LG B2050, Service Manual

Page: 17 / 133

3. TECHNICAL BRIEF
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(2) Transmitter Part
The transmit (Tx) section consists of an I/Q baseband upconverter, and offset phase-locked loop
(OPLL) and two output buffers that can drive external poweramplifiers (PA), one for the GSM 850
(824-849 MHz) and E-GSM 900 (880-915 MHz) bands and one for the DCS 1800 (1710-1785
MHz) and PCS 1900 (1850-1910MHz) bands.
A. IF Modulator
The basebandconverter(BBC) within the GSM chipset generates I and Q baseband signals for
the Transmit vector modulator. The modulator provides more than 40dBc of carrier and unwant-
ed sideband rejection and produces a GMSK modulated signal. The baseband software is able
to cancel out differential DC offsets in the I/Q baseband signals caused by imperfections in the
D/A converters.
The Tx-Modulator implements a quadrature modulator. A quadrature mixer upconverts the dif-
ferential in-phase (TXIP, TXIN) and quadrature(TXQP, TXQN) signals with the IFLO to generate
a SSB IF signal that is filtered and used as the reference input to the OPLL.
The IFLO frequency is generated between 766 and 896 MHz and internally divided by 2 to gen-
erate the quadrature LO signals for the quadrature modulator, resulting in an IF between 383
and 448 MHz. For the E-GSM 900 band, two different IFLO frequencies are required for spur
management. Therefore, the IF PLL must be programmed per channel in the E-GSM 900 band.
Figure 3-3 SI4205 TRANSMITTER PART
GSM PA
PA
RFOG
RFOG
DCS/PCS