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Only for training and service purposes
LGE Internal Use Only
3. TECHNICAL BRIEF
LGE Internal Use Only
3.14.1.3 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 the 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.