Gsm Mode - LG GT540 Service Manual

3.2 GSM MODE

3.2.1 GSM RECEIVER
The GSM-850, GSM-900, GSM-1800, and GSM-1900 receiver inputs of RTR6285 are connected directly to the
transceiver front-end Module. GSM-850, GSM-900, GSM-1800, and GSM-1900 receiver inputs use differential
configurations to improve common-mode rejection and second-order non-linearity performance. For example
Figure 1-2 shows receiver input topologies for DCS and PCS (GSM-850/900 have the same receiver input topologies).
The balance between the complementary signals is critical and must be maintained from the RF filter outputs all the
way into the IC pins.
Since GSM-850, GSM-900, GSM-1800, and GSM-1900 signals are time-division duplex (the handset can only receive
or transmit at one time), switches are used to separate Rx and Tx signals in place of frequency duplexers – this is
accomplished in the switch module. The GSM-850, GSM-900, GSM-1800, and GSM-1900 receive signals are routed to
the RTR6285 through band selection filters and matching networks that transform single-ended 50-Ω sources to
differential impedances optimized for gain and noise figure. The RTR input uses a differential configuration to
improve second-order intermodulation and common mode rejection performance. The RTR6285 input stages
include MSM-controlled gain adjustments that maximize receiver dynamic range.
The amplifier outputs drive the RF ports of the quadrature RF-to-baseband downconverters. The downconverted
baseband outputs are multiplexed and routed to lowpass filters (one I and one Q) having passband and stopband
characteristics suitable for GMSK or 8-PSK processing. These filter circuits include DC offset corrections. The filter
outputs are buffered and passed on to the MSM7227 IC for further processing asshown in Figure 1-4.
Copyright © 2010 LG Electronics. Inc. All right reserved.
Only for training and service purposes
[Figure 1-2] DCS and PCS Receiver Inputs Topologies
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3. TECHNICAL BRIEF
LGE Internal Use Only