NSB–6
System Module
PAMS Technical Documentation
Page 46
Issue 1 06/2000
E
Nokia Mobile Phones Ltd.
PA
DIR.COUPLER
DETECTOR
ERROR
AMPLIFIER
RF_OUT
RF_IN
TXC
R1
R2
DOMINATING
POLE
K
K
K
K
det
cp
PA
= –R1/R2
C
R
AGC strategy
AGC–amplifier is used to maintain output level of the receiver in certain
range. AGC has to be set before each received burst, this is called pre–
monitoring. Receiver is switched on roughly 280 us before the burst be-
gins, DSP measures received signal level and adjusts AGC–amplifiers via
serial bus from COBBA GJP.
There is 50 dB accurate gain control (10 dB steps) and one larger step
(~30 dB) in LNA. LNA AGC step size depends on channel with certain
amount.
RSSI must be measured accurately on range –48...–110 dBm. After –48
dBm level MS reports to base station the same reading.
Production calibration is done with two RF–levels, LNA gain step is not
calibrated.
AFC function
AFC is used to lock the transceivers clock to frequency of the base sta-
tion. AFC–voltage is generated in COBBA with 11 bit DA–converter.
There is a RC–filter in AFC control line to reduce the noise from the con-
verter. Settling time requirement for the RC–network comes from signal-
ling, how often PSW (pure sine wave) slots occur. They are repeated after
10 frames, meaning that there is PSW in every 46 ms. AFC tracks base
station frequency continously, so transceiver has got a stable frequency,
because changes in VCTCXO–output don’t occur so fast (temperature).
Settling time requirement comes also from the start up–time allowed.
When transceiver is in sleep mode and ”wakes” up to receive mode, there
