The schematic shows how a commercial mixer is wired into the circuit.
A homebrew one can be used, at a considerable cost savings, but the trifilar transformers can be a problem. However, after doing a couple of mixers, the rest go a lot easier.
The 455 kHz Mixers are followed by a diplexer and a 455 kHz ceramic filter. The ones used are Murata SFU455A's. Input and output impedance is 1500 ohms.
I attempted to match the impedance with a 5 turn to 20 turn transformer wound on a FT50-43 ferrite core. Resistors are used in the data sheet to do the impedance matching for the filters, but I found no difference with or without the resistors, so I left them out.
Practically any 455kHz ceramic filter will work here, and it could even be left out if you can't find one. Different ceramic filters may have different input and output impedance, but I have never found it to be critical.
The coupling transformer could be changed to match the new impedance for a different filter, but you might be cutting hairs where so little difference is noted that it wouldn't be worth the effort.
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