HC5513 datasheet(11/20 Pages) RENESAS

11 / 20 page

HC5513

FN3963 Rev.12.00

Page 11 of 20

August 2003

Transhybrid Circuit

The purpose of the transhybrid circuit is to remove the receive

signal (VRX) from the transmit signal (VTX), thereby preventing

an echo on the transmit side. This is accomplished by using an

external op amp (usually part of the CODEC) and by the

inversion of the signal from the 4-wire receive port (RSN) to the

4-wire transmit port (VTX). Figure 17 shows the transhybrid

circuit. The input signal will be subtracted from the output

signal if I1 equals I2. Node analysis yields the following

equation:

The value of ZB is then:

Where VRX/VTX equals 1/ A4-4.

Therefore:

Example:

Given: RTX = 20k, ZRX = 280k, ZT = 562k(standard

value), RF = 20 and ZL= 600,

The value of ZB = 18.7k.

Supervisory Functions

The loop current, ground key and the ring trip detector outputs

are multiplexed to a single logic output pin called DET. See

Table 1 to determine the active detector for a given logic input.

For further discussion of the logic circuitry see section titled

“Digital Logic Inputs”.

Before proceeding with an explanation of the loop current

detector, ground key detector and later the longitudinal

impedance, it is important to understand the difference between

a “metallic” and “longitudinal” loop currents. Figure 18 illustrates

3 different types of loop current encountered.

Case 1 illustrates the metallic loop current. The definition of a

metallic loop current is when equal currents flow out of tip and

into ring. Loop current is a metallic current.

Cases 2 and 3 illustrate the longitudinal loop current. The

definition of a longitudinal loop current is a common mode

VTX

RSN

TIP

RING

ZTR

VTR

VTX

1000

VTX

ZRX

HC5513

A = 4

VRX

A = 250

FIGURE 16. SIMPLIFIED AC TRANSMISSION CIRCUIT

VTX

RTX

-----------

VRX

-----------

(EQ. 21)

– TX

VRX

VTX

-----------



(EQ. 22)

RTX

ZRX

-----------



1000

-------------

2RF ZL

ZL 2RF

--------------------------------------------



(EQ. 23)

HC5513

VTX

RSN

RTX

RFB

CODEC/

FILTER

VTX

ZRX

VRX

FIGURE 17. TRANSHYBRID CIRCUIT

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