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3. TECHNICAL BRIEF
(3) Frequency Synthesizer
Figure 3-4 Block Diagram of Frequency Synthesizer part of SI4205
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
= N x f
out
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-GSM 900 bands. For transmit mode, the RF2 and IF PLL phase
detector update rates are always f =200 kHz.
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