XRT59L921 Datasheet, PDF (26/32 Pages) Exar Corporation – TWENTY-ONE CHANNEL E1 LINE INTERFACE UNIT

XRT59L921
TWENTY-ONE CHANNEL E1 LINE INTERFACE UNIT
REV. 1.2.1
2.3 The Receive Equalizer Bock
After a given Channel (within the XRT59L921) has received the incoming line signal, via the RTIP_n and
RRing_n input pins, the first block that this signal will pass through is the Receive Equalizer block.
As the line signal is transmitted from a given Transmitting terminal, the pulse shapes (at that location) are
basically square. Hence, these pulses consist of a combination of low and high frequency Fourier components.
As this line signal travels from the transmitting terminal (via the coaxial cable or twisted pair) to the receiving
terminal, it will be subjected to frequency-dependent loss. In other words, the higher frequency components of
the signal will be subjected to a greater amount of attenuation than the lower frequency components. If this line
signal travels over reasonably long cable lengths, then the shape of the pulses (which were originally square)
will be distorted and with inter-symbol interference increases.
The purpose of this block is to equalize the incoming distorted signal, due to cable loss. In essence, the
Receive Equalizer block accomplishes this by subjecting the received line signal to frequency-dependent
amplification (which attempts to counter the frequency-dependent loss that the line signal has experienced). By
doing this, the Receive Equalizer is attempting to restore the shape of the line signal so that the received data
can be recovered reliably.
2.4 The Peak Detector and Slicer Block
After the incoming line signal has passed through the Receive Equalizer block, it will next be routed to the
Slicer block. The purpose of the Slicer block is to quantify a given bit-period (or symbol) within the incoming
line signal as either a “1” or a “0”.
2.5 The LOS Detector block
The LOS Detector block, within each channel (of the XRT59L921) was specifically designed to comply with the
LOS Declaration/Clearance requirements per ITU-T G.775. As a consequence, the channel will declare a LOS
Condition, (by driving the RxLOS output pin “High”) if the received line signal amplitude drops to –20dB or
below. Further, the channel will clear the LOS Condition if the signal amplitude rises back up to –15dB or
above. Figure 13 presents an illustration that depicts the signal levels at which a given channel (within the
XRT59L921) will assert and clear LOS.
FIGURE 13. ILLUSTRATION OF THE SIGNAL LEVELS THAT THE RECEIVER SECTIONS (WITHIN XRT59L921) WILL
DECLARE AND CLEAR LOS
0dB
-6dB
-9dB
LOS Signal Must be Cleared
Minimum Cable Loss for E1
LOS Signal Must be Cleared or Declared
-35dB
LOS Signal Must be Declared
Timing Requirements associated with Declaring and Clearing the LOS Indicator. The XRT59L921 was
designed to meet the ITU-T G.775 specification timing requirements for declaring and clearing the LOS
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