BenQ
Rev 1.0
1.T/R switch:
U706 is a front-end switch device for GSM/DCS.
These devices include 1 di 2 sw 2 LPFs. The following
table shows the three operating modes.
Mode
Vc1 (pin 5)
Vc2 (pin 8)
EGSM TX
L
H
DCS TX
H
L
EGSM Rx and DCS
Rx
L L
These four control signals are generated from U707 Dual Buffer, which
controlled by EGSM_T/R (U1 Pin B10) and DCS_T/R (U1 pin E9).
2.PA and power control circuit:
PA (U708), which integrates the power level control circuit, is
controlled by signal BS1 (U1 Pin B13), Tx_ON (U1 Pin E6) and RAMP
(U3 Pin F9). The purpose of the BS1 is for band select control. The
RAMP signal is for power level control. The Tx_ON signal is for power
saving control. For detailed signal waveform, please refer to TX signal
path diagram.
3.Transceiver:
U601 is a highly integrated RF transceiver IC for GSM900, DCS1800
and PCS1900 triple-band cellular systems.
Receiver Operation
As HD155156NP supports triple band, the front-end incorporates
three LNAs and mixers. The incoming RF signals are mixed directly
down to I/Q baseband by the front-end block. This incorporates three
LNAs / three buffers and Gilbert Cell mixer blocks optimized for
operation at 900MHz, 1800MHz, 1900MHz respectively. Two closely
matched baseband amplifier chains follow the front-end block. These
include distributed lowpass filtering; three switched gain stages and one
fixed gain stage.
68
In addition, the baseband section integrates A/D and D/A converters
that provide automatic on-chip correction of DC offsets. The three
switched gain stages in each channel are DC coupled and provide
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