User Guide

PAMS

Technical Documentation

NSB–1

System Module

Page 3 – 35

Original 06/98

VCO

CHARGE

PUMP

PHASE

DET.

f_out

Kvco

M = A(P+1) + (N–A)P=

ref

f_out /

freq.

reference

= NP+A

AGC

The purpose of the AGC–amplifier is to maintain a constant output level

from the receiver. The receiver is switched on approximately 150 s before

the burst begins, DSP measures the receive signal level and adjusts the

TXC–DAC (which controls Receive Controlled Gain Amplifier) or it switches

on/off the LNA with the FRAC control line. The Receive Controlled Gain

Amplifier has 57 dB of continuos gain control (37 dB to –20 dB) while the

gain in the LNA is a digital step and is either 15 dB or –24 dB.

The requirement for receive signal level (RSSI) under static conditions is

that the MS shall measure and report to the BS over the range –48 dBm

to –110 dBm. For RF levels above –48 dBm, the MS must report to BS

the same reading, so above this level the AGC is not required. Because

of the RSSI requirements, the gain step in LNA is ”ON” ( FRAC = ”0”) for

receive levels below –43 dBm. This leaves the AGC in PLUSSA to adjust

the gain to desired value (50mVp–p). This is accomplished in DSP by

measuring the receive IQ level after the selectivity filtering (IF–filters,

Σ∆±converter and FIR–filter in DSP). This results in an AGC dynamic

range of 50 dB with the remaining 7 dB for gain variations in RX–chain

(for calibration). For RF levels below –95 dBm, the output level of the re-

ceiver drops dB by dB with a level of 7.1 mVp–p @ –110 dBm for

DCS1900.

This strategy is chosen because it is necessary to roll off the AGC in

PLUSSA early so that the signal is not saturated in selectivity tests but

cannot roll off too early as this will sacrifice the signal to noise ratio thus

requiring a larger AGC dynamic range. The 50 mVp–p target level is set,

because the RX–DAC in COBBA_GJ will saturate at 1.4 Vp–p. This re-