Chapter 3. Circuit Description; Rf Transmit/Receive Part - LG PX8700 Service Manual

PX8700

CHAPTER 3. Circuit Description

1. RF Transmit/Receive Part

1.1 Overview
The TX and RX part employs the Direct-Conversion system. The TX and RX frequencies are
respectively 824.04~848.97 and 869.04~893.97 for cellular and 1850~1910 and 1930~1990 for
PCS. The block diagram is shown in [Figure 1-1]. RF signals received through the antenna are
seperated by the Triplexer.
RF Signal fed into the low noise amplifier in RFR6500(LNA) through the duplexer. Then, they are
fed into Mixer in RFR6500. In RFR6500, the RF signal is changed into baseband signal directly.
Then, this signal is changed into digital signal by the analog to digital converter (ADC, A/D
Converter), and the digital circuit part of the MSM(Mobile Station Modem) 6500 processes the
data from ADC. The digital processing part is a demodulator.
In the case of transmission, RFT6150 receives OQPSK-modulated anlaog signal from the
MSM6550.
The RFT6150 connects directly with MSM6550 using an analog baseband interface. In RFT6150,
the baseband quadrature signals are upconverted to the Cellular or PCS frequency bands and
amplified to provide signal drive capability to the power amp.
After that, the RF signal is amplified by the Power Amp in order to have enough power for
radiation. Finally, the RF signal is sent out to the cell site via the antenna after going through
the duplexer.
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