Agc Strategy - Nokia NSE–3 SERIES Service Manual

PAMS
Technical Documentation

AGC strategy

AGC–amplifier is used to maintain output level of the receiver almost
constant. AGC has to be set before each received burst, this is called
pre–monitoring. Receiver is switched on roughly 150 ms before the burst
begins, DSP measures received signal level and adjusts RXC, which con-
trols RX AGC–amplifier or it switches off the LNA with PDATA0 control
line. This pre–monitoring is done in three phases and this sets the settling
times for RX AGC. Pre–monitoring is required because of linear receiver,
received signal must be in full swing, no clipping is allowed and because
DSP doesn't know, what is the level going to be in next burst.
There is at least 60 dB accurate gain control ( continous, analog ) and
one digital step in LNA. It is typically about 30...35 dB.
RSSI must be measured on range –48...–110 dBm. After –48 dBm level
MS reports to base station the same reading.
Because of RSSI–requirements, gain step in LNA is used roughly on –45
dBm RF–level and up to –10 dBm input RF–level accurate AGC is used
to 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 level
by measuring RX IQ–level after all selectivity filtering ( meaning IF–filters,
Σ∆±converter and FIR–filter in DSP). So 50 dB accurate AGC dynamic
range is required. Remaining 10 dB is for gain variations in RX–chain ( for
calibration ). Below –95 dBm RF–levels, output level of the receiver drops
dB by dB. At –95 dBm level output of the receiver gives 50 mVpp
differentially. This is the target value for DSP. Below this it drops down to
ca. 9 mVpp differentially @ –110 dBm RF–level.
This strategy is chosen because we have to roll off the AGC in PLUSSA
early enough, that it won't saturate in selectivity tests. Also we can't start
too 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 is
required to have margin for +/– 200 kHz faded adjacent channel ( ca. 19
dB ) and extra 9 dB for pre–monitoring.
Production calibration is done with two RF–levels, LNA gain step is not
calibrated. Gain changes in the receiver are taken off from the dynamic
range of accurate AGC. Variable gain stage in PLUSSA is designed in a
way, that it is capable of compensating itself, there is good enough mar-
gin in AGC.
Original 11/97
NSE–3
System Module
Page 3 – 55