Only For Training And Service Purposes - LG KF305 Service Manual

3.7.2.3 Main Frequency Synthesizer
The AD6548 has a single fast-locking fractional synthesizer used for VCO control in both receive and
transmit mode. The entire system including VCO, tank, fractional N dividers, sigma delta
compensation, charge pump and loop filters are fully integrated.
The only external component is the frequency reference. The synthesizer is controlled via the serial
interface. The VCO is fed into the respective dividers to generate the appropriate LO frequencies for
the RX and TX bands.
Fractional N Dividers
The fractional N divider allows the PLL system to have a smaller step size than theb comparison
frequency which is set by the external reference to 26 MHz. This feature allows all the GSM
frequency band rasters to be achieved, with fast lock times and good phase noise characteristics.
The divider section consists of a dual modulus 8/9 prescaler, integer M & A dividers, and fractional
N system based on sigma-delta modulation to generate the required fractional divide ratio. The
Denominator of the fractional divider can be set to 3 different values, (1040, 1170, 1235), depending
on the mode of operation. For example a denominator of 1040 with an input fraction F maintains an
average value of F/1040 allowing 25 kHz steps when operated at a reference of 26 MHz.
The Overall count value is thus:
8*M + A + Fraction
Where: M is 4 bits, but the MSB is set to 1. A is 3 Bits. Fraction is N/ Denominator.
The Denominator is set to one of 3 values: 1040, 1170, 1235. N is a 11 bit value.
Values for M, A and N are loaded from serial interface word, but the denominator is automatically
set according to the mode.
Refer to the Programming Procedures for more details.
Phase Frequency Detector/Charge Pump
A Phase Frequency Detector (PFD) is used for the PLL phase detector. The charge pump is
designed such that good matching of up and down currents is achieved over a wide output
operating range. The charge pump output is internally routed to the integrated synthesizer loop filter.
Synthesizer Loop filter
To minimize complexity of the external PCB layout the Main Synthesizer loop filter is also fully
integrated into the IC. No external components or adjustments are required.
Voltage Controlled Oscillator
The integrated voltage controlled oscillator (VCO) is a complete self-calibrating subsystem. This
employs a fully automated digital self-calibration function to ensure optimum phase noise
performance over the entire frequency range. The VCO generates frequencies between 2520MHz
and 2985MHz as required to operate in the four GSM bands for RX and TX.
Copyright © 2009 LG Electronics. Inc. All right reserved.

Only for training and service purposes

3. TECHNICAL BRIEF
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LGE Internal Use Only