LG TracFone LG840G Service Manual page 26

3. TECHNICAL BRIEF
Shared UMTS/GSM configuration (four GSM LNAs)
In this configuration, the GSM 850 receive path shares the UMTS 850 receiver front-end path (including LNA).
Beginning at the antenna switch output, the GSM signal is routed through the UMTS850 duplexer to the shared LNA
input at pins L22 and K22. Likewise, the GSM 1900 receive path shares the UMTS 1900 front-end, including pins J23 and
H23.
The GSM 900 and GSM 1800 bands have dedicated receive paths from the antenna switch outputs to the QSC LNA
inputs.
Each band has its own band-select filter that drives its LNA input. All four GSM bands include input filtering: the 850 and
1900 bands share the UMTS duplexer filtering, while the 900 and 1800 bands have dedicated bandpass filters.
The filter functions suppress out-of-band received signals and the handset's GSM transmitter leakage.
Transmit power suppression must be adequate to avoid overdriving the GSM Rx chain. Like the UMTS paths, the GSM
paths use a differential configuration into their LNAs, and thus equire differential matching networks.
The internal GSM receivers are functionally identical to the UMTS receivers: although there are multiple GSM 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,
producing two analog outputs: in-phase (I) and quadrature (Q). The GSM baseband signals drive lowpass filters whose
passband and stopband characteristics are optimized for the active GSM 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.
The Rx LO signal is delivered to the downconverter circuits from the LO generation and distribution circuits as described
in next section.
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