AMD
For DC-coupled interconnections in which the distance between the TAXIchip set and
the optical module is less than 3″, transmission line terminations are not necessary. All
that is required is the appropriate ECL pull-down as shown in Figure 5-7(1) . On these
short line lengths, elimination of line reflections is not critical. However, without any
increase in complexity or power consumption, line reflections can be reduced simply by
locating the pull-down resistor, RE, at the end of the line instead of at the beginning. This
reduces the reflection coefficient at the end of the line, and therefore, reducing the
magnitude of the reflections.
Fig u re 5 -7
DC-Co u p le d TAXl-Fib e r Op t ic In t e rfa c e (Un t e rm in a t e d )
+5 V
+5 V
Optical Fiber
TAXI
TX
TAXI
RX
+
ODL
TX
+
ODL
RX
L < 3″, Z0
L < 3″, Z0
+
–
+
186 ≤ RE ≤ 7.2 (Z0)
–
–
–
RE
RE
RE
RE
12330E-14
Note:
If the DC-coupled interconnection is longer than 3″, transmission line terminations are necessary. For this
case, the suggested configuration is shown in Figure 5-8. Note that the line termination network also provides
the desired pull-down to VCC – 2 V, sufficiently below the output LOW level of VCC – 1.8 V
Fig u re 5 -8
DC-Co u p le d TAXl-Fib e r Op t ic In t e rfa c e
+5 V
+5 V
Optical Fiber
R1
L>3″, Z0
R1
R1
L>3″, Z0
R1
R2
TAXI
TX
TAXI
RX
+
ODL
TX
+
ODL
RX
+
+
–
–
–
–
R1R2
= Z0
R2
R2
R2
R1+ R2
5R2
= 3 V
R1+ R2
12330E-15
Note:
If the optical and TAXI power and ground planes are decoupled as shown in Chapter 6, AC coupling is always
recommended to allow for variations in power and ground plane voltages. AC coupling is discussed in
Section 5.5.2.
(1) Adequate bypass capacitors have been omitted from this and the following figures to simplify the drawings.
67
TAXIchip Integrated Circuits Technical Manual
AMD [ AMD ]
