The filter is followed by an AGC amplifier with adjustable gain. The signal is further amplified before it is fed
to balanced analog IQ output pins. Analogue output pins are accompanied by reference voltage output, which
sets the DC level for the AD converter in BB ASIC.
GSM receiver
As GSM Rx branches are functionally identical, the following description is applicable to all of them.
The received signal goes from the GSM antenna to the single feed switch, which routes the received GSM
signal to a front-end module, containing a PIN diode switch matrix. The switch matrix is used for the GSM
band as well as for the Rx/Tx selection.
The switch matrix is followed by integrated LNA stages in the RF ASIC. The LNAs are followed by demodulators
which down-convert the signal to baseband I and Q signals.
In the BB-chain there are two adjustable gain stages and one mode specific gain. The first one called BB-gain
is just after the demodulator. It has a 12 dB gain range with 6dB steps. After the BB-gain there is the channel
select filter.
The main AGC amplifier precedes the channel selection filter. It has 30 dB of gain control range with 3 dB
steps. The final amplifier stage is used as an output buffer for IQ signals. This stage is set to 0 dB gain in the
GSM mode.
Transmitter
Introduction to transmitter functionality
Transmitter functions are implemented in the RF ASIC N7500.
The RF ASICs contain a BB frequency low pass filter, which is tuneable according the signal bandwidth of the
system in use. In addition, they both contain three separate RF paths (GSM850/900, GSM1800/1900 and
WCDMA) comprising of a final frequency IQ modulator and VGA amplifiers.
WCDMA transmitter
In the transmitter side, an analogue I/Q modulated signal is fed from the digital baseband into the RF ASIC
and through the low pass filter. After the filter the signal is fed to the I/Q 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 GSM1800/1900 signals. The WCDMA branch is selected by biasing the appropriate modulator.
The modulator is followed by two VGA stages. The signal then exits the RF ASIC via a balanced line. The signal
is band pass filtered by a SAW filter before it is fed to WCDMA PA module.
After the PA, the transmitted WCDMA signal is fed through the duplex filter to the single feed antenna switch
and then to the antenna.
WCDMA power control
WCDMA TX power control is performed by the two VGA stages in the RF ASIC.
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.
WCDMA PA module
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) and one bias current input. Bias current is
generated by 5 bit DA (DAC201) converter in RF ASIC N7500. The converter is controlled via RFBus.
In production the PA quiescent current can be tuned to target value by adjusting the bias current of PA output
stage. The bias current is also used as PA on/off controls.
RM-132; RM-133
System Module
Nokia Customer Care
Issue 1
COMPANY CONFIDENTIAL
Page 9 –37
Copyright © 2006 Nokia. All rights reserved.
