Nokia NSE–3 SERIES Service Manual

Contents

Nokia NSE–3 SERIES Service Manual Manual pdf 59 page image

PAMSTechnical DocumentationNSE–3System ModulePage 3 – 53Original 11/97TransmitterTransmitter chain consists of IQ–modulator, upconversion mixer, poweramplifier and there is 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 PLUSSA. Itgenerates modulated TX IF–frequency, which is VHF–synthesizer outputdivided by two, meaning 116 MHz. There is also an AGC–amplifier inPLUSSA, but it is not used in GSM. Output is set to maximum with a 5–bitmessage in control register. AGC–amplifier is used in other digital sys-tems, because PLUSSA is a core IC. After PLUSSA signal is attenuatedand filtered for upconversion into final TX–frequency in CRFU_1a. Up-conversion mixer in CRFU_1a is a so called image reject mixer. It is ableto attenuate unwanted sideband in the upconverter output. Mixer itself isa double balanced Gilbert cell. Phase shifters required for image rejectionare also integrated. Local signal needed in upconversion is generated bythe UHF–synthesizer, but buffers for the mixer are integrated intoCRFU_1a. Output of the upconverter is buffered and matching networkmakes a single ended 50 ohm impedance.Next stage is TX interstage filter, which attenuates unwanted signals fromthe upconverter, mainly LO–leakage and image frequency from the up-converter. Also it attenuates wideband noise. This bandpass filter is aSAW–filter.After TX SAW–filter, there is a discrete transistor stage. Function of thisblock is to reduce the AM–content. This feature is realized with saturatedoperation of the V640 transistor. Typical input level into this amplifier ishigher than output level.The final amplication is realized with third IC, power amplifier is a MMIC.It has got a 50 ohm input, output requires an external matching network.MMIC contains three amplifier stages and interstage matchings. Alsothere is a gain control, which is controlled with a power control loop. PAhas got over 35 dB power gain and it is able to produce 2.5 W into outputwith 0 dBm input level. Gain control range is over 35 dB to get desiredpower levels and power ramping up and down.Harmonics generated by the nonlinear PA ( class AB ) are filtered out withthe matching network and lowpass/bandstop filtering in the duplexer.Bandstop is required because of wideband noise located on RX–band.Power control circuitry consists of power detector in the PA output and er-ror amplifier in PLUSSA. There is a directional coupler connected be-tween PA output and duplex filter. It takes a sample from the forward go-ing power with certain ratio. This signal is rectified in a schottky–diodeand it produces a DC–signal signal after filtering. This peak–detector islinear on absolute scale, except it saturates on very low and high powerlevels – it produces a S–shape curve.