Wcdma Transmitter; Gsm Transmitter; Frequency Synthesizers - Nokia n80 User Manual

RM-91; RM-92
Nokia Customer Care System Module
The RF ASICs contain a BB frequency low pass filter, which is tunable 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 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.
PA DCDC converter
The DCDC converter is controlled by 5 bit DA (DAC101) converter in RF ASIC N7500. The converter is controlled
via RFBus. The DCDC converter limits the lowest supply voltage to 1.3 V. At the highest power levels, the DCDC
converter output settles nominally to 3.1 V.

GSM transmitter

An analog IQ modulated signal is fed to the RF ASIC from Digital BB. It is first low pass filtered then routed to
the GSM modulator. The appropriate routing after the modulator is selected by biasing either GSM850/
EGSM900 or GSM1800/1900 variable gain amplifier.
After the VGA stage, the signal exits the RF ASIC. In case of GSM1800/1900, the signal goes directly to the GSM
PA module. In case of GSM850/EGSM900, the PA module is preceded by a switchable SAW filter. After the filter,
the signal is fed to GSM TX front-end module (TXFEM), which also contains the antenna switch.
GSM PA module
The TXFEM module contains two separate amplifier chains, one for GSM850/EGSM900 and another for
GSM1800/1900. Both amplifiers have a battery supply connection and two bias current inputs.

Frequency synthesizers

There are two separate synthesizers for WCDMA Rx and WCDMA Tx / GSM Tx / Rx. This allows the duplex
separation to be set according the network settings in the WCDMA mode.
The VCOs are locked by PLLs into a reference oscillator, VCTCXO (Voltage Controlled Temperature Compensated
Crystal Oscillator).
The PLLs are located in RF ASICs and controlled via RFBus.
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