Frequency Synthesizers - Nokia 6680 RM-36 Service Manual

RM-36
Nokia Customer Care System Module
GSM PA module
A single GSM/EDGE PA module contains two separate amplifier chains, one for EGSM900 (and GSM850) and
another for GSM1800/1900. Both amplifiers have a battery supply connection and two bias current inputs. The
bias current for final amplifier stage is adjusted according the power level in use in order to optimise efficiency.
The bias currents are also used as on/off switching signals for PAs.
In the EDGE mode, PA linearity has to be higher than in GMSK mode because of envelope variations of the 8-PSK
modulations. This is achieved by increasing the bias currents compared to the GMSK mode and setting a
dedicated Vmode control signal up. Increasing bias currents improves the linearity of the amplifiers, but it also
tends to unnecessarily increase the gain of the PA. Vmode control aims to keep the gain of the amplifiers down.
The bias current needed for the maximum and the lowest output powers is specified by a PA manufacturer.
The current for the intermediate power levels is then linearly adjusted between these two values.

Frequency synthesizers

RF has separate synthesizers for Rx and Tx. Both synthesizers consist of:
• PLL
• loop filter
• VCO
• balun
The VCO frequencies are locked by PLLs into a reference oscillator, VCTCXO.
The PLLs are located in N7500 and N7501 respectively and controlled via RFBus. PLL charge pump charges or
discharges the integrator capacitor in the loop filter depending on the phase of the measured frequency
compared to the phase of the reference frequency. The integrator output voltage is connected to the control
input of the VCO.
The VCOs operate at the channel frequency multiplied by two in the upper bands (for example, GSM1800/1900/
WCDMA) and by four in EGSM900 (and GSM850, if applicable). The required frequency dividers required for
modulators are integrated in N7501 and those for demodulators in N7500. The dividers are controlled via RFBus.
Figure 131 Phase locked loop in N7500 and N7501 (PLL)
Reference oscillators
As a reference oscillator for the frequency synthesizers a 38.4MHz VCTCXO (voltage controlled temperature
compensated crystal oscillator) is used.
Page 9–52 Company Confidential Issue 1
Copyright ©2005 Nokia. All Rights Reserved.