LG T5100, Service Manual, Page: 12 / 136

2. TECHNICAL BRIEF
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2.2.1. RF Front End
Antenna switch and dual saw filters are integrated in FEM (FL700).
The Received RF signals (GSM 925MHz ~ 960MHz, DCS 1805MHz ~ 1880MHz) are fed into the
antenna and Mobile switch. An antenna matching circuit is between the antenna and the mobile
switch. The Antenna Switch in FEM (FL700) is used to control the Rx and Tx paths. And, the input
signals VC1, VC2 and VC3 of FEM are directly connected to base-band controller to switch either Tx
or Rx path on. FEM (FL700) supports for dualFEM control logics given below Table 3.1
The receiver part uses a direct conversion receiver architecture that allows for the on-chip integration
of the DC offset compensation system, eliminating DC offset from the demodulated signal.
The TRF6151 integrates dual differential input LNAs that are matched to the 100 ohm in EGSM and
100 ohm in DCS balanced-output SAW filters through external LC matching networks.
2.2.2 Demodulator and base-band processing
In direct conversion receiver there is only one mixer down-converting received RF signal to BB signal
directly. TRF6151 has 4 Gain steps. Front end and VGA high gain mode is 66dB, Front-end
intermediate Gain and VGA high gain mode is 60dB, Front-end low gain and VGA high gain mode is
46dB, Front-end and VGA low gain mode is 20dB.
The Rx gain setting is done in the AGC algorithm. The nominal gain of the receiver is set as a function
of the expected signal strength at the antenna input so that a desired level is reached at the Rx I/Q.
The receiver chain in TRF6151 has variable gain parts, LNA, Mixer, LPF, VGA, VGA Filter, Auxiliary
gain control and VGA. The gain settings can be adjustable via 3-wire bus control lines. The base-band
signals pass via integrated low-pass filters to the base-band A/D converters.
The remainder of the channel filtering is performed by the baseband chipset. The demodulator
contains switches to maintain the sense of the baseband I/Q outputs with respect to the incoming RF
signal on EGSM900 and DCS1800.
2.3 Synthesizer Part
The RF main chipset includes a fully integrated VCO with an on-chip LC tank. A fractional-N
synthesizer can phase lock the local oscillator used in both transmit and receive path to a precision
frequency reference input.
The fractional-N operation offers low phase noise and fast setting times, allowing for multiple slot
applications such as GPRS. The counter and mode settings of the synthesizer are also programmed
via 3-wire interface.
Switch Mode VC1 VC2 VC3
EGSM Tx High Low Low
EGSM/DCS RX Low Low Low
DCS TX Low High High
Table 1. Control Logics