Gsm Mode; Gsm Receiverv - LG U8500 Service Manual

3. TECHNICAL BRIEF

3.2 GSM Mode

3.2.1 GSM Receiver
The Dual-mode U8500's receiver functions are split between the two RFICs as follows:
• UMTS-2100 operation uses the RFR6250 Receiver ICs to implement the receive signal path,
accepting an RF input and delivering analog baseband outputs (I and Q).
• EGSM-900, DCS-1800, and PCS-1900 modes both use the RTR6250 IC only.
Each mode has independent front-end circuits and down-converters, but they share common
baseband circuits (with only one mode active at a time). All receiver control functions are beginning
with SBI2-controlled parameters.
The EGSM, DCS, and PCS receiver inputs of RTR6250 are connected directly to the transceiver front-
end circuits(filters and antenna switch module). EGSM, DCS, and PCS receiver inputs are similar to
the RFR6200 UMTS Rx input in that they also use differential configurations to improve common-
mode rejection and second-order non-linearity performance.
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 EGSM, DCS, and PCS 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 EGSM, DCS, and PCS receive signals are routed to the RTR6250 through band selection filters
and matching networks that transform single-ended 50-Ωsources to differential impedances optimized
for gain and noise figure. Similar to the RFR, the RTR input uses a differential configuration to improve
second-order inter-modulation and common mode rejection performance. The RTR6250 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 processing. These filter circuits
include DC offset corrections.
The filter outputs are buffered and passed on to the MSM6250 IC for further processing (an interface
shared with the RFR6200 UMTS receiver outputs).
2 The RFIC operating modes and circuit parameters are MSM-controlled through the proprietary 3-line Serial Bus Interface (SBI). The Application
Programming Interface (API) is used to implement SBI commands. The API is documented in AMSS Software - please see applicable AMSS
Software documentation for details.
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