Technical Brief; Tx Module (Sky77518, U501); Only For Training And Service Purposes - LG KP235 Service Manual

3. TECHNICAL BRIEF

3.1 TX module (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 DCS 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 DCS PA block, impedance-matching circuitry for 50
ohm input and output impedances, TX harmonics filtering, high linearity and low insertion loss PHEMT
RF switches, 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 DCS band. Both PA blocks share common power supply pads to distribute current. The
output of each PA block and the outputs to the two receiver 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 3.1 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
TX_VCO while the VCO is being tuned prior to the transmit burst.
The SKY77518 is compatible with logic levels from 1.2V to VCC for BS and TX_EN pads, depending
on the level applied to the VLOGIC pad. This feature provids additional flexibility for the designer in the
selection of FEM interface control logic.
Copyright © 2008 LG Electronics. Inc. All right reserved.

Only for training and service purposes

Figure 3.1 Functional Block Diagram
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3. TECHNICAL BRIEF
LGE Internal Use Only