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I'm finding myself needing to do a fair bit of analog filter design. Mostly passive LC ladder filters. I honestly don't know much about how to design them by hand, I've been using the Genesys tool.

I'm also getting into using a lot of free software for my work. I find the work flow is generally faster and more enjoyable. So, I've been looking for a free software analog filter design tool. It's quite easy to design digital filters with Python, but there doesn't seem to be much for analog filter design (and I mean choosing the values of capacitors and inductors).

So, am I missing something? Is it straightforward to realize an analog filter with these tools? If not, why does nothing like this exist? What are the typical algorithmic methods for designing analog filters?

*EDIT: What I mean is free as in freedom software. I'm aware of all the vendor provided tools, I don't like them. I usually find them much more difficult to work with. And, I'm also just curious now about why there doesn't seem to be much open software in this area.

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    \$\begingroup\$ I used webench by texas instruments for active filters and Elsie for passive ones. Both are free. Give them a try. \$\endgroup\$
    – Plutonium smuggler
    Commented Jan 7, 2015 at 15:05
  • \$\begingroup\$ Maybe you should clarify if what you're looking for is free-as-in-free-beer software (plenty of which exists from chip vendors and has been suggested below) but which may well be supplied without sources or if you're looking for open-source software. \$\endgroup\$
    – got trolled too much this week
    Commented Jan 7, 2015 at 15:37
  • \$\begingroup\$ As for the "algorithmic methods", any book on filters will teach you the design principles... which you can implement in any software you want. Many books will use MATLAB (which is not free) as the software aid for the calculations. You can translate MATLAB code with almost no changes to GNU Octave (which is free and open-source). \$\endgroup\$
    – got trolled too much this week
    Commented Jan 7, 2015 at 15:50
  • \$\begingroup\$ There is a significant amount of free software on the IowaHills.com site. There is software for RF filters, Op Amp filters, Digital filters, as well as a Smith Chart. \$\endgroup\$
    – user5108_Dan
    Commented Aug 12, 2017 at 15:33

4 Answers 4

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There are a number of online tools though they are often limited. Here is a list of online and free desktop tools

Active filter web based tools

Active filter desktop design tools

Passive filters

Simulation

I would always recommend simulating the software generated filter with real life simulation models of the components you intend to use for your implementation.

Use a different simulation tool that the generation tool to limit the possibility of software bugs effecting your design.

If you are not dealing with purely sinusoidal signals remember to consider the step and impulse response.

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  • \$\begingroup\$ Note that the AD filter wizard can produce designs that have very bad impulse response; not sure if they now have an option to limit the Q factor. \$\endgroup\$
    – Peter Smith
    Commented Nov 29, 2018 at 16:59
  • \$\begingroup\$ Thanks peter, I updated the answer to make sure people don't just belive the proposed solution. \$\endgroup\$
    – Jay M
    Commented Nov 29, 2018 at 17:58
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Some of the IC vendors offer helpful tools; last time I did a bit of simple RC filtering I used Tina-TI which is a SPICE-based simulator. It has a menu in it which allows you to compute the frequency response of a subcircuit and view a graph. SPICE will certainly allow you to confirm a filter design once you've chosen values.

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Her is a very versatile PASSIVE filter design tool: "AADE" from AADE.com. Using this program you can design all classical RLC filter topologies.

For ACTIVE filter projects I prefer FilterPro (www.ti.com), which is available for free.

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I use microcap and there is a student version available. My colleagues use LTSpice because it is free but, when I want to go straight for values in things like sallen key op-amp filters or LCR filters I use this webpage from Okawa.

They do RC, RL, RLC (3 types), sallen key HP and LP (2nd and 3rd order with and without gain) and multiple feedback filters (2nd and 3rd order) - covers most folks needs I reckon.

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