LG TracFone LG840G Service Manual page 21

3. TECHNICAL BRIEF
Each UMTS duplexer provides a differential output signal that is compatible with its QSC LNA input.
The duplexer-to-LNA interface requires a differential matching network (MN) that optimizes the power transfer into the
LNA.
Although there are three UMTS LNAs, only one is active at a time. The active gain-stepped LNA output drives a shared
quadrature downconverter directly — an off-chip inter-stage filter is not required. The elimination of this filter is achieved
by a combination of factors:
- New on-chip QSC processing
- Higher performance achieved by the differential duplexer-to-LNA interface
Greater duplexer suppression of Tx leakage
The downconverter's RF circuitry includes another gain-stepped amplifier that supplements the LNA gain steps to further
extend the receiver dynamic range. The downconverter translates the active LNA's RF signal directly to baseband, produc-
ing two analog outputs: in-phase (I) and quadrature (Q). The UMTS baseband signals are routed to lowpass filters whose
passband and stopband characteristics are optimized for the active WCDMA waveform. Both filter outputs are buffered
to drive their analog-to-digital converters for digitization. The digital baseband outputs are routed to QSC baseband
circuits for further processing.
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