LG 512W, Service Manual

Page: 19 / 104

3. TECHNICAL BRIEF
- 18 -
A. TX IF Modulator
The TX I and Q signals from BB analog chipset, AD6521(U2), #F9, E9, C9, D9 are fed to #17~20 of
the HD155128TF TX modulator(U301), where they are then modulated onto either a TX IF of
270MHz(for GSM-TX) or 135MHz(for DCS-TX) by the quadrature mixer inside U301. The signal TX
LO IF(270MHz,135MHz) is input from U301. This signal is produced by the external 1,080 MHz IF
VCO of synthesizer Si4133G_XM2(U302), which has been divided down by a factor of 4 for GSM-
TX or 8 for DCS-TX. This division factor is set by the control signal of BB digital chipset,
AD6522(U3) #D14, D13, F11.
The modulator provides more than 40 dBc of carrier and unwanted side-band rejection and
produces a GMSK modulated signal. The BB software is able to cancel out differential DC offsets in
the I/Q BB signals caused by imperfections in the D/A converters. The TX-Modulator implements a
quadrature modulator. The IF-frequency input signal is split into two precise orthogonal carriers,
which are multiplied by the BB modulation signal IT/ITX and QT/QTX. It is used as reference signal
for the OPLL.
Figure 3-6. TX IF Modulator and OPLL Circuit.