Nokia RM-42 Service Manual page 468

RM-42
Nokia Customer Care System Module
The signal is first low pass filtered with filter corner frequency set to approximately 200 kHz. After the filter, the
signal is routed to the GSM modulator.
The appropriate routing after the modulator is selected by biasing either EGSM900 (/GSM850) or GSM1800/1900
variable gain amplifier. The amplifier gives 40 dB of power control dynamic range.
After the VGA stage the signal exits N7501. In case of GSM1800/1900 the signal goes directly to the GSM PA
module. In case of EGSM900 (and GSM850), the PA module is preceded by a SAW filter. After the filter, the signal
is fed to GSM PA module. Finally the signal is routed via antenna switch to the antenna.
Figure 125 GSM transmitter
GSM power control
A closed control loop comprise an integrated power detector (in PA module) and an error amplifier. The error
amplifier resides in N7501, and it controls the transmitter power of GSM.
Detector output from the PA gives a DC level proportional to the output power. The DC voltage is fed to the
negative input of the error amplifier, where it is compared to the level of the reference signal, TXC. TXC is got
from the BB circuitry. The output of the error amplifier is fed to a buffer amplifier, which in turn steers the VGA
amplifier.
The TXC signal also contains the output power ramp waveform, which is optimized in order to meet the transient
spectrum and burst timing requirements. PA is switched on and off by changing the bias currents. As a result
the output power ramping and final power level of the transmitter are set in a controlled manner.
During EDGE operation 8-PSK modulation is utilized. In the 8-PSK modulation, there are envelope variations
during the data transmission. This presents extra requirement to the linearity of the PA. Therefore the PA is set
to a dedicated EDGE mode by setting a specific mode control signal up (Vmode). The bias currents are also
adjusted in order to improve the linearity.
Because of the 8-PSK modulation, the power control loop has to be opened during the data transmission in
EDGE mode. Otherwise a part of the envelope variations could be canceled out by control loop and signal
information contents and spectrum would be deteriorated. Loop is opened with a dedicated TXA-signal via
RFBus. When the power is ramped up, a modulating bit sequence producing a constant envelope waveform is
used and the power control loop is closed. Once the wanted power level has been reached, the loop is opened
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