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3. TECHNICAL BRIEF
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Figure. 3.5.3 TRANSMITTER CHAIN BLOCK DIAGRAM
3. TECHNICAL BRIEF
3.5.2.2 Transmitter
The GMSK transmitter supports power class 4 for GSM850 or GSM900 as well as power class 1 for
DCS1800 or PCS1900. The digital transmitter architecture is based on a fractional-N sigma-delta
synthesizer for constant envelope GMSK modulation. This configuration allows a very low power design
without any external components.
Up- and down-ramping is performed via the ramping DAC connected to VRAMP.
RF synthesizer
The RF subsystem contains a fractional-N sigma-delta synthesizer for the frequency synthesis. Respective
to the chosen band of operation the phase locked loop (PLL) operates at twice or forth of the target signal
frequency. In receive operation mode the divided output signal of the digital controlled oscillator output
(DCO) serves as local oscillator signal for the balanced mixer. For transmit operation the fractional-N
sigma-delta synthesizer is used as modulation loop to process the phase/frequency signal. The 26 MHz
reference signal of the phase detector incorporated in the PLL is provided by the reference oscillator.