Introduction To Transmitter Functionality; Wcdma Transmitter - Nokia RM-42 Service Manual

RM-42
Nokia Customer Care System Module

Introduction to transmitter functionality

Transmitter functions are implemented in the RF ASIC N7501. It contains a BB frequency low pass filter, which
is tunable according to the signal bandwidth of the system in use.
In addition, N7501 contains separate RF paths comprising a final frequency IQ modulator and VGA amplifiers.
In order to eliminate the effect of process variations on the low pass filter characteristics, a tuning procedure
is carried out in production. The same tunings must be performed if the RF ASIC N7501 is changed.

WCDMA transmitter

In the transmitter side, an analogue I/Q modulated signal is received from the digital baseband into N7501 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 EGSM900 and GSM1800/1900 signals.
The modulator is followed by two VGA stages giving 85 dB of gain control range. The signal then exits N7501
via a balanced line. In order to attenuate the out of band noise of the transmitter the signal is band pass filtered
by a SAW filter before it is fed to the WCDMA PA module.
After the PA the transmitted WCDMA signal is fed through an isolator and a duplex filter to the antenna.
Figure 123 WCDMA transmitter
WCDMA power control
WCDMA Tx power control is accomplished by the two VGA amplifier stages in N7501 Tx ASIC.
The VGAs have a common temperature compensation circuit and one voltage mode analogue input for gain
control (TXC).
The gain of VGA amplifier chain is controlled by a DA converter in BB. The same DA converter is shared by GSM
Tx power control function.
It is required that phone can measure its output power in high power levels. A sample of the output power is
taken by a capacitor between the power amplifier and the isolator and fed to a diode power detector. The
output of the detector is low pass filtered and the voltage is then AD converted in BB. The power detector
circuitry is calibrated in manufacturing.
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