LG MG300d, Service Manual

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3. TECHNICAL BRIEF
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(2) Synthesizer Section
The SKY74400 includes a fully integrated UHF VCO and 3rd order loop filter. A single sigma-delta
fractional-N synthesizer phaselocks the Local Oscillator (LO) used in both transmit and receive paths
to a precision frequency reference input. Fractional-N operation offers low phase noise and fast
settling times, allowing for multiple slot applications such as GPRS.
The SKY74400 frequency stepping function with a 3 Hz resolution allows quad band operation in both
transmit and receive bands using a fully integrated UHF VCO. The fine synthesizer resolution allows
direct compensation or adjustment for reference frequencyerrors.
The generated frequency is given by the following equation:
where: fvco = Generated VCO frequency
N = N-divider ratio, integer part
FN = Fractional setting
R = R-divider ratio
fref = Reference frequency
A. UHF VCO Frequency Setting
To tune the receiver’s receive frequency ( fRX ), the VCO frequency ( fVCO ) is set according to the
following equations:
GSM850/EGSM900:
DCS1800/PCS1900:
B. Digital Frequency Centering
The SKY74400 re-centers the UHF VCO frequency range each time the synthesizer is programmed.
This proprietary Skyworks technique, called Digital Frequency Centering (DFC) extends the VCO
frequency coverage, speeds up settling time, and ensures robust performance since the VCO is
always operated at the center of its tuning range. Each time the synthesizer is programmed, the DFC
circuit is activated and the VCO is centered to the programmed frequency in less than 20 µs. After this,
normal Phase Locked Loop (PLL) operation is resumed and the fine settling of the frequency is
finalized. DFC typically adjusts the VCO center frequency to within a few MHz and no more than 5
MHz offset, and presets the tuning voltage to the center of the range before the PLL takes over.
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