LG KE990 Service Manual - Page 19

3. TECHNICAL BRIEF 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 band upconverters and downconverters; with the help of these LO generation and distribution circuits, true zero-IF architecture is employed in all GSM 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 RTR6235 IC. The synthesizer (PLL1) creates the transceiver LOs that support four GSM band receivers and transmitters including: GSM850, EGSM900, 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 LGE Internal Use Only - 20 - Copyright © 2008 LG Electronics. Inc. All right reserved. Only for training and service purposes