LG U880 Service Manual page 17

3. TECHNICAL BRIEF
A generic, high-level functional block diagram of U880 is shown in Figure 1-1. One antenna collects
base station forward link signals and radiates handset reverse link signals. The antenna connects with
receive and transmit paths through a switch module (plus a duplexer for UMTS-2100 operation).
UMTS band signals at the antenna are switched to the relevant UMTS duplexer.
The UMTS receive band signals are amplified by the front-end LNAs of the RFR6250 IC before
passing through a band-pass filter and being applied to the mixer inputs of the RFR6250 IC. On-chip
circuits down-convert the received signal directly from RF to baseband using radioOne Zero-IF
techniques. Generation and distribution of the UMTS LO, for the down-converter, is performed entirely
on-chip (except for the loop filter).
The RFR6250 IC outputs analog baseband signals for processing by the MSM device. This baseband
interface is shared with the RTR6250 GSM receiver outputs, but is separate from the GPS baseband
interface.
EGSM, DCS and PCS receive signals from the antenna switch module pass through their band-pass
filters, then are applied to the RTR6250 IC. In a similar fashion to the UMTS paths, RTR6250 IC
circuits down-convert the received signals directly from RF to baseband.
The GSM LO for multiband down conversion is entirely generated within the RTR6250 IC (PLL and
distribution functions) with exception of the off-chip loop filter. The RTR analog baseband outputs are
routed to the MSM6250 IC for further processing (an interface shared with the RFR UMTS receive
paths).
The UMTS transmit path begins with analog baseband signals from the MSM device that drive the
RTR6250 IC. Integrated PLL and VCO circuits generate the Tx LO used in the quadrature upconverter
that translates baseband signals directly to RF. The RTR6250 output driver stages deliver fairly high-
level signals that are filtered and applied to the power amplifiers (PA). The PA output is routed to the
antenna through a duplexer and switch module.
The shared EGSM-900, DCS-1800, and PCS-1900 transmit path begins with the same baseband
interface from the MSM6250 IC that is used for the UMTS band. A single EGSM/DCS/PCS quadrature
upconverter translates the GMSK-modulated signal to a convenient intermediate frequency (IF) that
forms one input to an offset phase-locked loop (OPLL).
OPLL functions are split between the RTR6250 IC and off-chip loop filter and dual Tx VCO circuits,
and translate the GMSK-modulated signal to the desired EGSM-900, DCS-1800 or PCS-1900 channel
frequency. This signal is applied to a dual power amplifier (only one is active at a time).
The enabled path continues with the PA, an automated power control (APC) circuit that samples the
transmit power and adjusts its level, the switch module (which includes a band-appropriate lowpass
filter), and the antenna.
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