A filter is a device used for extracting useful parts of a signal and rejecting unwanted ones. This tool allows a user to study filters of upto second order in time and frequency domains. Commonly encountered filters are the bass and treble controls on a music system.

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How to use the tool

This tool simulates the unfiltered and filtered signals in the time domain. It also has a Bode plot for study of the frequency domain behaviour and a pole-zero plot. To use the tool, select the order of the filter from the drop down box in cell B10. Key in the filter parameters in the specified cells. Key in a function that represents the time domain signal in the cell B8. This is be similar to entering a function in the Function Calculus Tool. The following screenshots show the entries and outputs for a second order Butterworth filter where the input signal is SIN (0.8x) + 0.5 SIN (10x). Notice how well the high frequency terms are rejected in the output. (This is an example of a low pass filter).

Step and Impulse Responses

Response of filters to Step and Impulse signals is a very common way of studying filters in the time domain. To generate a step response, key in "1" into the cell B8 and to generate the impulse response, key in the function "=IF(x=0,1,0)". The following figure shows the step response for the 2nd order Butterworth filter.