LG G7100 Service Manual page 19

A. IF Modulator
The baseband converter(BBC) within the GSM chipset generates I and Q baseband signals for the
transmit vector modulator. The modulator provides more than 40dBc of carrier and unwanted side-
band rejection and produces a GMSK modulated signal. The baseband software is able to cancel out
differential DC offsets in the I/Q baseband signals caused by imperfections in the D/A converters.
The TX-Modulator implements a quadrature modulator. The IF-frequency input signal is split into two
precise orthogonal carriers, which are multiplied by the baseband modulation signal IT/ITX and
QT/QTX. It is used as reference signal for the OPLL.
B. OPLL
The offset mixer down converts the feedback Tx RF signal using LO to generate a IF modulating
signal. The IF signal goes via external passive bandpass filter to one port of the phase detector.
The other side of the phase detector input is LO signal. The phase detector generates an error
current proportional to the phase difference between the modulated signal from the offset mixer
and the reference signal from the LO. The error current is filtered by a second order low-pass filter
to generate an output voltage which depends on the GMSK modulation and the desired channel
frequency. This voltage controls the transmit VCO such that the VCO output signal, centered on
the correct RF channel, is frequency modulated with the original GMSK data. The OPLL acts as a
tracking narrowband band pass filter tuned to the desired channel frequency. This reduces the
wideband noise floor of the modulation and up-conversion process and provides significant
filtering of spurious products.
C. Power amplifier
The CX77315[U601] is designed in a compact form factor for quad-band cellular handsets comprising
GSM850/900, DCS1800, and PCS1900 operation. The PAM also supports Class 12 General Packet
Radio Service(GPRS) multi-slot operation. The module consists of separate GSM850/900 PA and
DCS1800/PCS1900 PA blocks, impedance-matching circuitry for 50 ohm input and output
impedances, and a Power Amplifier Control (PAC) block with and internal current-sense resistor.
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3. TECHNICAL BRIEF