First of all, I needed to design a low-pass filter for the Raspberry Pi. To 'clean up' the output. It is a square wave after all, with only odd harmonics. A simple task for a Cauer [elliptic] filter.
So I sat down and opened my filter book. Decided on a design I had used many times before. This one 'notches' out harmonics providing very good suppression.
Having developed a spreadsheet to calculate the values, I took a careful look at the components. You generally have two choices, one uses capacitors the other uses coils. The lowest cost version is where you make the coils from scavenged wire. The capacitors can be recovered from old radio equipment. And no they don't have to be Silver-Mica types. Ceramic will be fine.
[I will make the spreadsheet available at a later stage. This will allow the constructor to design the cut-off frequency and impedance to his requirement. Not mine.]
Then it hit me!
It is possible to use ferrites for the coils. But they are neither 'cheap' or readily available here in SA. So I started the secondary phase of this project...
Coil Making in the 21st Century [in South Africa]
- Getting hold of the wire. [do not steal!]
- Transformer Disassembly.
- ATX Power Supply Toroids.
- Having got your wire, how do you wind it?
- Measuring your coil.
During this 'process', I came across a lot of reference works. Some are available on the Internet. A lot I had photocopied in the 70's. Now scanned they provide valuable references. Quite a few are IEEE papers. If you want them nowadays, you will have to pay $ for the privilege.