Certainly the most critical part of the new receiver, was (and is) the bandpass filter. After all, the idea was to reject the interference from the OrbComm satellites. Theoretically, it would be even impossible to do it (one of the operating frequencies is so close to NOAA-12 that they actually overlap.
Anyway, a fairly sharp Tchebyshev filter was designed, simulated and constructed, and, on the way, a lot was learned about LC-filters while building. Schoolbooks is one thing, making it work quite another...
Click on the image to obtain a larger version. There's also a PDF version of the circuit.
Left is the circuit of the bandpass filter. A few remarks about
|One view of the filter. The blue trimmer C tunes the coupling section. The white thing behind the couple L is foam double-sided adhesive tape, to fix the toroid to the wall.|
|Another view. The black trimmer Cs at each end are the tuning caps. The brown caps are Mica 680 pF capacitors.|
|This version was constructed by Jeff, VE3AK. He commented that the filter could not be measured using a function generator - it was too narrow! He used a potcore for the coupling inductor (center).|
|Filter response in dB||Filter response (magnitude)|
Response of the filter, simulated with NGSpice.
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|(c) John Coppens ON6JC/LW3HAZ|