Nokia NSB-7 Series, Руководство по обслуживанию

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PAMS Technical Documentation System Module
NSB-7
Issue 1 06/00 ã Nokia Mobile Phones Ltd. Page 57
decreases gain at high frequencies to insure phase margins high enough.
Figure 17: Power Control Loop Diagram
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
µ s before the burst begins. DSP measures received signal level and
adjusts AGC amplifier via serial bus from COBBA GJP.
There is a 50 dB accurate gain control (10 dB steps) and one larger step (~30 dB) in LNA.
LNA AGC gain step size depends on a channel with some amount.
RSSI must be measured accurately on range of -48...-110 dBm. Above -48 dBm level MS
reports to base station the same reading.
Production calibration is done by two RF levels. The LNA gain step is not calibrated.
AFC function
AFC is used to lock the transceivers clock to the frequency of base station. AFC voltage is
generated by COBBA asic with a 11 bit DA converter. In the AFC control line a RC filter is
used to reduce noise coming from the converter. Settling time requirement for the RC
network comes from signalling, i.e. how often PSW (Pure Sine Wave) slots occur. The
PSW is repeated in every 10th frame. It means there is a PSW every 46 ms. The AFC
tracks base station frequency continuously. This way the transceiver gets a stable fre-
quency. Temperature does not affect to VCTCXO frequency very fast.
Settling time requirement comes also from the start-up time allowed. When the trans-
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