63
where:
V
TR
= Is the AC metallic voltage between tip and ring,
including the voltage drop across the fuse resistors R
F
.
V
TX
= Is the AC metallic voltage. Either at the ground
referenced 4-wire side or the SLIC tip and ring terminals.
I
M
= Is the AC metallic current.
R
F
= Is a fuse resistor.
Z
T
= Is used to set the SLIC’s 2-wire impedance.
V
RX
= Is the analog ground referenced receive signal.
Z
RX
= Is used to set the 4-wire to 2-wire gain.
E
G
= Is the AC open circuit voltage.
Z
L
= Is the line impedance.
(AC) 2-Wire Impedance
The AC 2-wire impedance (Z
TR
) is the impedance looking
into the SLIC, including the fuse resistors, and is calculated
as follows:
Let V
RX
= 0. Then from Equation 10
Z
TR
is defined as:
V
M
Substituting in Equation 9 for V
TR
Substituting in Equation 12 for V
TX
Therefore
Equation 16 can now be used to match the SLIC’s
impedance to any known line impedance (Z
TR
).
Example:
Calculate Z
T
to make Z
TR
= 600
in series with 2.16
μ
F.
R
F
= 20
:
-----------------------------------------
+
Z
T
= 560k
in series with 2.16nF.
(AC) 2-Wire to 4-Wire Gain
The 2-wire to 4-wire gain is equal to V
TX
/ V
TR
.
From Equations 9 and 10 with V
RX
= 0:
V
TR
(AC) 4-Wire to 2-Wire Gain
The 4-wire to 2-wire gain is equal to V
TR
/V
RX
.
From Equations 9, 10 and 11 with E
G
= 0:
For applications where the 2-wire impedance (Z
TR
,
V
RX
Equation 15) is chosen to equal the line impedance (Z
L
), the
expression for A
4-2
simplifies to:
Z
RX
(AC) 4-Wire to 4-Wire Gain
The 4-wire to 4-wire gain is equal to V
TX
/V
RX
.
From Equations 9, 10 and 11 with E
G
= 0:
V
TX
Z
T
I
------------
=
(EQ. 12)
Z
TR
-----------
=
(EQ. 13)
Z
TR
V
M
----------
2R
-----------------------
I
M
M
+
=
(EQ. 14)
Z
TR
Z
------------
2R
F
+
=
(EQ. 15)
Z
T
1000
Z
TR
2R
F
–
(
)
=
(EQ. 16)
Z
T
1000
600
j
ω
2.16
10
6
–
2
20
–
=
A
2
4
–
-----------
Z
1000
T
F
--------------------------+
=
=
(EQ. 17)
A
4
2
–
-----------
Z
RX
-----------
–
Z
------------
2R
F
Z
L
+
+
--------------------------------------------
=
=
(EQ. 18)
A
4
2
–
-----------
–
1
2
--
=
(EQ. 19)
A
4
4
–
V
RX
-----------
Z
RX
-----------
–
Z
2R
+
------------
2R
F
Z
L
+
+
--------------------------------------------
=
=
(EQ. 20)
V
TX
RSN
TIP
RING
I
M
1000
Z
TR
V
TR
E
G
-
V
TX
I
M
V
TX
Z
RX
1
HC5513
R
F
R
F
A = 4
+
-
+
-
+
+
-
Z
T
+
-
V
RX
+
-
A = 250
A = 250
I
M
Z
L
FIGURE 16. SIMPLIFIED AC TRANSMISSION CIRCUIT
HC5513