Rf Synthesizer; Vctcxo - LG KE820 Service Manual

3. TECHNICAL BRIEF

3.23. 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.

3.24. VCTCXO

The VCTCXO (X401) supply 26MHz reference clock and controlled by AFC input to generate a strict
system clock. The 26MHz clock is used to Transceiver(U402), Bluetooth chip(U102) and S-Gold2
(U102).
2V7_VCXO
R449
0
X401
3 1
OUT VCONT
4 2
VCC GND
C431
26MHz
1uF
Figure 40 VCTCXO Schematic
- 66 -
R425 620
AFC
C430 C455
22p NA