LG TracFone LG840G Service Manual page 19

3. TECHNICAL BRIEF
The active upconverter's output is at the desired RF channel frequency and drives the QSC output stages.
These RF circuits include multiple variable-gain stages that provide transmit AGC control.
A greater than 81 dB gain control range is realized using information from the on-chip Tx power detector combined with
a control signal from the baseband circuits. The wide range of driver amplifier output levels is achieved while supporting
the WCDMA standard's requirements for ACLR, spurious emissions, Rx-band noise, etc.
The high-band driver amplifier output is followed by a SPDT switch that provides two high-band outputs,
thereby fulfilling the tri-band UMTS requirement. The low-band output port is driven directly by its output amplifier.
All three output ports are single-ended with 50ohm nominal impedance. Each requires a matching network to interface
with its band-pass filter.
Each of the three UMTS Tx output chains are functionally identical: the QSC Tx output drives the PA through the band-
pass filter and a matching network; a directional coupler provides a sample of the PA output signal; the through path of
the coupler is routed to the Tx port of the duplexer; the duplexer antenna port is connected to the antenna switch; and
the switch is connected to the antenna.
The coupler outputs provide a low-level sample of the active transmitter's Tx power. An on-chip power-detector circuit
provides a Tx power estimate that assists in setting the transmit gains and helps ensure that the maximum allowed
output power is not exceeded. The three UMTS couplers (one for each band) use a daisy-chain configuration that allows
them to share the single, on-chip power detector circuit.
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