LG U370, Service Manual, Page: 25 / 179

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Only for training and service purposes
3.5.2 VCTCXO (X100 : DSA321SCE-19.2M)
The Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) provides the
reference frequency for all RFIC synthesizers as well as clock generation functions within the
MSM6245 IC. The oscillator frequency is controlled by the MSM6245 IC. TRK_LO_ADJ pulse density
modulated signal in the same manner as the transmit gain control TX_AGC_ADJ. A two-pole RC
lowpass filter is recommended on this control line.
The PM6650 IC controls the handset power-up sequence, including a special VCTCXO warm-up
interval before other circuits are turned on. This warm-up interval (as well as other TCXO controller
functions) is enabled by the MSM TCXO_EN line . The PM6650 IC VREG_TCXO regulated output
voltage is used to power the VCTCXO and is enabled before most other regulated outputs.
Any GSM mode power control circuits within the MSM6245 IC require a reference voltage for proper
operation and sufficient accuracy. Connecting the PM6650 IC REF_OUT directly to the MSM6245 IC
GSM_PA_PWR_CTL_REF provides this reference. This sensitive analog signal needs a 0.1 µF low
frequency filter near to MSM side, and isolate from digital logic and clock traces with ground on both
sides, plus ground above and below if routed on internal layers.
3.5.3 Front-End Module (U100 : D5011)
This equipment uses a single antenna to support all handset operating modes, with an antenna switch
module select the operating frequency and band. UMTS operation requires simultaneous reception
and transmission, so the UMTS Rx/Tx connection is routed to a duplexer that separates receive and
transmit signals. The active connection is MSM-selected by three control lines (GPIO[9], GPIO[10]).
Two GPIO are programmed to be ANT_SEL0_N, ANT_SEL1_N) respectively.
3. TECHNICAL BRIEF
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[Table 1.2] Front End Module control logic