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From: MRW on 6 Jan 2006 09:12
Hi,

I was going to the appliation note for the TEA5768 and I can't seem to
figure out how they came up with this matching network:

http://img391.imageshack.us/my.php?image=tea5768rfmatchckt1bd.jpg

The image shows 3 pin connections to an LNA. The application note
states that "the input impedance of the LNA is (100 ohm || 4pF) each
pin (referenced to ground)." It also mentions the following:
"The antenna-input signal is fed into the balanced FM-RF inputs (pin 35
and pin 37) via a RF matching impedance circuit. A series capacitor,
two parallel capacitors and a coil (L1) build this circuit. The coil
used in this circuit should have a minimal Q factor of 30. Its
tolerance is ± 5%. Together with the impedance of the LNA, the RF
matching circuit forms a low Q band pass filter."

Can anyone give me any tips on how they came up with this matching
network?

I tried to do the following.
1). Found the input impedance by using 88 MHz as my frequency. I picked
this because the frequency response chart showed a peak at this
frequency.

However, I cannot seem to proceed after that because I get confused by
the two RF inputs. The matching networks that I've worked with only
assumed one RF input.

Can I just ignore the two RF inputs and combine the two capacitors
(27pF and 47pF) in parallel then have this total capacitance be in
parallel of the 120nH inductor?


Thanks!

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