LG CU720 Black, Руководство по обслуживанию

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3.4 LO generation and distribution circuits
The integrated LO generation and distribution circuits are driven by internal VCOs to support various
modes to yield highly flexible quadrature LO outputs that drive all GSM/EDGE and UMTS band
upconverters and downconverters; with the help of these LO generation and distribution circuits, true
zero-IF architecture is employed in all GSM and UMTS band receivers and transmitters to translate the
signal directly from RF to baseband and from baseband to RF.
Two fully functional fractional-N synthesizers, including VCOs and loop filters, are integrated within the
RTR6275 IC. The first synthesizer (PLL1) creates the transceiver LOs that support the UMTS
2100/1900/1800 transmitter, and all four GSM band receivers and transmitters including: GSM 850,
GSM 900, GSM 1800, and GSM 1900. The second synthesizer (PLL2) provides the LO for the UMTS
2100/1900/1800 receiver. An external TCXO input signal is required to provide the synthesizer
frequency reference to which the PLL is phase and frequency locked. The RTR6275 IC integrates
most of PLL loop filter components on-chip except two off-chip loop filter series capacitors, and
significantly reduces off-chip component requirement. With the integrated fractional-N PLL
synthesizers, the RTR6275 has the advantages of more flexible loop bandwidth control, fast lock time,
and low-integrated phase error.
3.5 Off-chip RF Components
3.5.1 Front-End Module
3. TECHNICAL BRIEF
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