LG U890, Service Manual

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3.4 LO Phase-locked Loop
Most LO functions are fully integrated on-chip, do not require user adjustment, and need not be
considered by handset designers. QUALCOMM has established and implemented frequency plans
and LO generation schemes that support the radioOne 6250-IIseries chipset while requiring minimal
off-chip design effort. Only one area requires handset designer attention: the loop filters of each
phase-locked loop (PLL).
3.4.1 UMTS Rx PLL (PLL2)
UMTS Rx LO functional blocks are distributed between the RFR6250 IC, RTR6250 IC, and external
UMTS_RX_CH_VCO and loop filter components (Figure 3.4.1-1). The external UMTS_RX_CH_VCO
must be enabled for UMTS Rx operation and disabled otherwise; a dedicated MSM6250A IC signal
(UHF_VCO_EN ) enables the VCO.
The RFR6250 IC accommodates single-ended or differential LO inputs; if singleended, either pin can
be active. AC-couple the inactive pin to ground using an appropriately valued capacitor (12 pF is used
in U890). The 27 pF capacitor should be used to AC-couple the active pin to the VCO signal. Using
only the selected VCO signal, the RFR6250 IC LO generation and distribution circuits create the
necessary LO signals for the active quadrature downconverter.
A sample of the downconverter LO is buffered and routed from RFR6250 IC pin 19 to RTR6250 IC pin
32 (RX_VCO_IN). This signal requires a terminating resistor near the RTR6250 IC input pin and an AC
coupling capacitor that assures the internal RTR6250 IC biasing is not disrupted in the example. Good
microstrip or stripline controlled-impedance techniques must be used
3. TECHNICAL BRIEF
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Figure 3.4.1-1 UMTS Rx PLL functional block diagram