Project II (Part 2) – Filters

The three basic components to any filter include capacitors, resistors, and inductors. Using these components, up to four different kinds of filters can be created. Operational amplifiers are often used in conjunction with filters in order to amplify the output of the filter.

High Pass & Low Pass Filters

As the name suggests, high frequencies in a high pass filter are passed through while cutting off the lower frequencies. The circuit (pictured at right) usually implements an inductor and a resistor, although combinations of a resistor and capacitor are also used. Depending on the values of these components, you can pick the frequency where the lower frequencies begin to get cut off.

Low pass filters are similar to high pass filters except that they block high frequencies while leaving the lower frequencies intact. Usually a capacitor and resistor are used in series with the output of the filter being across the capacitor (see image to left). Changing the values of the capacitor and resistor changes the frequency at which the filter starts eliminating frequencies.

Since inductors are hard to find in stores, I will try not to use them when making the projects. If you have inductors, you can feel free to use them and experiment with them in filter circuits.

Band-Pass & Band-Stop Filters

These filters utilize both the capacitor and the inductor that are sometimes used in conjunction with resistors. Changing the values of the components will change the frequencies at which they are cut or passed. Although, how the capacitor and inductor are placed in the circuit dictates whether a circuit is a band pass or band stop filter.

If the inductor and capacitor appear in parallel to each other, the circuit represents a band-pass filter. The image to the left illustrates the output of a band-pass filter. Notice that frequencies below f_L and frequencies above f_H are cut. Ideally, none of the frequencies below f_L and above f_H would be passed through, but in reality, trace amounts of the frequencies end up being passed.

If the inductor and capacitor are in series with each other, the circuit represents a band-stop filter. The output appears opposite of the band-pass filter and cuts out frequencies between f_L and f_H. If the difference between f_L and f_H is small, the filter is often referred to as a notch filter.

The next blog entry will focus on calculating capacitor and resistor values and how to filter your music and cut out higher and lower frequencies.

Resources:

http://en.wikipedia.org/wiki/High_pass_filter

http://en.wikipedia.org/wiki/Low-pass_filter

http://en.wikipedia.org/wiki/Band-pass_filter

http://en.wikipedia.org/wiki/Band-stop_filter

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