3. H/W Circuit Description
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3.1.4. Transmitter Part
The transmit (TX) section consists of an I/Q baseband upconverter, an offset phase-locked loop
(OPLL) and two output buffers that can drive external power amplifiers (PA), one for the GSM 850
(824 to 849 MHz) band and one for the DCS 1800 (1710 to 1785 MHz) and PCS 1900 (1850 to 1910
MHz) bands. The OPLL requires no external duplexer to attenuate transmitter noise or spurious
signals in the receive band, saving both cost and power. Additionally, the output of the transmit VCO
(TXVCO) is a constant-envelope signal that reduces the problem of spectral spreading caused by non-
linearity 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 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.
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 band. 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 band, 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, and should be set to the recommended settings detailed in the
register description.
Figure 3. Transmitter Block Diagram
Содержание L1150
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