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G310 Service Manual
4.2.4 Transmit Section
4.2.4.1 An Overview of Transmit Section
The transmit (TX) section consists of an I/Q baseband upconverter, an offset phase-locked loop
(OPLL) and two 50 ohms output buffers that can drive external power amplifiers (PA), one for the
EGSM 900 (880 – 915 MHz) bands and one for the PCS 1900 (1850 – 1910 MHz) bands. The OPLL
requires no external duplexer to attenuate transmitter noise and spurious signals in the receive
band, saving both cost and power. Additionally, the output of the transmit VCO (TXVCO) is a
constant-envelope signal which reduces the problem of spectral spreading caused by nonlinearity in
the PA.
A quadrature mixer upconverts the differential in-phase (TXIP,TXIN) and quadrature(TXQP,TXQN)
signals with the IFLO to generate a SSB IF signal which is fitered and usesd as the reference input
to the OPLL. The Si4133T generates the IFLO frequency between 766 and 896 MHz. The IFLO is
divided by two to generate the quadrature LO signals for the quadrature modulator, resulting in an IF
between 383 and 448 MHz.
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 two for the generate the RFLO frequency between 1272 and 1483 MHz. To allow a single
VCO to be used for the RFLO, high-side injection is used for the EGSM 900 bands, and low-side
injection is used for the DCS 1800 and PCS 1900 bands. The I and Q signals are automatically
swapped within the Si4200 when switching bands.
Low-pass fitlers 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.
Fig.4-8. Transmitter block diagram
19
PANTECH
R&D CONFIDENTIAL
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