XRT81L27 Datasheet, PDF (27/30 Pages) Exar Corporation – SEVEN CHANNEL E1 LINE INTERFACE UNIT WITH CLOCK RECOVERY

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XRT81L27
SEVEN CHANNEL E1 LINE INTERFACE UNIT WITH CLOCK RECOVERY
REV. 1.1.0
3.2 CAPACITIVE-COUPLING THE RECEIVER TO THE
LINE
Figure 15 presents a recommended method to use
when capacitive-coupling the Receive Section to the
line.
FIGURE 15. CAPACITIVE - COUPLED RECEIVE SECTIONS OF THE XRT81L27 TO THE LINE (FOR BALANCED 120Ω
APPLICATIONS)
RPOS1
RNEG1
RClk1
RLOS1
RTIP1
C1
0.1uF
RRing1
C2
0.1uF
30.1 Ω
30.1 Ω
3.3 THE RECEIVE EQUALIZER BOCK
After a given Channel (within the XRT81L27) has re-
ceived the incoming line signal, via the RTIP_n
(where _n is the channel number) and RRING_n in-
put pins, the first block that this signal will pass
through is the Receive Equalizer block.
As the line signal is transmitted from a given Trans-
mitting terminal, the pulse shapes (at that location)
are basically square. Hence, these pulses consist of a
combination of low and high frequency Fourier com-
ponents. As this line signal travels from the transmit-
ting terminal (via the coaxial cable or twisted pair) to
the receiving terminal, it will be subjected to frequen-
cy-dependent loss. The higher frequency compo-
nents of the signal will be subjected to a greater
amount of attenuation than the lower frequency com-
ponents. If this line signal travels over reasonably
long cable lengths, then the original square shape of
the pulses will be distorted and with inter-symbol in-
terference 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 sub-
jecting the received line signal to frequency-depen-
dent amplification (which attempts to counter the fre-
quency-dependent loss that the line signal has expe-
rienced). By doing this, the Receive Equalizer is at-
tempting to restore the shape of the line signal so that
the received data can be recovered reliably.
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