Using the information from Example, design a filter that blocks only the highest frequency...

Using the information from Example, design a filter that blocks only the highest frequency from passing, while avoiding the other frequencies to pass.

APPLICATION EXAMPLE

Engine Noise Removal

An inventor is wondering why her recently built engine is making noise. She can detect three distinct audible frequencies. She wants to hear them one at a time, so that she can fix each problem separately. She connects a microphone to the engine, which sends the noise through an electric filter and an amplifier before reaching her headphones. The equation that defines the noise is:

Design a filter that allows the lowest frequency to pass, but not the others.

SOLUTION

For this problem, a low-pass filter is needed with a break frequency at, or slightly above, the lowest frequency described above. The lowest frequency is 250 Hz (500p = 2p * 250). Because the signal should be attenuated 3 dB at the break frequency, set the break frequency to 300 Hz. Therefore, the transfer function is:

The filter is shown in Figure.

1 fB = =300 Hz

2pRC

Using the break frequency equation, values for R and C can be determined. There are an infinite number of possibilities. One possibility is to use C = 10 μF.

FIGURE Filter for Example.