User Guide

GSM PA module

A single GSM/EDGE PA module contains two separate amplifier chains, one for EGSM900 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.

PA detection

It is possible to use PAs manufactured by different vendors. Because of this it is possible to set manufacturer

specific bias values for the PA. PA is detected by DSP SW in manufacturing phase. If PA is changed this detection

routine must be rerun before Tx calibrations. Components R7518, R7522, R7528 and R7534 are part of PA version

detection circuitry.

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 GSM1800/1900/WCDMA and by four in EGSM900.

The required frequency dividers required for modulators are integrated in N7501 and those for demodulators

in N7500. The dividers are controlled via RFBus.

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