MarkAren posted to sci.electronics.design:

On Oct 21, 8:27 pm, wrote:
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outdated application circuit.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
Hi Mike,

Digging for data on the Toko MC117 series...
http://81.149.89.17/Pages/MOLD/page91.htm

The E523FN-2000242 isn't shown, but there does appear to be a
relationship between PN and inductance (2xxxxxx indicates 2.5 turns)
which seems to be in the 55-66nH range.

Hope that helps a little bit.

Regards,

Mark


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


http://www.nxp.com/acrobat_download/...ts/BB804_3.pdf

Just a wild guess, but the 38 pF may be the capacitance required to be
resonant at 100 MHz. Inductors below 1 uH rarely have parallel
capacitances above 1 pF. And remember that 10 pF is a variable
trimmer.

Put this into your assumptions and see how it works.