Transmitter Part - LG F1200 Service Manual

3. TECHNICAL BRIEF

3.4 Transmitter Part

The Transmitter part contains Si4206[U102] active parts, Power Amp Module[U100] and
Antenna Switch Module[FL100]. The transmit section of Si4206[U102] consists of an I/Q
baseband pconverter, an offset phaselocked loop (OPLL) and two 50 ohm output buffers that
can drive external power amplifiers. The RF GMSK outputs from the transmit VCO are fed
directly to the RF power amplifiers. The peak output power and the profile of the transmitted
burst are controlled by means of incorporated power control circuits inside of PA and DAC
output from the Baseband Controller. The PA outputs pass to the antenna connector via
Antenna Switch Module.
(1) 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 I/Q signals with the IFLO to generate a SSB IF
signal, which is filtered and used as the reference input to the OPLL. The Si4206 [U102]
generates the IFLO frequency. The IFLO is divided by two to generate the quadrature LO
signals for the quadrature modulator.
(2) 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 E-GSM 900 bands. The Si4206 generates the RFLO frequency between
1327 and 1402 MHz. To allow a single VCO to be used for the RFLO, high-side injection is used
for the E-GSM 900 bands, and low-side injection is used for the DCS 1800 and PCS 1900
bands. 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. The OPLL requires no external duplexer to
attenuate transmitter noise and spurious signals in the receive band. Additionally, the output of
the transmit VCO (TXVCO) is a constant-envelope signal which reduces the problem of spectral
spreading caused by non-linearity in the PA.
Figure 4. TRANSMITTER PART Block Diagram
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