Gsm Mode - LG -P500 Service Manual

3. TECHNICAL BRIEF

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 1-2.
LGE Internal Use Only
[Figure 1-2] DCS and PCS Receiver Inputs Topologies
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