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3. TECHNICAL BRIEF
3. TECHNICAL BRIEF
The SKY77518 is a transmit and receive front-end module (FEM) with Integrated PowerAmplifier 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 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 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 TX-VCO 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.
3.1 SKY77518 TX–RX iPAC™ FEM for Dual-Band GSM/GPRS
(SKY77518, U500)