Only For Training And Service Purposes - LG KC910 Service Manual

3. TECHNICAL BRIEF
Since 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-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 inter-modulation 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 MSM6280
IC for further processing as shown in Figure 1-4.
3.2.2 GSM TRANSMITTER
The RTR6285 transmitter outputs(HB_RF_OUT1 and LB_RF_OUT1) include on-chip output matching
inductors. 50ohm output impedance is achieved by adding a series capacitor at the output pins. The
capacitor value may be optimized for specific applications and PCB characteristics based on pass-band
symmetry about the band center frequency as shown in Figure 1-3.
The RTR6285 IC is able to support GSM 900 and GSM 1800/1900 mode transmitting. This design
guideline shows a tri-band GSM application. Both high-band and low band outputs are followed by
resistive pads to ensure that the load presented to the outputs remains close to 50ohm.
LGE Internal Use Only
[Figure 1-3] GSM Transmitter Outputs Topologies
Copyright © 2007 LG Electronics. Inc. All right reserved.
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Only for training and service purposes