Nokia Mobile Phones NSE-5 Series Service Manual page 72

NSE–5
System Module
Transmitter
The transmitter consists of an IQ-modulator that is common for the
GSM900 and the GSM1800 chain, two image rejection upconversion
mixers, two power amplifiers and a power control loop.
Common transmitter part
The I- and Q-signals are generated by the COBBA in baseband. After the
post filtering (RC-network) they are fed into the IQ-modulator in the
SUMMA.
GSM900 transmitter
The IQ–modulator generates a modulated TX IF-signal centered at 116
MHz, which is the VHF-synthesizer output divided by four. The
TX-amplifier in the SUMMA has two selectable gain levels. The output,
which is balanced, is set to maximum via a control register in the SUMMA.
After the SUMMA there is a bandpass LC-filter for reduction of noise and
harmonics before the signal is upconverted to the final TX-frequency. Both
the input and output of the bandpass LC–filter are balanced. The
upconversion mixer, which is located in the CRFU3, is a so–called image
rejection mixer. It is able to attenuate unwanted frequency components in
the upconverter output. The mixer type is a double balanced Gilbert cell.
The phase shifters required for image rejection are also integrated. The
local oscillator signal needed for the upconversion, is generated by the
UHF-synthesizer, but buffers for the mixer are integrated in the CRFU3.
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Figure 18. Transmitter Block Diagram
PAMS
Technical Documentation
Issue 1 07/99