User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 83
Issue 1 07/99
GSM1800 Transmitter
The IQ–modulator generates a modulated TX IF-signal centered at 232
MHz, which is the VHF-synthesizer output divided by two. The
TX-amplifier in the SUMMA has two selectable gain levels. The output
(single-ended) is set to maximum via a control register in the SUMMA.
After the SUMMA there is a SAW filter for reduction of noise and
harmonics before the signal is fed for upconversion into the final
TX-frequency in the CRFU3. The input of the SAW filter is single ended
but the output is balanced. The upconversion mixer for GSM1800 is an
image rejection mixer as well as the one for GSM900. The local oscillator
signal needed in the upconversion is generated by the UHF-synthesizer.
Buffers for the mixer are integrated into the CRFU3. The output of the
upconverter is single ended and requires external matching to 50
impedance.
Then the GSM1800 TX signal passes through the first attenuator in the
GSM1800 TX chain. This attenuator ensures the right input level to the
buffer, also called pre–amplifier, which will be mentioned later.
The next stage is the first TX interstage filter, which attenuates unwanted
frequency components from the upconverter. 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.
To ensure enough power gain in the GSM1800 TX chain the TX signal
then passes through the buffer (pre amplifier). The buffer is driven into
saturation to compensate for variations in CRFU3 output level and ripple
in the first TX interstage filter and to ensure constant input level at the
GSM1800 PA.
The next stage is the second TX interstage filter, which attenuates
unwanted frequency components from the buffer. The interstage filter also
attenuates wideband noise. Both interstage filters is the same type of
bandpass SAW-filter.
Between the second interstage filter and the GSM1800 PA the second
attenuator is placed. The attenuator ensures both stability of the PA
because of konstant 50 on the PA input and the right input level.
After the second attenuator in the GSM1800 TX chain, the TX–signal is
fed into the input of the GSM1800 PA. The GSM1800 PA contains three
amplifier stages, interstage, input and output matchings. The PA has more
than 33 dB power gain and the maximum output power is approx. 33 dBm
at an input level of 0 dBm. The gain control range is over 35 dB to get the
desired power levels and power ramping up and down.
The GSM1800 transmitter has no duplexer, but a TX/RX switch instead.
This is due to space limitations.