LG KU950 Service Manual page 25

3. TECHNICAL BRIEF
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 MSM6275 IC signal
(UHF_VCO_EN ) enables the VCO.
Figure 3.4.1-1 UMTS Rx PLL functional block diagram
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 KU950). 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.
Most UMTS Rx PLL circuits are included within the RTR6250 IC: reference divider, phase detector,
charge pump, feedback divider, and digital logic that generate LOCK status. The buffered 19.2 MHz
TCXO signal provides the synthesizer input (REF), the frequency reference to which the PLL is phase
and frequency locked. The reference is divided by the R-Counter to create a fixed frequency input to
the phase detector, FR. The other phase detector input (FV) varies as the loop acquires lock, and is
generated by dividing the RX_VCO_IN frequency using the feedback path.s NCounter.
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