LG U310 Service Manual page 26

3. TECHNICAL BRIEF
The VCO output drives LO generation and distribution circuits (within the RFR6250 IC) to create the
necessary LO signals for Rx quadrature down-converter for either UMTS band.
By definition, a Zero-IF down-converter requires FRF equal to FLO, and the RFR6250 design achieves
this without allowing FVCO to equal FRF.
The tuning signal is routed off-chip for filtering, allowing optimization for different applications.
3.4.2 Transceiver PLL (PLL1)
All LO functional blocks for the other handset modes(UMTS Tx, EGSM Tx/Rx, DCS Tx/Rx, PCS
Tx/Rx) are integrated into the RTR6250 IC except the loop filter components (Figure 3.4.2-1). On-chip
circuits include reference divider, phase detector, charge pump, VCO, feedback divider, and digital
logic status. The functional description given in Section 3.4.1 for the UMTS Rx PLL applies to the
Transceiver PLL as well.
Figure 3.4.2-1 Transceiver PLL functional block diagram
The off-chip loop filter allows optimization of key PLL performance characteristics (stability, transitory
response, settling time, and phase noise) for different applications Guidelines are provided in the next
subsection for proper implementation of this critical circuit.
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