C.phase Frequency Detector (Pfd); D.loop Filter; E.tx Vco; F.feedback Down-Converting Mixer - LG KG110 Service Manual

3. TECHNICAL BRIEF
C. Phase Frequency Detector (PFD)
The PFD ensures that the transmitted signal contains the required modulation and is
accurately locked to the desired GSM channel. The downconverted feedback signal from
the TXVCO and the Quadrature Modulator output are phase compared by the PFD.
The PFD charge pump generates a current pulse proportional to the difference in phase
which is applied to the loop filter.
D. Loop filter
To minimize complexity of the external PCB layout the TX loop filter is fully integrated into
the IC. At power up the filter is automatically calibrated as part of the baseband filter cal,
eliminating process tolerances. The calibration is fully integrated and requires no
extra programming.

E.TX VCO

The Transmit Voltage Controlled Oscillator (TX VCO) and tank components are a fully
integrated subsystem. The subsystem includes PA drivers so the outputs are used to directly
drive the external PAs. The low noise oscillator design and internal filtering mean that external
TX SAW filters are not required. In Low band operation the TX VCO output is divided by two
and filtered. The TX VCO is automatically calibrated to ensure optimum performance over its
operating frequency of 1648 to 1910 MHz.
F. Feedback Down-Converting Mixer
The feedback down converting mixer is used to translate the TX VCO output frequency to
the TX IF. An integrated band pass filter exists between the mixer and the PFD to filter the
mixers unwanted side band and higher order mixing products.

G.Transmit Frequency Plan

Unlike many other translation loop modulators the AD6548 uses only a single VCO source to
derive the local oscillator signal for both the Feedback Down-Converting Mixer and the
Quadrature modulator. Therefore there is a fixed relationship between the Tx IF frequency and
the LO VCO frequency .This ratio was chosen to minimize VCO tuning range, TX IF frequency
variation and ensure excellent transmit spectral mask performance. The Feedback-Down
Converting Mixer operates low side injection for the high bands and high side injection for the
low bands. The final relationship between the transmitted TX frequency frequency and the LO
VCO frequency is different between the two bands. These relationships are taken account of in
the synthesizer architecture and programming.
H. Main Frequency Synthesizer
The AD6548/9 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 a low cost crystal for the 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.
I. 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
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