LG KS10, Service Manual

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LGE Internal Use Only Copyright © 2007 LG Electronics. Inc. All right reserved.
Only for training and service purposes
3. TECHNICAL BRIEF
3.2.2 GSM Transmitter
The shared GSM Low-band (EGSM900) 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-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 1.2.2-1.
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.
Figure 1.2.2-1 Offset phase-locked loop interfaces