User Guide

GSM transmitter

N7505 receives an analogue IQ modulated signal from digital BB, which is first low pass filtered with filter

corner frequency set to approximately 200 kHz. After the filter, the signal is routed to the GSM modulator.

The amplifier gives 40 dB of power control dynamic range.

The appropriate routing after the modulator is selected by biasing either EGSM900 (/GSM850) or

GSM1800/1900 variable gain amplifier. The amplifier gives 40 dB of power control dynamic range.

After the VGA stage the signal exits N7505. In case of GSM1800/1900 the signal goes directly to the FEM thru

a balun, which converts single ended signal into balanced signal needed to feed the FEM. In case of GSM850

and EGSM900 the PA module is preceded by a SAW filter.

TX SAW Z7503 is also dual packaged so that the correct band is selected with control voltage coming from

N7505. SAW also performs the unbalanced to balanced signal transformation. After the filter, the signal is

fed to FEM. Finally the signal is routed to the antenna.

Figure 67 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.

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.

Frequency synthesizers

RF ASIC N7505 has a single synthesizer that is used to generate right frequencies for both TX and RX.

Reference oscillators

A 38.4MHz VCTCXO is used as a reference oscillator for the frequency synthesizers.

RM-198

System Module Nokia Customer Care

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