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All the tools I have used so far design for matched impedance, I need something,preferabely free which gives me this option. the requirement is for a higher ( >=4) order filter, lossless, for a 8 ohm (or more) load, and very small source resistance. changing the values works ( in ELSIE) for up to 4th order, with a little bit of adjustment, but for higher orders I do not get reasonable results.

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    \$\begingroup\$ Achieving results comparable to the nice plots that design software gives as output becomes difficult (often requiring hand-tweaking) for filter orders above four. \$\endgroup\$ – glen_geek May 7 '17 at 13:48
  • \$\begingroup\$ I am expecting to do some tweaking, but I was hoping to find a design tool which allows for mismatched impedances in the design phase. what I did so far was design a 4rth order filtr using ELSIE, to 8 ohms , adjust the input impedance, and then tweak the other elements until I got a reasonable result,which I then tested with a simulator. \$\endgroup\$ – Mordechai Salomon May 7 '17 at 13:59
  • \$\begingroup\$ What filter might that be? Butterworth? Chebyshev? I or II? Bessel? Cauer/elliptic? Papoulis? ...? If software is your only interest, I doubt you'll see one, else you might find books that tell you how to -- theoretically -- derive your transfer functions in order to make Cauer of Foster topologies. I say "theoretically" because, as @glen_geek says, you'll never get to see the nice response in real life -- tolerance is a tritch. \$\endgroup\$ – a concerned citizen Jan 20 at 8:05
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I've never found those filter tools particularly flexible either. There are some on line too. I've tried to use them for various things and eventually revert to a simulator. The important aspect of this technique is knowing the filter topology. You can get that from any theory source. If you know what components to put together, you might as well use the simulator. This is especially valid if you're going to use one anyway for testing. Yes it might take some effort for a 6th order Butterworth, but it can be done. There's a table in Horowitz & Hill for VCVS active filters up to 8 stages that might be useful.

This is yesterday's simulation of an very simple output filter for the LM2596 chip. It's used in those £2 Chinese DC converter modules. The output filter is from the datasheet, and you'll see very a low input impedance, and a 200mA supply current through a 25 Ohm output impedance. I don't know how this could be designed on line via a templated circuit. In this case, the output of the DC module is triangular which is very unlikely to be catered for in a prescribed template. It's not like your active filters, but illustrates the flexibility afforded.

output filter

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Sounds like you may have better luck with a "Crossover design" program (Plenty available), or failing that the "Handbook of filter synthesis" by Zverev is good for the weird and the wonderful.

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