program to add matching sections at either end to transform the impedance to the values you require, by filling in the optional fields below. A typical value for Q is 3, and for f0 is the centre frequency (for a bandpass filter) or the corner frequency (for a lowpass or highpass filter).

For further details, see the book ``Solid State Design for the Radio Amateur'' by W. Hayward & D. DeMaw (ARRL, 1986) [out of print].

To create a matching network on its own (without a filter), specify an ``order 0'' filter. For a really rather boring design, specify ``order 0'' and no matching network.

Transform source impedance to ohms
Transform termination impedance to ohms
Q of LCC matching network:
f0 of LCC matching network:


Graph Plotting Parameters:

You may enter the frequency limits for the response graph. If you leave these fields blank, suitable values will be chosen for you.
Plot from Hz to Hz

By default, the frequency response graph has a linear magnitude scale. If that is what you want, leave the following box blank. If you want a logarithmic magnitude scale in dB, enter the lower limit of the magnitude scale in dB here (e.g. -80).
Lower limit (dB), or blank for linear scale:


Submit Form:

When you've filled in the form, press ``Submit'':
Submit form:     Reset form: