Designing passive LC-filters typically involves looking up prototype filter component values in a table in reference books like “Handbook of Filter Synthesis” by Zverev or “Design of Microwave Filters, Impedance-Matching Networks, and Coupling Structures” by Matthaei et. al., then transforming the values to produce a filter with the desired impedance and cut-off frequency. This can be a bit tedious and error prone, so when I found out about the classic Matthaei book, which is downloadable via the link above, and that it contains the math required to calculate the prototype filter component value tables, I set out to create a spreadsheet to help with the filter design.

In addition to calculating the (previously) magical prototype tables for Butterworth and Chebyshev (with user-specified pass-band ripple) filters, the spreadsheet also performs the frequency and impedance transformation for filters of orders from 1 to 10.

To make the design process even quicker and better, I added a feature to create LTSpice schematics of the selected filter so that the filter properties can be simulated (and perhaps manually adapted to standard component values and to include parasitics) using LTSpice. I used the SI prefix formatting function I wrote about in the previous blog post to write out the component values in a pretty manner.

The usage of the spreadsheet should be fairly self-explanatory, but there are also usage instructions on the first tab. Basically, the user should fill out the values in yellow cells and leave the rest alone. I did not lock any cells, since I often get annoyed by spreadsheets with locked cells and I encourage others to modify and improve it.

Here is a link to the Excel 2002 file:

FilterSynthesis_v1.0.xls

Make sure that macros are enabled if you want to use the LTSpice export features.

Here are some screenshots