LG KG195 Service Manual page 56

4.2.19.5 Voltage Control Crystal Oscillator
Voltage Control Crystal Oscillator (VCXO) consists of an amplifier, a buffer, and a programmable
capacitor array. The VCXO provides the MT6129 with a selectable reference frequency of either 13
MHz or 26 MHz. The amplifier is designed to be in series resonance with a standard 26 MHz crystal.
The crystal is connected from the input pin XTAL of amplifier to ground through a series load
capacitance. The buffer provides a typical 600mVpp voltage swing at either 13 MHz or 26 MHz. It is
designed to drive a tuned load to improve harmonic contents and reduce the oscillator current
consumption. The capacitor array, from 0.0625 pF to 4 pF in steps of 0.0625 pF, is used to shunt the
series load capacitor for coarse tuning and remove any fixed offsets due to crystal manufacturing
variations. An internal varactor that provides fine tuning combines with the capacitor array. As an
alternative, the reference frequency can be provided by an external 26 MHz VCTCXO module.
When pin VCXOCXR is tied to the VCCVCXO supply, the XTAL pin will accept an external signal.
Furthermore, the VCXO control pin can be tied to VCCVCXO to prevent the current leakage during the
sleep mode operation.
4.2.19.6 Regulator
The MT612X internal regulators provide low noise, stable, temperature and independent supply
voltages to critical blocks in the transceiver. An internal P-channel MOSFET pass transistor is used to
achieve a low dropout (LDO) voltage of less than 150 mV in all regulators.
Copyright © 2007 LG Electronics. Inc. All right reserved.
Only for training and service purposes
4. Circuit Description
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