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 analogue 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 switch matrix. It routes the
output GSM signal to the single feed antenna switch and then to the antenna.
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.
Reference oscillator
A 38.4MHz VCTCXO is used as a reference oscillator for the frequency synthesizers.
The frequency of the reference oscillator is locked into the frequency of the base station with the help of an
AFC voltage, which is generated in BB by DSP and converted by dedicated DAC.
RM-132; RM-133
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COMPANY CONFIDENTIAL
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