LG KG920, Service Manual

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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 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.
Figure. 3-3 SI4210TRANSMITTER PART