LG GS500 Manual page 54

4. RF CIRCUIT TECHNICAL BRIEF
4.2.3 GSM RF Synthesizer
The SMARTi 3GE contains a fractional-N sigma-delta synthesizer for the frequency synthesis in the RX
operation mode. For TX operation mode the fractional-N sigma-delta synthesizer is used as Sigma-Delta
modulation loop to process the phase/frequency signal. The 26MHz reference signal is provided by the
internal crystal oscillator. This frequency serves as comparison frequency of the phase detector and as clock
frequency for all digital circuitry.
The divider in the feedback path of the synthesizer is carried out as a multi-modulus divider (MMD). The loop
filter is fully integrated and the loop bandwidth is about 100 kHz to allow the transfer of the phase
modulation. The loop bandwidth is automatically adjusted prior to each slot (OLGA2). To overcome the
statistical spread of the loopfilter element values an automatic loopfilter adjustment (ALFA) is performed
before each synthesizer startup.
The fully integrated quad-band VCO is designed for the four GSM bands (850, 900, 1800, 1900 MHz) and
operates at double or four times transmit or receive frequency. To cover the wide frequency range the VCO is
automatically aligned by a binary automatic band selection (BABS) before each synthesizer startup.
4.2.4 Reference Oscillator
The SMARTi 3GE comprises three 26MHz reference frequency outputs for the GSM baseband, for the 3G RF
and for other subsystems (GPS, Bluetooth, etc.) as well as an reference frequency input for application of an
external VCXO module.
4.2.5 Front End Module Control
Implemented in the SMARTi 3GE are two outputs for direct control of GSM front end modules with two logic
input pins to select RX- and TX-mode as well as low- and highband operation.
LGE Internal Use Only
GS500 service manual
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