Lo Generation And Distribution Circuits - LG KF690 Service Manual

3.3 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; with the help of these
LO generation and distribution circuits, true zero-IF architecture is employed in all GSM 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
RTR6235 IC. The first synthesizer (PLL1) creates the transceiver LOs that support all four GSM band
receivers and transmitters including: GSM900, DCS1800, and PCS1900. An external TCXO input
signal is required to provide the synthesizer frequency reference to which the PLL is phase and
frequency locked. The RTR6235 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 RTR6235 has the advantages of more flexible loop
bandwidth control, fast lock time, and low-integrated phase error
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3. TECHNICAL BRIEF
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