This tool takes the filter specification selected in previous steps and computes real-world component values. The original specification can be modified and the effects of those modifications explored on a mag-phase diagram.
Modifications are grouped into four categories: top-level specification (incl. Butterworth, etc., order, fC/fH, and Q), pole location (F0n, Qn) and topology, independent component values and finally dependent component values. Note: Changes entered into text fields must be accepted by hitting "Enter", or they will not take effect.
The top-most sub-panel of the tool allows the filter specification to be changed. Changing any of the parameters found here recomputes all pole-zero locations and component values. It also enables the "Validate" button at right, which allows you to go back and check that your modifications still meet the original specification.
After the specification parameters have been selected, the section immediately below displays a set of boxes corresponding to the stages that will be cascaded to implement the specified filter function. Up to 8th-order filters can be designed with this tool, and since each stage can have a pair of complementary poles, from 1-4 stages are displayed. Each stage is 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, however, usually the element of interest is the pull-down menu at the bottom of each stage which selects an implmenting topology for the stage. A default topology has already been assigned. Changing the topology should display a corresponding schematic in the third section immediately below the pole-zero section. Note: Changes to this and subsequent sections make it impossible to use the Validate button to check the result.
The next panel has two independent displays selected by tab buttons: a stage schematic with circuit design parameters ("Circuit") vs. an overall filter mag-phase chart ("Mag-Phase"). Each stage sub-circuit needs several independent design parameters to be entered in the area 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 immediately below this one. Note: Design parameters must be entered separately for each stage by selecting the stage and then entering the design values.
The overall 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 real response are both plotted: green for the magnitude and red for the phase with the real-world response in darker colors and usually hidden behind the ideal plots. Click "Zoom" for an enlarged view, and use drag-zoom for even more detail.
Component values for each component in each stage can be manually entered and the effect seen on the overall mag-phase response. The effect of component variation is also 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. Also, please note that the "Tolerance" menu currently only demonstrates the effect of a limited selection of component values but not the additional deviation resulting from manufacturing tolerances.
When you are satisfied with the specification, pole-locations, topologies and component values, click either "SPICE" or "Schematics and BOM" for saveable design data.
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.