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XRT79L71
PRELIMINARY
181
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
NOTE: For a more in-depth example on how to interface the XRT79L71 to the outside world, please see Appendix A for the
XRT79L71 Evaluation Board Schematic Design.
4.3.1.2
The Automatic Gain Control Block
The AGC (or Automatic Gain Control) Block is the very first sub-lock to receive either a DS3 or E3 signal from
RTIP/RRING input pins. For DS3 Applications, the purpose of the AGC block is two-fold.
To compensate for any flat-loss that the incoming line signal may have experienced as it travels from the
source (e.g., remote) to the destination (e.g., local) terminal.
To function as a part of the ALOS (Analog LOS) Detector
Each of these roles/purposes is briefly discussed below.
4.3.1.2.1
Compensating for Flat Loss
As the name of this sub-block implies, the AGC (Automatic Gain Control) block functions by automatically
adjusting its ampification gain (of the incoming line signal) in order to insure that this amplitude of the incoming
line signal (after being amplified by the AGC block) is within a certain desirable range for optimal internal signal
processing within the remainder of the Receive DS3/E3 LIU Block circuitry.
Therefore, if the incoming DS3 or E3 line signal is of an amplitude that is less than this desired amplitude, then
the AGC block will increase the amplitude of the incoming DS3/E3 signal by providing the appropriate amount
of Gain to this signal.
If the amplitude of the incoming DS3 or E3 signal were to suddenly increase, then the AGC block will seek to
make sure that the amplitude of this internal signal remains within the desirable range by lowering its gain to
the appropriate level.
Conversely, if the amplitude of the incoming DS3 or E3 signal were to suddenly
decrease, then the AGC block will seek to make sure that the amplitude of this internal signal remains within
the desirable range by increasing its gain to the appropriate level.
4.3.1.2.2
Functioning as a Part of the ALOS Detector
If the amplitude of the incoming DS3 signal were to drop to a very low level, then the AGC block will respond by
increasing its gain (in order to drive the internal signal level back to within the desirable range). However, if the
AGC block increases its gain beyond a certain amount, then the Receive DS3/E3 LIU Block will conclude that
the amplitude of the incoming DS3 or E3 signal is simply too low and that it should declare the LOS (Loss of
Signal) defect condition.
FIGURE 84. SCHEMATIC DESIGN, DEPICTING HOW TO INTERFACE THE RECEIVE DS3/E3 LIU BLOCK (OF THE
XRT79L71) TO THE LINE
C1
0.1uF
T3
T3001
1
6
3
4
R4
37.4
R3
37.4
J3
BNC
1
2
U3
XRT79L71
R14
R13
RTIP
RRING