HC5517_00 Datasheet, PDF (14/19 Pages) Intersil Corporation – 3 REN Ringing SLIC For ISDN Modem/TA and WLL

HC5517
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
(A₁) that is used to invert the pulse metering signal for later
cancellation. The components required for pulse metering
are C₆ and R₅, are shown in Figure 11. The pulse metering
signal is AC coupled to prevent a DC offset on the input of
the internal amplifier. The value of C₆ should be 10µF. The
expression for the voltage at OUT1 is given in Equation 38
V_PM
V_TX
C₆ R₅
C₈ R₉
TO EXTERNAL
TRANSHYBRID AMP
R₈
OUT₁
-IN₁
- ^A1
+
FIGURE 11. PULSE METERING PHASE SHIFT AMPLIFIER DESIGN
.
V_OUT1 = –V_{T X} • ^RR-----⁸⁹- – V_PM • ^RR-----⁸⁵-
(EQ. 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 C₆ and C₈ are
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 R₅
in Equation 40.
V_OUT1(injected) = V_PM • ^RR-----⁸⁵- = 1
(EQ. 39)
R₅ = R₈
(EQ. 40)
The ratio of R₈ to R₅ is set equal to one and results in unity
gain of the pulse metering signal from 4-wire side to 2-wire
side. The value of R₈ is considered to be a constant since it
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_TX is the 2-wire feedback used to
drive the internal amplifier (A1) which in turn drives the
OUT1 pin of the SLIC. The voltage measured at V_TX is
related to the loop impedance as follows:
V_TX = ^–---_R-²---⁰-_L--⁰-- • V_PM • G_PM
(EQ. 41)
14
For a 600Ω termination and a pulse metering gain (G_PM) of
1, the feedback voltage (V_TX) is equal to one third the
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_PM)
must be accounted for in the transhybrid balance circuit.
The polarity of the signal at OUT1 (Equation 38) is opposite
of V_PM allowing the circuit of Figure 12 to perform the final
stage of transhybrid cancellation.
V_RX
R_{2 R3}
R₁
OUT₁
V_{PM R4}
-
+
V_TXO
CA741C
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_RX signal has been omitted for clarity. All reference
designators refer to components of Figures 11 and 12.
V_TXO
=
–R₈
•

–

V_{---R--T--9--X-}
–
-^V--_R--^P---₅-^M--_{}
•
RR_-----13-
–

V

P
M
•
RR_{-----14-}
(EQ. 42)
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_TXO equals zero
when a signal is injected on the 4-wire side, V_TXO is set to
zero and the resulting equation is shown below.
0
=
R₈
•



V_-----T---X--
R₉
+ ^V---_R--^P---₅-^M--_{}
•
-^R----¹-
R₃
–

V_PM

•
RR_{-----14-}
(EQ. 43)
Rearranging terms of Equation 43 and solving for R₄ results
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.
R₄
=



RR_-----83-
•
^{–----2--R-0---0-}L^{----••-----GR----}9^-P---M---
+
R^--1---₅^- _{}
–1
(EQ. 44)
The value of R₄ (Figure 12) is 12.37kΩ given the following
set of values:
R₈ = 40kΩ
R₉ = 40kΩ
R_L = 600Ω
R₃ = 8.25kΩ
R₅ = 40kΩ
G_PM = 1
Substituting the same values into Equation 41 and Equation 42,
it can be shown that the signal at OUT₁ is equal to -2/3V_PM.
This result, along with Equation 44 where R₃ equals to 2/3R4,
indicates the signal levels into the transhybrid amplifier are
equalized by the amplifier gains and opposite in polarity,
thereby achieving transhybrid balance at V_TXO.