LG C3400 Service Manual page 15

3. TECHNICAL BRIEF
(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 power amplifiers (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 baseband converter(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
unwanted 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
differential 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
generate 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.
B. OPLL
The OPLL consists of a feedback mixer, a phase detector, a loop filter, and a fully integrated
TXVCO. The TXVCO is centered between the DCS 1800 and PCS 1900 bands, and its
output is divided by 2 for the GSM 850 and E-GSM 900 bands. The RFLO frequency is
generated between 1272 and 1483 MHz. To allow a single VCO to be used for the RFLO,
high-side injection is used for the GSM 850 and E-GSM 900 bands, and low-side injection is
used for the DCS 1800 and PCS 1900 bands. The I and Q signals are automatically swapped
when switching bands. Additionally, the SWAP bit in register 03h can be used to manually
exchange the I and Q signals.
Low-pass filters before the OPLL phase detector reduce the harmonic content of the
quadrature modulator and feedback mixer outputs. The cutoff frequency of the filters is
programmable with the FIF[3:0] bits in register 04h (Figure 3-3), and should be set to the
recommended settings detailed in the register description.
Figure. 3-3 SI4205 TRANSMITTER PART
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