LG AR140B, Руководство по обслуживанию

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Only for training and service purposes
3. Technical Brief
3.20. 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 (OLGA²). 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.
3.21. 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 31. DCXO Schematic
14
13
2
1
1
1
2
S
Y
S
F
O
X
X
O
X
GND1
FSYS1
26MHZ
X400 NX3225SA
2 1
3
4
GND2 HOT2
HOT1 GND1