XRT7300 datasheet(30/55 Pages) EXAR

30 / 55 page

XRT7300

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E3/DS3/STS-1 LINE INTERFACE UNIT

REV. 1.1.1

interface the Receive Section of the XRT7300 to the

line as shown in Figure 16 or Figure 17.

3.2

THE RECEIVE EQUALIZER BLOCK

After the XRT7300 has received the incoming line

signal via the RTIP and RRING input pins, the first

block that this signal passes through is the AGC cir-

cuit followed by the Receive Equalizer.

As the line signal is transmitted from a given transmit-

ting terminal, the pulse shapes at that location are ba-

sically square. Hence, these pulses consist of a com-

bination of “Low” and “High” frequency Fourier com-

ponents. As this line signal travels from the transmit-

ting terminal via the coaxial cable to the receiving

terminal, it is subjected to frequency-dependent loss.

In other words, the higher-frequency components of

the signal is subjected to a greater amount of attenu-

ation than the lower-frequency components. If this

line signal travels over reasonably long cable lengths

(e.g., greater than 450 feet), then the shape of the

pulses (which were originally square) is distorted and

inter-symbol interference increases.

The purpose of the Receive Equalizer is to equalize

the distortion of the incoming signal due to cable loss.

The Receive Equalizer accomplishes this by subject-

ing the received line signal to frequency-dependent

amplification (which attempts to counter the frequen-

cy dependent loss that the line signal has experi-

enced) and to restore the shape of the line signal so

FIGURE 16. RECOMMENDED SCHEMATIC FOR INTERFACING THE RECEIVE SECTION OF THE XRT7300 TO THE LINE

(TRANSFORMER-COUPLING)

BNC

1:1

37.5

Ω

37.5

Ω

RTIP

RRING

0.01uF

RPOS

RNEG

RCLK1

RLOS

RLOL

RxLOS

RxLOL

RxPOS

RxNEG

RxLineClk

FIGURE 17. RECOMMENDED SCHEMATIC FOR INTERFACING THE RECEIVE SECTION OF THE XRT7300 TO THE LINE

(CAPACITIVE-COUPLING)

Ω

RTIP

RRING

0.01uF

Receive Line Signal

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