LG CU500 - Cell Phone, Service Manual

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3. TECHNICAL BRIEF
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3.2.2 GSM Transmitter
The shared GSM Low-band (GSM850/GSM900) and High-band (DCS1800, PCS1900) transmit path begins
with the baseband inputs from the MSM6275 IC. These differential analog input signals are buffered,
lowpass filtered, corrected for DC offsets then applied to the GSM quadrature upconverter. The upconverter
LO signals are generated from the transceiver VCO signal by the LO distribution and generation circuits
within RTR6250. This upconverter translates the GMSK or 8-PSK-modulated signal to a convenient
intermediate frequency (IF) that forms one input to a frequency/phase detector circuit. This IF signal is the
reference input to an offset phase-locked loop (OPLL) circuit as shown in Figure 3.2.2-1.
The feedback path of this OPLL circuit includes a downconversion from the RF output frequency range
to the IF range. The two inputs to this downconversion mixer are formed as follows:
1. The dual Tx VCO output (operating in the desired RF output frequency range) is buffered within the
RTR6250 IC then applied to the mixer RF port.
2. The LO Generation and Distribution circuits that deliver the transmit path.s LO for the baseband-to-IF
upconversion also provides the .offset LO. signal that is applied to the feedback path.s mixer LO port.
The mixer IF port output is the offset feedback signal - the variable input to the frequency/phase
detector circuit. The detector compares its variable input to its reference input and generates an error
signal that is lowpass filtered by the loop filter and applied to the dual Tx VCO tuning port to force the
VCO output in the direction that minimizes errors. As mentioned earlier, the VCO output is connected to
the feedback path thereby creating a closed-loop control system that will force frequency and phase
errors between the variable and reference inputs to zero.
Figure 3.2.2-1 Offset phase-locked loop interfaces