User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 82
Issue 1 07/99
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.
The output of the upconverter is single–ended and requires an external
matching.
The next stage is the TX interstage filter, which attenuates unwanted
frequency components from the upconverter further. These unwanted
component mainly originates from LO-leakage and insufficient
suppression of the image frequency in the upconversion. The interstage
filter attenuates wideband noise too. The filter is a bandpass SAW-filter.
Between the interstage filter and the GSM900 PA an attenuator is placed.
The attenuator ensures both stability of the GSM900 PA because of
konstant 50 on the PA input and the right input level.
After the attenuator, the TX-signal is fed to the input of the GSM900 PA,
which is a MMIC consisting of three amplifier stages and an interstage
matching. It has a 50 input and output impedance. The gain control is
integrated in the PA and is controlled with a power control loop circuit. The
PA has more than 35 dB power gain and the maximum output power is
approx. 35 dBm at an input level of 0 dBm. The gain control range is over
35 dB to ensure the desired power levels and power ramping up and
down. The harmonics generated by the nonlinear PA (class AB) are
filtered out with the duplexer.
After the duplexer the signal is fed to the diplexer. There is a directional
coupler connected between the PA output and the duplex filter input to
provide feedback for the power loop.