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TAXIchip Integrated Circuits Technical Manual

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.

Figure 5-7

DC-Coupled TAXl-Fiber Optic Interface (Unterminated)

12330E-14

TAXI

–

ODL

–

L < 3

″, Z0

+5 V

186

≤ RE ≤ 7.2 (Z0)

TAXI

–

ODL

–

L < 3

″, Z0

+5 V

Optical Fiber

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

Figure 5-8

DC-Coupled TAXl-Fiber Optic Interface

12330E-15

TAXI

–

ODL

–

L>3

″, Z0

+5 V

TAXI

–

ODL

–

+5 V

Optical Fiber

R1R2

R1+ R2

= Z0

5R2

R1+ R2

= 3 V

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.

L>3

″, Z0

(1) Adequate bypass capacitors have been omitted from this and the following figures to simplify the drawings.

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