LG B2100, Service Manual

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Service Manual /
21
3. TECHNICAL BRIEF
(3) Frequency Synthesizer
The Aero I transceiver integrates two complete PLLs including VCOs, varactors, resonators, loop filters,
reference and VCO dividers, and phase detectors. The RF PLL uses two multiplexed VCOs. The RF1 VCO is
used for receive mode, and the RF2 VCO is used for transmit mode. The IF PLL is used only during transmit
mode. All VCO tuning inductors are also integrated. The IF and RF output frequencies are set by
programming the N-Divider registers, NRF1, NRF2 and NIF. Programming the N-Divider register for either
RF1 or RF2 automatically selects the proper VCO. The output frequency of each PLL is as follows:
f
out
= N x f
ø
The DIV2 bit in register 31h controls a programmable divider at the XIN pin to allow either a 13 or 26 MHz
reference frequency. For receive mode, the RF1 PLL phase detector update rate (f) should be programmed
f = 100 kHz for DCS 1800 or PCS 1900 bands, and f = 200 kHz for GSM 850 and E-GSM900 bands. For
transmit mode, the RF2 and IF PLL phase detector update rates are always f =200 kHz.
Figure. 3-4 SI4205 FREQUENCY SYNTHESIZER PART