On Oct 21, 2:26 pm, JosephKK wrote:
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outdated application circuit.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/...5759_3.pdfhttp...

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.

Ah, that makes a lot of sense. I was under the impression that these
coils had built-in capacitors, but after comparison with other Toko
datasheets, the ones that have caps say "available with internal
capacitance".

With the new numbers, the capacitance ratio is sufficient, but the
required inductance comes to around 100 nH. That sounds reasonable to
me.

Thanks for the help,
Mike