Instructions
Filters are initially specified by overall type and function in the first gray region at the top of the applet. Low/High/Bandpass, Bessel/Butterworth/Chebyshev/etc, order and corner frequency, fC, must be chosen first. When you are satisfied with changes made to a form field (or fields), hit ENTER to accept those changes and recalculate.
After the gross parameters have been set, the section immediately below displays a set of boxes corresponding to the stages that must be cascaded to implement the system function specified in the previous step. Up to 8th-order filters can be designed with this tool, and since each stage can have a pair of complementary poles, 1-4 stages are displayed. Each stage can be individually selected and edited by clicking on the border area or stage label. A stage is selected when a blue box highlights it.
The pole/zero locations can be manually edited (see below), however, usually the element of interest is the pull down menu at the bottom of each stage which allows selection of a concrete implementation topology. When a filter is first specified, default topologies are assigned to each stage. Changing the stage topology should display a corresponding schematic in a third section immediately below.
Each topology needs several key design parameters to be entered in an area to the right of the schematic so as to calculate the other component values in the stage. Although default values are provided, they will probably need to be customized for your application. Once those values have been entered, the other component values appear in the fourth section below the schematic/mag-phase panel. Design parameters must be entered separately for each stage by selecting the stage and then entering the design values.
The system magnitude-phase response of the entire filter can be plotted by clicking on the "Mag-Phase" tab next to the "Circuit" tab of the third section. The ideal and component-modified system responses are both plotted: green for the magnitude and red for the phase with the component-modified response in darker colors and usually hidden behind the ideal plots. Note: The component-modified response does not include opamp behavior and so real-world performance can deviate substantially from the shown response, most commonly at higher frequencies.
Component values for each component in each stage can be manually entered and the effect evaluated on the system mag-phase response. The effect of component variation is quantitatively shown by the pole-zero deviations in the fifth panel at the bottom of the applet. The effects of component quantization can also be explored by choosing a setting from the "Tolerance" menu although the exact component values continue to be shown in the component fields. Also, please note that the "Tolerance" menu currently only demonstrates the effect of a limited selection of component values, not normal manufacturing/parametric deviation from those values.
Printing this page will result in printing the filter schematic when in schematic mode, or printing of the mag-phase plot when in that mode. Printing is currently only supported from Microsoft Internet Explorer.
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