LG -A258 Service Manual page 39

3. TECHNICAL BRIEF
3.6.2.2 Transmitter
The GMSK transmitter supports power class 4 for GSM850 or GSM900 as well as power class 1 for DCS1800 or
PCS1900. The digital transmitter architecture is based on a fractional-N sigma-delta synthesizer for constant
envelope GMSK modulation. This configuration allows a very low power design without any external
components. Up- and down-ramping is performed via the ramping DAC connected to VRAMP.
RF synthesizer
The RF subsystem contains a fractional-N sigma-delta synthesizer for the frequency synthesis. Respective to
the chosen band of operation the phase locked loop (PLL) operates at twice or forth of the target signal
frequency. In receive operation mode the divided output signal of the digital controlled oscillator output
(DCO) serves as local oscillator signal for the balanced mixer. For transmit operation the fractional-N sigma-
delta synthesizer is used as modulation loop to process the phase/frequency signal. The 26 MHz reference
signal of the phase detector incorporated in the PLL is provided by the reference oscillator.
3.6.2.3 Front-end/PA Control Interface
Two outputs (FE1, FE2) for direct control of antenna switch modules enable to select RX- and TX-mode as well
as low- and high-band operation. An extra band select signal PABS for the power amplifier is used, to support
discrete PA and switching modules. Time accurate power dissipation of the PA is achieved by the control
signal PAEN. A minor set of power amplifiers require a bias voltage to enhance power efficiency. Support of
this power amplifiers is achieved by the implemented bias DAC.
3.6.2.4 Power Supply
To increase power efficiency less sensitive parts of the RF subsystem are supplied by the DCDC converter
situated in the PMU subsystem. Conversion of the 1.8 V output voltage of the DCDC to the 1.3 V circuit supply
voltages is achieved by several Low-DropOut regulators (LDO). One embedded direct-to-battery LDO
provides the 2.5 V supply voltage for the remaining circuits. This voltage is also used to generate a clean 1.4 V
supply voltage via an additional LDO.
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