HC5517 Intersil Corporation, HC5517 Datasheet - Page 14

• Manufacturer Part Number: HC5517
• Description: 3 REN Ringing SLIC For ISDN Modem/TA and WLL
• Manufacturer: Intersil Corporation

Inverting Amplifier (A1)
The pulse metering signal is injected in the -IN1 pin of the
SLIC. This pin is the inverting input of the internal amplifier
(A
cancellation. The components required for pulse metering
are C
signal is AC coupled to prevent a DC offset on the input of
the internal amplifier. The value of C
expression for the voltage at OUT1 is given in Equation 38.
The first term is the gain of the feedback voltage from the
2-wire side and the second term is the gain of the injected
pulse metering signal. The effects of C
negligible and therefore omitted from the analysis.
The injected pulse metering output term of Equation 38 is
shown below in Equation 39 and rearranged to solve for R
in Equation 40.
The ratio of R
gain of the pulse metering signal from 4-wire side to 2-wire
side. The value of R
is selected based on impedance matching requirements.
Cancellation of the Pulse Metering Signal
The transhybrid cancellation technique that is used for the
voice signal is also implemented for pulse metering. The
technique is to drive the transhybrid amplifier with the signal
that is injected on the 4-wire side, then adjust its level to
match the amplitude of the feedback signal, and cancel the
signals at the summing node of an amplifier.
NOTE: The CA741C operational amplifier is used in the application
as a “stand in” for the operational amplifier that is traditionally located
in the CODEC, where transhybrid cancellation is performed.
Referring to Figure 3, V
drive the internal amplifier (A1) which in turn drives the
OUT1 pin of the SLIC. The voltage measured at V
related to the loop impedance as follows:
FIGURE 11. PULSE METERING PHASE SHIFT AMPLIFIER
V
V
R
V
OUT1
OUT1
5
TX
1
) that is used to invert the pulse metering signal for later
=
V
=
6
V
PM
R
TX
and R
8
–
------------ - V
=
injected
R
200
–
L
V
DESIGN
TX
C
C
5
8
6
8
, are shown in Figure 11. The pulse metering
PM
to R
R
------ - V
R
=
R
R
8
9
5
9
V
–
5
G
8
PM
is set equal to one and results in unity
PM
is considered to be a constant since it
-
PM
IN
1
TX
R
------ -
R
8
5
R
------ -
R
R
is the 2-wire feedback used to
73
-
+
8
5
8
=
A
1
1
6
TO EXTERNAL
TRANSHYBRID AMP
should be 10 F. The
OUT
6
1
and C
(EQ. 41)
(EQ. 38)
(EQ. 39)
(EQ. 40)
8
TX
are
is
5
HC5517
For a 600
1, the feedback voltage (V
injected pulse metering signal of the 4-wire side. Note,
depending upon the line impedance characteristics and the
degree of impedance matching, the pulse metering gain may
differ from the voice gain. The pulse metering gain (G
must be accounted for in the transhybrid balance circuit.
The polarity of the signal at OUT1 (Equation 38) is opposite
of V
stage of transhybrid cancellation.
FIGURE 12. CANCELLATION OF THE PULSE METERING SIGNAL
The following equations do not require much discussion.
They are based on inverting amplifier design theory. The
voice path V
ence designators refer to components of Figures 11 and 12.
The first term refers to the signal at OUT1 and the second
term refers to the 4-wire side pulse metering signal. Since
ideal transhybrid cancellation implies V
when a signal is injected on the 4-wire side, V
zero and the resulting equation is shown below.
Rearranging terms of Equation 43 and solving for R
in Equation 44. This is the only value to be calculated for the
transhybrid cancellation. All other values either exist in the
application circuit or have been calculated in previous
sections of this data sheet.
The value of R
set of values:
R
R
R
R
R
G
Substituting the same values into Equation 41 and Equation 42,
it can be shown that the signal at OUT
This result, along with Equation 44 where R
indicates the signal levels into the transhybrid amplifier are
equalized by the amplifier gains and opposite in polarity,
thereby achieving transhybrid balance at V
R
V
0
8
9
L
3
5
TXO
4
PM
=
= 40k
= 40k
= 600
= 8.25k
= 40k
=
PM
R
= 1
8
=
R
------ -
R
allowing the circuit of Figure 12 to perform the final
8
3
–
R
V
---------- -
R
8
TX
termination and a pulse metering gain (G
9
–
------------------------------- -
RX
OUT
V
200
V
R
PM
+
RX
–
4
L
V
----------- -
signal has been omitted for clarity. All refer-
V
---------- -
R
1
(Figure 12) is 12.37k
R
PM
TX
5
G
R
9
9
PM
–
R
R
R
V
----------- -
2
3
4
R
------ -
R
R
+
PM
1
3
5
------ -
R
1
–
5
V
TX
R
------ -
R
–
PM
1
1
3
) is equal to one third the
R
–
-
+
1
CA741C
R
------ -
R
V
1
4
PM
1
is equal to -2/3V
TXO
R
------ -
R
given the following
TXO
3
1
4
V
equals to 2/3R4,
TXO
.
TXO
equals zero
4
(EQ. 44)
is set to
(EQ. 42)
(EQ. 43)
results
PM
PM
PM
) of
)
.