User Guide

Figure 80 GSM transmitter

GSM power control

A closed control loop comprise an integrated power detector (in FEM) and an error amplifier. The error

amplifier resides in N7505 and it controls the transmitter power of GSM.

Detector output from the FEM gives a DC level proportional to the output power. The DC voltage is fed to the

negative input of the error amplifier where it is compared to the level of the reference signal, TXC. TXC is

received from the BB circuitry.

The output of the error amplifier is fed to a buffer amplifier which in turn steers the VGA amplifier.

FEM is switched on and off by changing the bias currents. As a result the output power ramping and final

power level of the transmitter are set in a controlled manner. During EDGE operation 8-PSK modulation is

utilised.

The bias currents are also adjusted. The power control loop is opened during the data transmission in both

GSMK and EDGE modes.

The loop is opened with a dedicated TXA-signal via RFBus. When the power is ramped up, a modulating bit

sequence producing a constant envelope waveform is used and the power control loop is closed. Once the

desired power level has been reached, the loop is opened and the power control voltage is kept constant.

When the active part of the burst is over the loop is again closed and the power is ramped down.

The power control loop is enabled and disabled in N7505 RF ASIC. In case of dual slot transmission the output

power is ramped down between the consecutive slots.

GSM FEM (front end module)

A single GSM/EDGE FEM 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. The bias currents are

also used as on/off switching signals for PAs.

RM-170; RM-171

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