FM radio broadcast signals are usually really strong compared to e.g. ham radio short wave signals. The FM signals can cause massive intermodulation in SDRs, LimeSDR in my case. So I designed a small prototype of a filter for these frequencies.
I used the free version of Ansoft Designer for the calculation of the filter. Parameters were:
- Legendre approximation
- Stop band from 80 to 120 MHz
- Order 5
After calculation I had to tune/round the values of Cs and Ls to existing components.
The input and output are SMA connectors. All other components are 0603 SMD.
The first prototype was ordered at dirtypcbs.
I soldered one board and measured with my Siglent SSA 3021X spectrum analyzer, see screenshot. You can see the corner frequencies at 73 and 140 MHz.