LG -P970 Service Manual page 30

3. TECHNICAL BRIEF
3.2.2 GSM TRANSMITTER
The transmitter takes the modulating symbols sent from baseband via DigRF Interface and converts them into a
RF signal. It is based on a polar modulator architecture where amplitude and phase (or alternatively frequency)
are first handled separately and afterwards are merged in the RF domain.
The signal flow can be understood when you look at Figure 3-4. The digital symbol stream enters either a 8PSK
or a GMSK modulator, depending on the desired mode. The GMSK signal is thereby differentially encoded as
postulated by GSM specification 45.004. If a generator runs out of symbols, it behaves as if an input series of
repeating dummy symbols has been applied. To simplify mode switching between 8PSK and GMSK, the two
generators are synchronized. In 8PSK mode, the output of the internal 8PSK signal generator is given in
Cartesian (I/Q) coordinates. The desired conversion into polar coordinates (amplitude and phase) is executed by
a CORDIC algorithm. For GMSK mode the CORDIC algorithm is not involved - the incoming GMSK symbols
directly determine the modulation frequency of the sigma-delta modulation loop.
In the phase path the phase signal is differentiated to obtain a frequency signal, which is fed into a preemphasis
filter to compensate for narrow PLL bandwidth. Afterwards it is applied to the Sigma-Delta modulation loop. In
this way the Sigma-Delta PLL shifts the phase information into RF domain.
The digital amplitude signal is multiplied with the ramping waveform, converted into an analog voltage and
filtered. After that it is mixed with the purely phase modulated RF carrier to gain an amplitude and phase
modulated output signal. The output power can be influenced directly by a PGA (programmable gain amplifier),
if no power control loop is needed, or by specifying a target output level, if a closed power loop for enhanced
output power accuracyis desired.
[Figure 3-4] 2G Transmitter Line Up
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