HC5513

FN3963 Rev.12.00

Page 11 of 20

August 2003

Transhybrid Circuit

The purpose of the transhybrid circuit is to remove the receive

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

circuit. The input signal will be subtracted from the output

equation:

Therefore:

Example:

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

RSN

TIP

RING

1000

1

HC5513

A = 4

+

-

+

-

+

-

+

-

+

-

+

-

A = 250

A = 250

FIGURE 16. SIMPLIFIED AC TRANSMISSION CIRCUIT

-----------

-----------

+

0

=

(EQ. 21)

R

-----------



=

(EQ. 22)

-----------



1000

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

++

+

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



=

(EQ. 23)

HC5513

RSN

CODEC/

FILTER

+

-

+

-

+

-

FIGURE 17. TRANSHYBRID CIRCUIT