LG -P500 Service Manual page 30

3.6.2 19.2MHz VCTCXO (X250, KT3225L19200DCW28RA0)
The Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) provides the reference
frequency for all RFIC synthesizers as well as clock generation functions within the MSM6285 IC. The oscillator
frequency is controlled by the MSM6285 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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Only for training and service purposes
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3. TECHNICAL BRIEF
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