GX500 Operational Description Revision A
LG Electronics 70/143 LGE Property
3.19 RF synthesizer
The transceiver 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
(OLGA). To overcome the statistical spread of the loop filter element values an automatic loop filter
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.
3.20 DCXO
The SMARTiPM contains a fully integrated 26MHz digitally controlled crystal oscillator (DCXO) with three
outputs for the system clock, one output for the GSM baseband and two additional for other subsystems (GPS,
Bluetooth, etc.).The only external part of the oscillator is the crystal itself. The frequency tuning is performed
along the selected subrange by programming the frequency control word (XO_TUNE) via the three wire bus
(“3Wbus”)
Figure 3-34 DCXO Schematic
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