tda5251f1 Infineon Technologies Corporation, tda5251f1 Datasheet - Page 38

• Manufacturer Part Number: tda5251f1
• Description: Ask/fsk 315mhz Wireless Transceiver
• Manufacturer: Infineon Technologies Corporation

TDA5251 F1
Version 1.0
Application
Confidential
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 0.97 / -17° (equals a resistor of
3.3kOhm in parallel to a capacitor of 1.5pF) 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 a DC-decoupling-capacitor. On the evalboard the most important matching components are
(shunt) L3 and (series)C7, C2. The capacitors is mainly a DC-decoupling-capacitor 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.
Preliminary Specification
38
2003-02-18