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3. TECHNICAL BRIEF
special processing is required in the baseband for offset compensation or extended dynamic range.
Compared to a direct-conversion architecture, the low-IF architecture has a much greater degree of
immunity to dc offsets that can arise from RF local oscillator(RFLO) self-mixing, 2nd order distortion of
blockers, and device 1/f noise.
(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.
Figure 3-3 Block Diagram of Transmitter part of SI4205
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
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