wrote in message
ups.com...
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available components (i.e. from Digikey or Mouser) into the outdated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

http://www.nxp.com/acrobat_download/...757_5759_3.pdf
http://www.nxp.com/acrobat_download/...ts/BB804_3.pdf

For L in uH and C in PF, the LC ratio at 88 MHz is 3.27. The LC ratio at 108
is 2.17. If Cmax - Cmin is 10 PF, then L= (3.27 - 2.17)/10 = .11 uh. That
means Cmax is 3.27/.11=29.7 PF and Cmin=2.17/.11=19.7PF. Sanity check:
Cmax - Cmin = 29.7 - 19.7 = 10 PF; qed.

If they say a 100 nH inductor has a capacitance of 38 PF, that is a garbage
statement.

Tam