Filters Tutorial

If you work with audio circuits but are not getting the desired results, you may need to add a filter to your project. A device may not work for a number of reasons, including your location and unique circumstances. A filter can sometimes make all the difference by enabling you to tune out strident or harsh peaks in the overall frequency response of the circuit. To understand how filters work, we must examine the components that are required to perform the task.

Capacitors

The single most important component in the fabrication of filters is the capacitor because of its frequency-selective characteristic. The illustration below shows the types of packages in which capacitors are found in the commercial market. Capacitors pass only AC signals, and large capacitors pass more of the audio spectrum than small capacitors. Very large capacitors pass the entire audio range. This comes in handy when you need to remove all traces of audio signals in a DC power supply.

Coils

Unlike capacitors, coils pass both AC and DC but resist high-frequency signals that make them a natural choice for a low-pass filter. Some coils are wrapped around a multi-layered metal core and are called chokes. If two or more coils share a common core, it is called a transformer.

A large capacitor wired across the output of a DC power source will eliminate AC signals. Add a coil and another large electrolytic capacitor and you have a pi filter. Pi filters (so named because of their resemblance to the mathematical symbol) are the most common filters in the world — they are included in every TV, computer, radio, and stereo system.

Since the pi filter eliminates all audio, it can be considered a no-pass filter.

High-Pass Filters

Other types of filters eliminate only a part of the audio spectrum. When sound researchers need to examine a high-frequency element, they run the signal through a small capacitor. This allows high frequencies to pass while blocking low frequencies.

A potentiometer in parallel enables the experimenter to adjust the amount of low-frequency energy that is allowed to pass.

With the addition of a shunting element, the filter becomes more effective and stable. The two-element filter is called a double-pole filter and is very popular with electronic music synthesizers.

By adding a potentiometer, the filter becomes adjustable. The additional resistor prevents the total shunting of the signal to ground.

The resistor should be as low as 470 Ω while the potentiometer should be as much as 1 MΩ. Since component values and conditions can vary, experiment with different values to meet your needs.

The effectiveness of a high-pass filter can be tested in the filter illustrated above. Switch S1 can switch both elements in or out of the circuit.

Added stages can refine the effectiveness of the filter, as you can see in the illustration below.

Two pass elements, R1 and R2, combined with two shunting elements, C1 and C2, make an impressive low-pass filter. The transistor provides amplification to compensate for the amount of signal loss through multiple components. Potentiometer R5 is a dual pot so it changes the resistance of both elements of the shunting circuits.

Series and Parallel Modes

Capacitors and resistors react quite differently in the parallel and series modes.

As you can see, parallel capacitors add up to a total capacitance reading. In series mode, the total capacitance is equal to only the smallest value. Resistors, on the other hand, add up in value when used in series mode and decrease in value to a little below the smallest value. The exact resistance value can be calculated using the formula in the above illustration. This can be a handy way to obtain a specific resistance value for a given purpose.

Voltage-Controlled Filters

For advanced tonal control, synthesizers sometimes use a variable voltage to make filter changes. This enables the user to assign control voltages to perform tasks that are impossible to do manually. The voltage-controlled filter can also be synchronized with other elements, such as voltage-controlled amplifiers, modulators, and other filters.

Cutoff Frequency

No filter can pass signals to a point and then pass nothing beyond that point. Instead, filters have a roll-off response. Single-pole filters, for the most part, have a three-dB/8ve signal depletion rate, while double-pole filters fall into the category of six dB/8ve. Amplified and feedback filters can reach 18 to 24 or more decibels. The cutoff frequency is the point where the filter takes effect.

Resonant Filters

Filters with 12, 18, and 24 dB can be achieved by using feedback elements that create a controlled resonance. Operational amplifier ICs are perfect for use in filters that use feedback to increase the roll-off rate. Above is an illustration of this type of filter.

Feedback is a productive way to increase the effects of filtering, but care must be taken to prevent the filter from going into the oscillation. Potentiometer R3 can help control the effects of feedback and pinpoint the desired cutoff point. The control voltage can also prevent oscillation and is adjusted through potentiometer R5.

Conclusion

There is no doubt that filters play a major role in modern electronics — power supplies, frequency band tuning and tone controls on radios and stereos, synthesizers, professional audio equipment, and the list goes on — and having a basic knowledge of them has become necessary to increase your design capabilities.