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3. TECHNICAL BRIEF
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 signals are automatically swapped when
switching bands. Additionally, the SWAP bit in register 03h can be used to manually
exchange the I and Qsignals.
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 (Figure 3-3), and should be set to the
recommended settings detailed in the register description.
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