"JosephKK" wrote in message
. net...
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outdated application circuit.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.

You are right. Actually , I did think the same as you did, but 100 nH and 38
PF works out to about 82 MHz, which is overkill. You only need to go as low
as about 87 if you want to include CH6 audio. He will want to use a varactor
with about a 3:1 capacitance ratio to allow for stray and fixed capacitance.
In my example a (5 - 15) PF should be doable.

Tam