Nokia NSE–3 SERIES Service Manual

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Nokia NSE–3 SERIES Service Manual Manual pdf 61 page image

PAMSTechnical DocumentationNSE–3System ModulePage 3 – 55Original 11/97AGC strategyAGC–amplifier is used to maintain output level of the receiver almostconstant. AGC has to be set before each received burst, this is calledpre–monitoring. Receiver is switched on roughly 150 ms before the burstbegins, DSP measures received signal level and adjusts RXC, which con-trols RX AGC–amplifier or it switches off the LNA with PDATA0 controlline. This pre–monitoring is done in three phases and this sets the settlingtimes for RX AGC. Pre–monitoring is required because of linear receiver,received signal must be in full swing, no clipping is allowed and becauseDSP doesn’t know, what is the level going to be in next burst.There is at least 60 dB accurate gain control ( continous, analog ) andone digital step in LNA. It is typically about 30...35 dB.RSSI must be measured on range –48...–110 dBm. After –48 dBm levelMS reports to base station the same reading.Because of RSSI–requirements, gain step in LNA is used roughly on –45dBm RF–level and up to –10 dBm input RF–level accurate AGC is usedto set RX output level. LNA is ON below –45 dBm. from –45 dBm down to–95 dBm this accurate AGC in PLUSSA is used to adjust the gain to de-sired value. RSSI–function is in DSP, but it works out received signal levelby measuring RX IQ–level after all selectivity filtering ( meaning IF–filters,Σ∆±converter and FIR–filter in DSP). So 50 dB accurate AGC dynamicrange is required. Remaining 10 dB is for gain variations in RX–chain ( forcalibration ). Below –95 dBm RF–levels, output level of the receiver dropsdB by dB. At –95 dBm level output of the receiver gives 50 mVppdifferentially. This is the target value for DSP. Below this it drops down toca. 9 mVpp differentially @ –110 dBm RF–level.This strategy is chosen because we have to roll off the AGC in PLUSSAearly enough, that it won’t saturate in selectivity tests. Also we can’t starttoo early, then we will sacrifice the signal to noise ratio and it would re-quire more accurate AGC dynamic range. 50 mVpp target level is set,because RX–DAC will saturate at 1.4 Vpp. This over 28 dB headroom isrequired to have margin for +/– 200 kHz faded adjacent channel ( ca. 19dB ) and extra 9 dB for pre–monitoring.Production calibration is done with two RF–levels, LNA gain step is notcalibrated. Gain changes in the receiver are taken off from the dynamicrange of accurate AGC. Variable gain stage in PLUSSA is designed in away, that it is capable of compensating itself, there is good enough mar-gin in AGC.