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XRT79L71
PRELIMINARY
357
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".
5.3.1.2
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 E3 application, the purpose of the AGC block is to compensate for flat-loss that
the incoming line signal may have experienced as it travels from the source (e.g., remote) terminal to the
destination (e.g., local) terminal.
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.
5.3.1.3
The Receive Equalizer Block
As a given pulse within a DS3 or E3 line rate signal travels from its "source" to the "destination" terminal via
coaxial cable, it will experience a "frequency-dependent" loss, in which the "high-frequency" portions of this
signal are more greatly attenuated than are the "lower-frequency" components of the signal. The result of this
"frequency-dependent" loss is manifested by a change in shape of a given pulse within this DS3 or E3 line
signal.
More specifically, one will typically note that (as a given pulse travels along the communication
medium) fast rising and falling edges give way to slower rising and falling edge. Pulses that originally of the
"square-wave" shape (at the output of the "source" terminal) become less "square" and more "rounded" in
shape as they travel along the coaxial cable. If the E3 data-stream travels a sufficiently long distance, then an
entity that is responsible for properly receiving a given incoming E3 signal, will have trouble receiving this
particular signal, due to phenomenon such as ISI (Inter-Symbol Interference).
FIGURE 166. 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