Lo Generation And Distribution Circuit - LG GT810h Service Manual

3. TECHNICAL BRIEF

3.5 LO GENERATION and DISTRIBUTION CIRCUIT

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, UMTS band and GPS
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 fraction-N
synthesizers, including VCOs and loop filters, are integrated within the RTR6285 IC.
In addition, the RTR6285 has a third synthesizer used for GPS operation. The first synthesizer (PLL1)
in the RTR6285 creates the transceiver Los that support the UMTS transmitter, and all four GSM band
receivers and transmitters including:
GSM850, GSM900, GSM1800, and GSM1900. The second synthesizer (PLL2) in the RTR6285 IC
provides the LO for the UMTS primary receiver. For the RTR6285 IC only, the second synthesizer also
provides the LO for the secondary UMTS receiver.
The third synthesizer (PLL3), only in the RTR6285 IC, provides the LO for the GPS receiver. An
external TCXO input signal is required to provide the synthesizer frequency reference to which the PLL
is phase and frequency locked. The RTR6285 ICs integrate most of the PLL loop filter components on-
chip except for three off-chip loop filter-series capacitors, which significantly reduces off-chip
component requirement. With the integrated fractional-N PLL synthesizers, the RTR6285 ICs have the
advantage of more flexible loop bandwidth control, fast lock time, and lowintegrated phase error.
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