Lo Generation And Distribution Circuit - LG GT540 Service Manual

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 up-converters and down-
converters; 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 RFto-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 low-integrated phase error.
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Only for training and service purposes
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3. TECHNICAL BRIEF
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