Nokia RM-42 Service Manual

Also see for RM-42: Service manual

Contents

The signal is first low pass filtered with filter corner frequency set to approximately 200 kHz. After the filter, thesignal is routed to the GSM modulator.The appropriate routing after the modulator is selected by biasing either EGSM900 (/GSM850) or GSM1800/1900variable 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 PAmodule. In case of EGSM900 (and GSM850), the PA module is preceded by a SAW filter. After the filter, the signalis fed to GSM PA module. Finally the signal is routed via antenna switch to the antenna.Figure 125 GSM transmitterGSM power controlA closed control loop comprise an integrated power detector (in PA module) and an error amplifier. The erroramplifier 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 thenegative input of the error amplifier, where it is compared to the level of the reference signal, TXC. TXC is gotfrom the BB circuitry. The output of the error amplifier is fed to a buffer amplifier, which in turn steers the VGAamplifier.The TXC signal also contains the output power ramp waveform, which is optimized in order to meet the transientspectrum and burst timing requirements. PA is switched on and off by changing the bias currents. As a resultthe 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 variationsduring the data transmission. This presents extra requirement to the linearity of the PA. Therefore the PA is setto a dedicated EDGE mode by setting a specific mode control signal up (Vmode). The bias currents are alsoadjusted in order to improve the linearity.Because of the 8-PSK modulation, the power control loop has to be opened during the data transmission inEDGE mode. Otherwise a part of the envelope variations could be canceled out by control loop and signalinformation contents and spectrum would be deteriorated. Loop is opened with a dedicated TXA-signal viaRFBus. When the power is ramped up, a modulating bit sequence producing a constant envelope waveform isused and the power control loop is closed. Once the wanted power level has been reached, the loop is openedRM-42Nokia Customer Care System ModulePage 9–50 Company Confidential 9241872 (Issue 1)Copyright ©2005 Nokia. All Rights Reserved.