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:
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 =B1 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!