Devlog 4

I finally started laying out the PCB for the SDR. This has honestly been a lot more difficult than I expected, especially with the filters.

I couldn’t find many good examples of how to actually lay out lumped component Butterworth filters. There are tons of examples of how to design them and calculate the component values, but not many showing how you are supposed to physically place all the inductors and capacitors on the board. So I’m mostly figuring it out as I go.

I also added a 2-way RF switch after the first filter bank that goes to an SMA connector. This means I can connect a VNA directly to the filters and actually see how well I made them instead of having to guess what is wrong if the SDR doesn’t work.

The control pins for the switches were also annoying because they are right in the middle of the filter bank. I had to figure out how to route them without having their return currents interfere with the RF return currents from the filters. I ended up moving the filters around a bit, which also let me shorten some of the RF traces.

I also had to add another connector for the Nucleo board because I had to flip the orientation of the second RF switch. Because of that, the second switch needs to be controlled separately from the first one.

Right now I’m just adding the stitching vias around the first filter bank. After that I’ll move on to the LNA and the second filter bank.