Data Sheet
April 2000
Lucent Technologies Inc.
41
L8560 Low-Power SLIC with Ringing
Applications
(continued)
Design Examples
(continued)
5-6400.b (F)
Figure 39. Interface Circuit Using First-Generation Codec (Blocking Capacitors Not Shown)
0.1
μ
F
R
TGS
V
TX
R
TGP
= 4.32 k
T
XI
V
ITR
R
T6
R
x
R
T3
R
HB
CODEC
OUTPUT
DRIVE
AMP
CODEC
OP AMP
–
+
19.2
C
N
R
N1
R
N2
R
GP
R
RCV
RCVN
RCVP
–I
T/R
207.36
C
G
Transmit Gain
Transmit gain will be specified as a gain from T/R to
PCM, T
X
(dB). Since PCM is referenced to 600
and
assumed to be 0 dB, and in the case of T/R being refer-
enced to some complex impedance other than 600
resistive, the effects of the impedance transformation
must be taken into account.
Again, specified complex termination impedance at T/R
is of the form:
5-6396(F)
First, calculate the equivalent resistance of this net-
work at the midband frequency of 1000 Hz.
R
EQ
=
Using R
EQ
, calculate the desired transmit gain, taking
into account the impedance transformation:
T
X
(dB) = T
X (specified[dB])
+ 20 log
T
X (specified[dB])
is the specified transmit gain. 600
is the
impedance at the PCM and R
EQ
is the impedance at
Tip and ring. 20 log
represents the power
loss/gain due to the impedance transformation.
Note in the case of a 600
pure resistive termination
at T/R 20 log
= 20 log
= 0.
Thus, there is no power loss/gain due to impedance
transformation and T
X
(dB) = T
X (specified[dB])
.
Finally, convert T
X
(dB) to a ratio, g
TX
:
T
X
(dB) = 20 log g
TX
The ratio of R
X
/R
T6
is used to set the transmit gain:
= g
TX
with a quad Lucent codec such as T7504:
R
X
< 200 k
R
2
C
R
1
------------f
(
)
2
C
12
R
1
R
22
1
2
π
f
(
+
R
1
R
2
+
+
)
2
R
22
C
12
2
R
22
C
1
2
π
f
(
1
)
2
R
22
C
12
+
-----------------f
2
+
R
EQ
600
R
EQ
600
R
EQ
600
600
600
R
T6
----------
19.2
207.36
1
M
----