I'm about to design a 433MHz transceiver and found a nice reference design using the TRC105 and the RF3608D SAW Filter.
Before designing the circuit I wanted to understand the impedance matching in the reference design, and figure that would be no problem on my smith chart… Now hours later I still don't get it.
In the datasheet for the RF3608D on page 2 is drawn a Application Circuit, which shows matching to a 50ohm antenna.
http://www.mouser.com/ds/2/347/rf3608d-33048.pdf
Looking at page 2 is shown an Application circuit.
I think the 220nH shunt inductor is to cancel the capacitive reactance, which I think is about -5.8ohm (read on page 7).
But I don't understand at all, what purpose is of the series LC circuit (5.6nH || 1.2pF)?
Best Answer
It's a parallel L-C circuit, tuned to about 2GHz, in series with the input.
Exactly why it's there isn't clear, but the broadband response on page 3 shows fairly poor rejection around 2GHz, so additional attenuation there may be important in some applications.