LG TracFone LG840G Service Manual page 24

3. TECHNICAL BRIEF
3.3.1 Transmitter Part
GSM/EDGE Transmitter Block
The QSC62x0 device supports quad-band GSM transmissions with two separate dual-band driver amplifier outputs; in
fact, most Tx active circuits are contained within the device. Both GSM transmit paths (Figure 3.3.1.1) begin with a single,
shared analog baseband signal from the device's baseband circuits — the same interface and baseband circuits used by
the UMTS transmitters. The GSM transmitters use the same quadrature upconverters as well — one for low band signals
and one for high band — with just one active at a time. The transmitter LO signals are generated by circuits described in
next section and delivered to the upconverter circuits at the correct frequency, with the proper phase relationship, and
with an adequate drive level.
The SPDT switches at each driver amplifier input allow selection of the output signal: either the actual GSM signal from
the upconverter or a test signal generated by the Tx LO synthesizer. The Tx output chain is functionally identical for both
the low band and the high band: the power amplifier is driven by the QSC device through a matching network and a
resistive attenuator; the PA output is routed to the antenna switch module whose output is connected to the antenna.
In addition to the through signal path, the QSC device also provides the PA ramp control signal that ensures smooth
transitions while the transmitter is turned on and off for GSM's burst transmissions.
The ramp signal is generated by one of the baseband circuit's Tx DACs. A switch after the baseband amplifier selects
whether the DAC output signal is routed to GSM PA module for ramping the PA up or down, or to the transmitter signal
path for data transmission.
25page Figure 3.3.1.1 p g g
Figure 3.3.1.1 QSC GSM transmitter signal paths functional diagram
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