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4.3. Receiver
4.3.1. Functional Description
ANTENNA
U502
ANTENNA
SWITCH
MODULE
The main building block for the receiver is the transceiver IC U500 which includes a direct conversion receiver with in-phase and
quadrature demodulation. The antenna passes the received signal to the antenna port of the Antenna Switch Module U502.
The U502 switches the signal to the corresponding receive port (EGSM 900, DCS 1800 or PCS 1900). The unbalanced output
signal from the U502 is routed through a 50 ohm PCB trace to the appropriate Rx SAW filter, FL502, FL503 or FL504.
The SAW filter converts the unbalanced signal to a balanced signal and also attenuates any out-of-band blocking signals.
The balanced output from the SAW filter are routed through 75 ohm balanced PCB traces to the LNA input of the transceiver
U500. A balanced matching network has been provided between the Rx SAW output and the LNA input in order to provide an
optimum LNA noise figure (NF). The output of the LNA is then converted directly to baseband frequency by a quadrature
demodulator mixer. The local oscillator (LO) signal presented to the mixer is provided by an internal VCO and it's frequency is
either divided by four (EGSM 900) or by two (DCS 1800 and PCS 1900) before reaching the mixer. After the RF signal is
converted into a baseband signal, the resulting in-phase and quadrature signals are fed into two baseband low pass filters.
These filters provide suppression of in-band blocking signals and adjacent channel interferers. The passband of the filters is
optimized for low group delay ripple. The baseband signal is then amplified by a Programmable variable Gain Amplifier (PGA)
within U500. The in-phase and quadrature baseband signals are then offset to a DC offset level of 1.35 V.
PROVISIONAL ISSUE
LNA
GSM 850
FL502
SAW
FILTER
LNA
EGSM 900
FL503
SAW
LNA
FILTER
DCS 1800
FL504
SAW
FILTER
LNA
PCS 1900
Figure 4.4: Receiver Block Diagram
U500
QUAD BAND TRANSCEIVER
PGA
VCO
AMP
1/4
1/2
– 4-4 –
IR
IRX
BASSBAND
LOW PASS
FILTER
QR
QRX
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