User Guide

Figure 83 GSM receiver after the antenna switch

Introduction to transmitter functionality

Transmitter functions are implemented in the ASIC N7500. It contains a BB frequency low pass filter, which

is tunable according to the signal bandwidth of the system in use. In addition N7500 contain three separate

RF paths (GSM850/900, GSM1800/1900 and WCDMA1900/2100) comprising of a final frequency IQ modulator

and VGA amplifiers.

WCDMA transmitter

In the transmitter side, an analogue I/Q modulated signal is received from the digital baseband into N7500

and fed through the low pass filter. The corner frequency of the filter is set to approximately 3 MHz.

After the filter the signal is fed to the IQ modulator, which converts the signal to final Tx frequency. There

are two separate I/Q modulators:.one for WCDMA and another for GSM850/EGSM900 and DCS1800/PCS1900

signals.

The WCDMA branch is selected by biasing the appropriate modulator. The modulator is followed by two VGA

stages giving (85 dB) of gain control range. The signal then exits N7500 via a balanced line.

After the PA the transmitted WCDMA signal is fed through a duplex filter to the antenna.

WCDMA power control

WCDMA TX power control is done by the two VGA stages in N7500 ASIC. The VGA have a common temperature

compensation circuit and one voltage mode analog input for gain control (TXC).

The gain of the VGA amplifier chain is controlled by a DA converter in BB. The same DA converter is shared by

GSM Tx power control function.

WCDMA PA module

The WCDMA PA is housed in a separate module having:

• a variable supply voltage input for the amplifier stages (Vcc11),

• a battery supply voltage for the bias circuits (Vcc12),

• one bias current inputs.

Bias currents are generated by 5-bit DA (DAC201) converter in N7500 RF ASIC. The converter is controlled via

RFBus.

PA DCDC converter

A 5 bit DA (DAC101) converter in N7500 RF ASIC controls the DC-DC converter. The converter is controlled via

RFBus. The DCDC converter limits the lowest supply voltage to 1.3 V. At highest power levels the DCDC converter

output settles nominally to 3.1 V.

RM-208

Nokia Customer Care System Module

Page 9 –30 COMPANY CONFIDENTIAL Issue 1