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JosephKK

Tam/WB2TT posted to sci.electronics.design:

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outdated application circuit.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

Perhaps stated more strongly than necessary, see my post.