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4. RF circuit technical brief
4.4 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 2 ). 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.
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Only for training and service purposes
Figure 35. Transmitter Architecture Block Diagram