LG -P990 Service Manual page 35

3. TECHNICAL BRIEF
3.6.2 26MHz VCTCXO (X101, DSA221SCL)
The Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) provides the reference
frequency for all RFIC synthesizers as well as clock generation functions within the PMB5703 IC. The oscillator
frequency is controlled by the PMB5703 ICs.
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 PM7540 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 PM7540 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 MSM7227 IC
require a reference voltage for proper operation and sufficient accuracy. Connecting the PM7540 IC REF_OUT
directly to the MSM7227 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.
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