Siemens C75 Service Repair Documentation page 9

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Company Confidential

Synthesizer: LO1

First local oscillator (LO1) consists of a PLL and VCO inside Bright (D903) and an internal

loop filter

RF PLL

The frequency-step is 400 kHz in GSM1800 mode and 800kHz in EGSM900 mode due to the

internal divider by two for GSM1800 and divider by four for EGSM900. To achieve the

required settling-time in GPRS operation, the PLL can operate in fastlock-mode a certain

period after programming to ensure a fast settling. After this the loopfilter and currents are

switched into normal-mode to get the necessary phasenoise-performance. The PLL is

controlled via the tree-wire-bus of Bright VI E.

RFVCO (LO1)

The first local oscillator is needed to generate frequencies which enable the transceiver IC to

demodulate the receiver signal and to perform the channel selection in the TX part. The VCO

module is switched on with the signal PLLON. The full oscillation range is divided into 256

sub-bands To do so, a control voltage for the LO1 is used, gained by a comparator. This

control voltage is a result of the comparison of the divided LO1 and the 26MHz reference

Signal. The division ratio of the dividers is programmed by the SGOLDlite, according to the

network channel requirements.

Service Repair Documentation

Level 2.5e – C75

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