User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 85
Issue 1 07/99
AGC
The purpose of the AGC-amplifier is to maintain a constant output level
from the receiver. To accomplish this, pre-monitoring is used. This
pre-monitoring is done in three phases and determines the settling time
for the RX AGC. The receiver is switched on approximately 150 s before
the burst begins and DSP measures the received signal level. The DSP
then adjusts the AGC-DAC in accordance with the measured signal level
and/or switches on/off the LNA with the front–end amplifier control line
(FRACTRL). The AGC amplifier has a 57 dB continuos controllable gain
(–17 dB to 40 dB) while the gain control of the LNA has two steps. That is
the gain in the LNA is either –16 dB or 15 dB.
The requirement for the received signal level under static conditions is that
the MS shall measure and report to the BS over the range -110 dBm to
-48 dBm. For RF levels above -48 dBm, the MS must report the same
signal level to the BS. Because of those requirements, the LNA is turned
”ON” (FRACTRL = ”0”) for received levels below -48 dBm. This leaves the
AGC in the SUMMA to adjust the gain to desired output value (56mVpp).
This is accomplished in DSP by measuring the received IQ level after the
selectivity filtering (IF-filters, Σ∆±converter and FIR-filter in DSP). For RF
levels below -94 dBm, the output level of the receiver drops dB by dB with
a level of 9 mVp-p @-110 dBm for GSM900 and 7.1 mVp-p @ -110 dBm
for GSM1800.
This strategy is chosen as a compromise between avoiding saturation
when strong interfering signals are present and not sacrificing the signal to
noise ratio. The 56 mVpp target level is set, because the RX-DAC in the
COBBA in baseband will saturate at 1.4 Vpp. This results in a headroom
of 28 dB which is sufficient for the +/- 200 kHz faded adjacent channel
(approximately 19 dB) and an extra 9 dB for pre-monitoring.
AFC function
In order to maintain the clock of the transceiver, i.e. the 13 MHz VCTCXO,
locked to the frequency of the base station an AFC (Automatic Frequency
Control) is used. The AFC reduces variations in the frequency of the
VCTCXO due to temperature drift. The AFC voltage is generated by
baseband with an 11 bit DAC in the COBBA. There is a RC-filter in the
AFC control line to reduce the noise from the converter.
The AFC voltage is obtained by means of Pure Sine Wave (PSW) slots,
which are a part of the signaling from the base station. The PSW slots are
repeated every 10 frames, meaning that there is a slot in every 46 ms.
Since changes in the VCTCXO -output frequency due to temperature
variations are relatively slow compared to the 46 ms, the transceiver has a
stable clock frequency.