Nokia Mobile Phones NSE-5 Series Service Manual page 73

PAMS
Technical Documentation
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
constant 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.
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.
Issue 1 07/99
NSE–5
System Module
Page 2 – 59