TDA5250D2_07 Datasheet, PDF (40/94 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

TDA5250 D2
Version 1.7
Application
RF I/O
C1
50 Ohm
SMA-connector
C2
VCC
C5
L1
L2
PA
C3
C4
R1
RX/TX
open or VCC
C7
C9
L3 ^C10
C6
LNI
LNIX
RX/TX
RX_Mode.wmf
Figure 3-2 RX-Mode
The RF-signal is able to run from the RF-input-SMA-connector to the LNA-input-pin LNI via C1, C2,
C7, L3 and C9. R1 does not affect the matching circuit due to its high resistance. The other input of
the differential LNA LNIX can always be AC-grounded using a large capacitor without any loss of
performance. In this case the differential LNA can be used as a single ended LNA, which is easier
to match. The S11 of the LNA at pin LNI on the evalboard is 0.945 / -47° (equals a resistor of
1.43kOhm in parallel to a capacitor of 1.6pF) for both high and low-gain-mode of the LNA. (pin LNIX
AC-grounded) This impedance has to be matched to 50 Ohm with the parts C9, L3, C7 and C2. C1
is DC-decoupling-capacitor. On the evalboard the most important matching components are (shunt)
L3 and (series) C2. The capacitors C7 and C9 are mainly DC-decoupling-capacitors and may be
used for some fine tuning of the matching circuit. A good CAE tool (featuring smith-chart) may be
used for the calculation of the values of the components. However, the final values of the matching
components always have to be found on the board because of the parasitics of the board, which
highly influence the matching circuit at RF.
Data Sheet
40
2007-02-26