LG U300, Service Manual

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3.3.2 Transmitter
The UMTS transmit path begins with analog baseband signals from the MSM device that drive the
RTR6250 IC. The RTR6250 IC provides all the UMTS transmitter active signal-path circuits except the
power amplifiers. Analog (I and Q) differential signals from the MSM device are buffered, filtered, and
applied to Baseband-to-RF quadrature upconverters. Gain control is implemented on-chip. The RF
outputs include an integrated matching inductor, reducing the off-chip matching network to a single
series capacitor.
The RTR6250 UMTS output is routed to its power amplifier through a bandpass filter, and delivers
fairly high-level signals that are filtered and applied to the PA. The PA device used in U300 is “Load
Insensitive PA”- no need to use isolator - and routed to the duplexer Tx port directly. Transmit power is
delivered from the duplexer to the antenna through the switch module.
The RTR6250 IC integrates LO generation and distribution circuits on-chip, substantially reducing off-
chip requirements. Various modes and programmable features result in a highly flexible transceiver LO
output that supports not only UMTS transmissions, but all EGSM900 and DCS1800/PCS1900 Rx and
Tx modes as well.
The UMTS Tx LO (PLL1) is generated almost entirely on-chip, requiring only the loop filter off-chip
(two capacitors and two resistors); all UMTS Tx VCO and PLL circuits are on-chip. An internal
RTR6250 switch routes the internal VCO signal to the LO generation and distribution circuits to create
the necessary UMTS Tx LO signals.
3. TECHNICAL BRIEF
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