Nokia NSB-6 Series Technical Documentation Manual

Also see for NSB-6 Series: Disassembly & troubleshooting instructions

Nokia NSB-6 Series Technical Documentation Manual Manual pdf 45 page image

NSB–6System ModulePAMS Technical DocumentationPage 45Issue 1 06/2000 Nokia Mobile Phones Ltd.TransmitterTransmitter chain consists of final frequency IQ–modulator, dualbandpower amplifier and a power control loop.I– and Q–signals are generated by baseband also in COBBA–ASIC. Afterpost filtering (RC–network) they go into IQ–modulator in HAGAR. LO–sig-nal for modulator is generated by VCO and is divided by 2 or by 4 de-pending on system mode, EGSM/GSM1900. After modulator the TX–sig-nal is amplified and buffered. There are separate outputs for both EGSMand GSM1900. HAGAR TX output level is 5 dBm minimum.Next TX signals are converted to single ended by discrete baluns. EGSMand GSM1900 branches are compined at a diplexer. In EGSM branchthere is a SAW filter before diplexer to attenuate unwanted signals andwideband noise from the Hagar IC.The final amplification is realized with dualband power amplifier. It hastwo 50 ohm inputs and two 50 ohm outputs. There are also separate gaincontrols, which is controlled with a power control loop in HAGAR. PA isable to produce over 2 W (4 dBm input level) in EGSM band and over 1W (4 dBm input level) in GSM1900 band into 50 ohm output. Gain controlrange is over 35 dB to get desired power levels and power ramping upand down.Harmonics generated by the nonlinear PA are filtered out with the diplexerinside the antenna switch–module.Power control circuitry consists of discrete power detector (common forEGSM and GSM1900) and error amplifier in HAGAR. There is a direc-tional coupler connected between PA output and antenna switch. It is adualband type and has input and outputs for both systems. Dir. couplertakes a sample from the forward going power with certain ratio. This sig-nal is rectified in a schottky–diode and it produces a DC–signal after filter-ing.This detected voltage is compared in the error–amplifier in HAGAR toTXC–voltage, which is generated by DA–converter in COBBA. TXC has araised cosine form (cos 4 – function), which reduces switching transients,when pulsing power up and down. Because dynamic range of the detec-tor is not wide enough to control the power (actually RF output voltage)over the whole range, there is a control named TXP to work under de-tected levels. Burst is enabled and set to rise with TXP until the outputlevel is high enough, that feedback loop works. Loop controls the outputvia the control pin in PA to the desired output level and burst has the wa-veform of TXC–ramps. Because feedback loops could be unstable, thisloop is compensated with a dominating pole. This pole decreases gain onhigher frequencies to get phase margins high enough. Power control loopin HAGAR has two outputs, one for both freq. bands.