LG KF750 Service Manual page 79

3. TECHNICAL BRIEF
C. Synthesizer
The synthesizer contains all the frequency generating blocks in the RF3000 circuit: The reference 26
MHz frequency generation is made in a external crystal oscillator, which is fully integrated in the circuit
(except for the crystal). The digital bus controls the center frequency of the XO. All RFfrequencies are
generated in a single 4GHz VCO, subsequently divided by 2 or 4 to arrive at the final frequency. To
cover the required frequency range, the integrated Voltage Controlled Oscillator (VCO) operates at
twice the frequency for band 1800/1900, and at four times the desired frequency for band 800/900.
The RF-VCO is locked to the XO in a fractional-N PLL consisting of a prescaler, a phase-frequency
detector, a charge-pump, a sigma-delta modulator and a fully integrated loop filter. Automatic trimming
of the VCO center frequency and the RC constant makes sure that the variation of the PLL dynamics
is sufficiently low.
The XO also has an external reference mode where a differential 26 MHz clock reference signal can
be supplied externally at the XoP and XoN pins. A single-ended clock reference buffer MCLK sources
a harmonically controlled square wave reference signal to be used in the baseband. An additional
single-ended clock is generated for use in the W-CDMA circuit. This output is activated by a control
signal in the digital bus.
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