Hello,
I've been "designing" a Philips TEA5757-based radio by fitting readily available components (i.e. from Digikey or Mouser) into the outdated application circuit. I'm currently stuck on the FM front-end tank circuit; I'm not quite sure I understand how it's done in the application diagram (page 27 of the TEA5757 datasheet). By my calculations, it shouldn't work.
The tank consists of a dual varactor (BB804), a 10 pF trimmer, and a RF coil that I cannot find the specs for (Toko MC117 E523FN-2000242). The schematic says the coil has 38 pF capacitance, and from comparison with current Toko coils and googling I'm guessing that it's an unshielded coil with a Q > 100 and an inductance < 100 nH. From the BB804 datasheet, each individual varactor has an effective range of
20-60 pF (generous assumption given 12V supply), so the series combination results in 10-30 pF.Altogether, the capacitance range is 48 to 68 pF, and 68/48 = 1.42. We need (108/88)^2 = 1.5 to tune the FM radio band. Stray capacitance and the trimmer don't help. I doubt Philips would provide a bum application diagram, so I must be missing something.
Thanks, Mike