Active Filter Workshop

Active Filter Design

Figure 1. Design Screen for Sallen & Key Filters


Active Filter Workshop is a suite of filter-design utility programs designed to operate in the Windows* environment. Types of filters handled include Sallen & Key filters, State-Variable filters, shelving filters, "Hard-Wired Parametric" filters and "All-Pass" delay filters. While Active Filter Workshop is primarily intended to be used by loudspeaker designers who need to design complementary electronics (crossovers, equalizers, etc.) it should have numerous other uses in engineering and education. Active Filter Workshop tries to provide the user with as much insight as possible by providing detailed schematics and showing the math necessary to determine component values.

Figure 2. Schematics and Transfer Functions for Third-Order Sallen & Key Lowpass Filter

Detailed Description:

Active Filter Workshop relies on an intuitive Windows* user interface to simplify the process of designing filters. Because direct access is provided to filter design parameters (coefficients, pole angles and the like) it is possible to design very complex filters, such as those having multiple-slopes, with a minimum of components. Another important feature of Active Filter Workshop is its handling of component values. In all but the "Hard-Wired Parametric" module the user selects common capacitor values and the program calculates the 1% resistor values required. This is important because there are many more available resistor values than capacitor values. A designer can design many types of filters with only a small stock of capacitor values; or, for that matter, whatever capacitors he may have on hand!

First through third order Sallen & Key filters are implemented using a single op-amp stage. Higher order Sallen & Key filters are multi-stage designs. State-Variable filters of order up to five are also handled. The schematics for the State-Variable filters have been laid out in a manner that helps with the user's intuitive understanding of the circuits. To design a filter the user selects the type (Butterworth, Chebychev, etc.), whether it's high-pass or low-pass, the order, the cutoff frequency and the desired capacitor value. Corresponding resistor values are calculated immediately. These can be copied or modified (entered manually or set to 1% values) and the resulting theoretical frequency-response and phase-response curves graphed.

Figure 3. Graph of Theoretical Filter Response

For equalization purposes two types of filters can be readily designed. First, simple high-pass and low-pass shelving filters are covered. The user enters the amount of boost required (in dB), the cutoff frequency and the desired capacitor value. Corresponding resistor values are calculated immediately. These can be copied or modified (manually or set to 1% values) and the resulting frequency-response and phase-response curves graphed.

Perhaps the most unique feature of Active Filter Workshop is the Hard- Wired Parametric filter designer. The original purpose of this module is the design of dedicated loudspeaker-specific equalizers, however it should be useful wherever non-adjustable parametric equalizers are required. The circuit is derived from an octave-band equalizer (National Semiconductor Applications Manual). The adjustment potentiometers have been omitted and only the number of filter sections required for a particular application are implemented. Each filter section requires one op-amp, two capacitors and two resistors. Symmetrical boost and cut filters are possible. Up to four filter sections can be designed simultaneously and their combined response displayed. Generally, boosts or cuts of up to 15 dB and Q's of up to 3 are possible. Opposing filters can be placed adjacent to each other if steeper slopes are required. Although there is significant interaction between the various filter sections useful filter circuits can be quickly and accurately designed. As an added feature, measured frequency-response data can be imported, graphed and inverted to provide a target for the designer. Components are calculated and selected in a manner that is consistent with the other parts of this program. As before, the frequency and phase response of the newly-designed filter can be graphed.

Finally, the All-Pass Delay filter design facilitates the design of multi-stage active group-delay filters. One possible application of this circuit is the time-alignment of the various drivers in a multi-way loudspeaker. Delays of up to 1 ms are readily attainable for lower frequencies with a reasonable number of stages. To design a delay the user enters the desired delay time, the cutoff (half-delay) frequency and the capacitor value. The number of stages required and the required resistor value are then calculated. The total group delay and phase- response can also be graphed.

Help screens are available throughout the program. They contain step-by- step instructions for each part of the program, making an instruction manual unnecessary. Helpful hints about component selection and circuit design are included, as are warnings about circuit instabilities, etc. If the user desires, he may print the help files which are in ASCII-text format. Design and Graph screens can be printed to the default Windows* printer.


Active Filter Workshop simplifies filter design and makes it more accessible. Both a design tool and a learning tool, it should find broad usage among engineers, students and hobbyists. It does, however assume basic skills on the part of the user in the implementation of op-amp circuits.

*Windows is a registered trademark of Microsoft Corporation.

This page Copyright (C) 1998 by Frank Ostrander. All Rights Reserved.