Transmitter - LG U8120 Service Manual

3. TECHNICAL BRIEF

3.7.2 Transmitter

A 4-bit sigma-delta bit stream comes from the Marita ASIC including both channel information and the
GMSK phase information. Via the 3-wire control bus also driven from Marita, the selection of
transmitter band is made. The 4bits from the bit stream provides the fine-tuning of the division ratio
before going to the divider of the used VCO (low band, 900MHz or high band, 1800MHz).
The modulated VCO signal is fed to the output buffer. One buffer is available for each of the low and
high bands. Trimming capability is included for best match versus the PA used.
The GSM/GPRS transceiver, Ingela, output is passed to the dual-band PA that after amplification
feeds the signal via a low pass filter to the antenna switch and further to the antenna.
The transmit block consists of two differential high power transmit output buffers with controllable
output power. The modulated transmit signal from the VCO buffer is amplified to a level suitable to
drive the external power amplifier. The buffer outputs are of open collector type and must be
terminated into a suitable load.
Figure 3-7 shows a block diagram of the transmitter block.
Figure 3-7. Block diagram for the transmitter.
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