AM7969-125JC datasheet(72/127 Pages) AMD

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TAXIchip Integrated Circuits Technical Manual

5.5.2 AC-Coupled TAXl-Fiber Optic Transceiver Interface

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.

Figure 5-9

AC-Coupled TAXl-Fiber Optical Interface

12330E-16

TAXI

–

ODL

–

L, Z0

+5 V

VEE

Vbb

VCC

TAXI

–

ODL

–

L, Z0

+5 V

VEE

VCC

R1R2

R1+ R2

= Z0

5R4

R3+ R4

= 3.7 V

R3R4

R3+ R4

Midpoint of Pseudo-

ECL Signal Swing

Vbb =

Midpoint of ODL

Signal Swing

VIH + VIL

= VEE +

(VCC – VEE) R2

R1 + R2

186 < RE < 7.2 (Z0)

C = Largest RF Capacitor Available

Optical Fiber

5.6 Interfacing to Coaxial Cable

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.

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