Transmitter Part - LG MG230 Service Manual

3.2.2 Transmitter Part

The Transmitter part contains AD6548 active parts and PAM .The transmit section of the AD6548
radio implements a translation loop modulator. This consists of a quadrature modulator, high speed
phase-frequency detector (PFD) with charge pump output, loop filter, TX VCO and a feedback down
converting mixer. The VCO output (divided by 2 for low band) is fed to the power amplifier with a
portion internally fed back into the down-converting feedback mixer to close the feedback loop.
A. Power Amplifier Module
The advanced quad-band Transmit Module designed for mobile handset applications provides full RF
transmit functionality.The GSM850/900 and DCS/PCS power amplifier blocks including power control
are combined with the low insertion loss quad-band pHEMT switch, Tx harmonics filtering, integrated
switch decoder, four receive ports, and full ESD protection. This architecture eliminates the need for
any PA-to-switch design effort for phone designers. All four Rx ports are frequency independent and
allow flexible routing to the transceiver. Fabricated in high-reliability InGaP HBT / pHEMT technology,
the module supports GPRS class 12 operation and provides 50 Ohms input and output impedances at
all RF input and output ports. The module control inputs are CMOS compatible and has no need for an
external reference voltage. With its excellent efficiency performance in all 4 bands, the power amplifier
and switch module contributes to the overall talk-time targets of next generation mobile handset
designs.
B. Quadrature Modulator
The Quadrature modulator takes the baseband I & Q signals and translates these into a GMSK signal
at the Transmit Intermediate Frequency (TX IF). After band-pass filtering and limiting the TX IF signal
is used as the reference input to the Phase Frequency Detector (PFD) of the transmit PLL.
C. Phase Frequency Detector (PFD)
The PFD ensures that the transmitted signal contains the required modulation and is accurately locked
to the desired GSM channel. The downconverted feedback signal from the TXVCO and the
Quadrature Modulator output are phase compared by the PFD. The PFD charge pump generates a
current pulse proportional to the difference in phase which is applied to the loop filter.
D. Loop filter
To minimize complexity of the external PCB layout the TX loop filter is fully integrated into the IC. At
power up the filter is automatically calibrated as part of the baseband filter cal, eliminating process
tolerances. The calibration is fully integrated and requires no extra programming.
E. TX VCO
The Transmit Voltage Controlled Oscillator (TX VCO) and tank components are a fully integrated
subsystem. The subsystem includes PA drivers so the outputs are used to directly drive the external
PAs. The low noise oscillator design and internal filtering mean that external TX SAW filters are not
required. In Low band operation the TX VCO output is divided by two and filtered. The TX VCO is
automatically calibrated to ensure optimum performance over its operating frequency of 1648 to 1910
MHz.
Copyright © 2007 LG Electronics. Inc. All right reserved.
Only for training and service purposes
3. TECHNICAL BRIEF
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