AMD
5 .5 .2 AC-Co u p le d TAXl-Fib e r Op t ic Tra n s c e ive r In t e rfa c e
Some applications will require the TAXIchip set to optical transceiver interconnection to
be AC-coupled. AC coupling should be used in the following situations: a) when the
TAXIchip set and optical components are driven by a common power supply but the
supply pins are decoupled using the scheme recommended in Chapter 6, and b) when
the TAXIchip set and optical components operate on different power supplies.
AC coupling via capacitors along with the necessary design equations is shown in
Figure 5-9. In this configuration, RE is the ECL output pull-down resistor, C provides the
AC coupling, the connection is made with a transmission line (coax, twisted pair,
microstrip) of length L and characteristic impedance, Z0, and R1 and R2 provide a
matched line termination and voltage bias to the midpoint of the optical component’s
logic swing (Vbb).
The configuration shown in Figure 5-9 is recommended for any line length, L, which
separates the TAXIchip set and the optical module. Although the matched line termina-
tion is not necessary for L<3″, the Vbb bias voltage is always needed for AC-coupled
links. Therefore, even for line lengths where matched line terminations are not neces-
sary (less than 3″), the resistors R1 and R2 can be chosen to give a matched load
without any added complexity.
Fig u re 5 -9
AC-Co u p le d TAXl-Fib e r Op t ic a l In t e rfa c e
VCC
+5 V
R1
R1
TAXI
TX
+
ODL
L, Z0
C
TX
+
–
Vbb
–
R1R2
R3R4
= Z0
C
=
R1+ R2
R3+ R4
R2
R2
RE
RE
VIH + VIL
2
(VCC – VEE) R2
R1 + R2
Midpoint of ODL
Signal Swing
=
Vbb
=
= VEE +
VEE
Optical Fiber
VCC
5R4
R3+ R4
Midpoint of Pseudo-
ECL Signal Swing
=
= 3.7 V
+5 V
R3
R3
186 < RE < 7.2 (Z0)
C = Largest RF Capacitor Available
TAXI
RX
+
ODL
RX
+
L, Z0
C
C
–
–
RE
RE
R4
R4
12330E-16
VEE
5 .6 In t e rfa c in g t o Co a x ia l Ca b le
In many applications, system cost can be reduced by using coaxial cable as the serial
media. Unlike optical fiber, which requires optical components between the fiber and the
TAXIchip set, coaxial cable can be connected directly to the TAXI SEROUT pins, giving
lower system costs.
68
TAXIchip Integrated Circuits Technical Manual
AMD [ AMD ]
