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3. TECHNICAL BRIEF
3.1 Power Amplifier (SKY77518, U501)
The SKY77518 is a transmit and receive front-end module (FEM) with Integrated Power Amplifier
Control (iPAC™) for dual-band cellular handsets comprising GSM900 and DCS1800 operation.
Designed in a low profile, compact form factor, the SKY77518 offers a complete Transmit VCO-to-
Antenna and Antenna-to-Receive SAW filter solution. The FEM also supports Class 12 General
Packet Radio Service (GPRS) multi-slot operation.
The module consists of a GSM900 PA block and a DCS1800 PA block, impedance-matching circuitry
for 50 Ω input and output impedances, TX harmonics filtering, high linearity and low insertion loss
PHEMT RF witches, diplexer and a Power Amplifier Control (PAC) block with internal current sense
resistor. A custom BiCMOS integrated circuit provides the internal PAC function and decoder circuitry
to control the RF switches. The two Heterojunction Bipolar Transistor (HBT) PA blocks are fabricated
onto a single Gallium Arsenide (GaAs) die. One PA block supports the GSM900 band and the other
PA block supports the DCS1800 band. Both PA blocks share common power supply pads to distribute
current. The output of each PA block and the outputs to the two receive pads are connected to the
antenna pad through PHEMT RF switches and a diplexer. The GaAs die, PHEMT die, Silicon (Si) die
and passive components are mounted on a multi-layer laminate substrate. The assembly is
encapsulated with plastic overmold.
Band selection and control of transmit and receive modes are performed using two external control
pads. Refer to the functional block diagram in Figure 1 below. The band select pad (BS) selects
between GSM and DCS modes of operation. The transmit enable (TX_EN) pad controls receive or
transmit mode of the respective RF switch (TX = logic 1). Proper timing between transmit enable
(TX_EN) and Analog Power Control (VRAMP) allows for high isolation between the antenna and
TXVCO while the VCO is being tuned prior to the transmit burst.
The SKY77518 is compatible with logic levels from 1.2 V to VCC for BS and TX_EN pads, depending
on the level applied to the VLOGIC pad. This feature provides additional flexibility for the designer in
the selection of FEM interface control logic.
3. TECHNICAL BRIEF
LGE Internal Use Only
Copyright © 2008 LG Electronics. Inc. All right reserved.
Only for training and service purposes
Figure 3.1 Functional Block Diagram
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