Selecting a Fiber-Optic Cable for the GPIB-110 Bus Extender

Bus extenders overcome the restrictions on cable length and device loading imposed by the ANSI/IEEE Standard 488.1-1987. By using one pair of bus extenders, you can increase the distance between devices up to 2 km and the number of devices to 28 -- 14 on each end -- without compromising the integrity of the GPIB or requiring you to alter your application. National Instruments offers two types of bus extenders that use parallel and serial protocols to extend the GPIB to longer distances.

The GPIB-110, which uses serial protocols, has separate hardware configurations for either fiber optic or coaxial transmission cables. The fiber optic configuration works with a variety of fiber optic cables. This document describes how to select a custom fiber optic cable for the GPIB-110. The following material assumes prior knowledge of optical fiber technology. For more information on bus extenders, refer to the related link Extending the IEEE 488 Bus.

1. Description
2. Fiber Optic Component Specifications
3. Example Power Budget Calculation
4. References

Description

The GPIB-110 uses the Honeywell HFM2010-224 transmitter and HFM1011-221 receiver to connect to standard SMA-style Amphenol 905 or 906 connectors. The transmitter produces a 300-micron diameter sweet spot optical output that connects to a variety of fibers.

Selecting a fiber optic cable depends on the ease of connection between the cable and the Honeywell transmitter and receiver. Although you can use the transmitter and receiver with a variety of fiber diameters, all published specifications assume a fiber with a 100-micron core. You need to consider the following key criteria in selecting a fiber optic cable:

• Fiber core/clad and cable diameter
• Numerical Aperture (NA)
• Attenuation
• Bandwidth
• Operating wavelength

The GPIB-110 requires a fiber optic cable with a signal bandwidth of at least 50 MHz. You can use either a duplex cable or two simplex fiber optic cables. The fiber core, NA, and operating wavelength must closely match that of the transmitter and receiver to minimize signal loss.

Once all of the factors match, you can select the cable attenuation per kilometer and calculate the power loss budget. The total power loss should be less than the power budget to ensure that enough power is delivered to the receiver for successful signal transmission. The excess power margin gives extra tolerance to the system against factors such as aging and mechanical stress of the cable that might contribute to the power loss.

Your connector choice depends on the fiber size you select. The metal SMA-style 905 or 906 connector can terminate fibers with core/cladding diameters ranging from 50/125 to 600/640 microns. The power loss budget must account for the loss of attenuation caused by the connectors. The Amphenol connector has a maximum loss of 2 dB per connector.

You can select connectors as well as the type of cable jacket that houses the fibers to satisfy environmental requirements.

Fiber Optic Component Specifications

The following tables list the specifications of the GPIB-110 fiber optic components and the fiber optic cable available from National Instruments.

Example Power Budget Calculation

Using the specifications from the previous section, the following GPIB-110 power budget calculation assumes 2 km of cable with an attenuation of 4.3 dB/km at a 820 nm wavelength using any Amphenol SMA-style 905 or 906 connector. You can increase the cable attenuation if you use a shorter cable length.

1. To determine the power budget, divide the minimum power rating of the transmitter by the minimum sensitivity level of the receiver.

Minimum Transmitter Power:
100 µW (820 nm, 100 micron core)

Minimum Receiver Sensitivity:
1.5 µW

Power Budget Formula:
10 log(100 µW ÷ 1.5µW) = 18 dB

2. To determine the total loss budget, add the maximum loss ratings of the cable and the connectors.

2 km cable at 4.3 dB/km attenuation	8.6 dB
2 Amphenol connectors	+4.0 dB
Total Loss Budget	12.6 dB

3. Now subtract the total loss budget calculation from the power budget. This is your excess power margin.

Power Budget	18.0 dB
Total Loss Budget	-12.6 dB
Excess Power Margin	5.4 dB

An excess power margin of 5.4 dB shows that this system can tolerate such factors as aging or mechanical stress of the cable that might contribute to power loss. Typical margins range from 3 to 6 dB.

References

In addition to the related links, the following documents contain information that supplements this document.

ANSI/IEEE Standard 488.1-1987, IEEE Standard Digital Interface for Programmable Instrumentation.

Amphenol Corporation Fiber Optic Products.
1925A Ohio Street, Lisle, IL 60532.
Form F122-00300, Issue 12/90-25M.

Amphenol Fiber Optic Designer's Handbook, 1983
Allied Corporation.

Fiber Optic Components Low Profile Receiver Module HFM1011 Series.
Honeywell Inc. Form 84-06615-0, 1-91.

Fiber Optic Transmitter Trilevel Transparent Code Low Profile Module HFM2010/HFM2025.
Honeywell Optoelectronics. Form 110-0277-000, 2-82.

Fiber Specification Guide.
Universal Fiber Optics, Inc.
PO Box 1909, Salem, VA 24153.
Related Links:
GPIB-110 User Manual

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