Selecting a Fiber-Optic Cable for GPIB-140 Series GPIB Extenders

The GPIB-140 series of GPIB extenders use fiber-optic cable to extend the distance between two GPIB systems up to 2 km. The transmitter and receiver on the GPIB-140 series can work with a variety of fiber-optic cables. This document gives specifications on National Instruments T7 and T8 cables, which can be used as a reference if a different fiber-optic cable is desired.

Introduction

The T7 and T8 cables available from National Instruments meet the needs of most customers using GPIB-140 series GPIB extenders.  In some circumstances it is desirable to have cable properties not available with these cables.  This could include heavy duty cables for direct in-ground burial or operation in harsh conditions.

Many factors differentiate fiber-optic cables, including those listed below:

• Operating wavelength
• Bandwidth
• Fiber core/cladding diameter
• Numerical Aperture
• Attenuation
  

The operating wavelength of the fiber-optic cable should closely match that of the transmitter and receiver for the link to work properly. The fiber attenuation should be calculated at the operating wavelength for a correct estimate of the total loss.

The bandwidth of the fiber is specified according to the signal frequency and the distance the signal is sent through the cable. The product of the signal frequency and traveling distance must not exceed the bandwidth-distance product of the fiber to maintain signal quality and avoid signal losses.

Core diameter gives a rough estimate of fiber performance – the smaller the core, the higher the bandwidth and the lower the loss. The four commonly used fibers have core/cladding diameters as follows:

Numerical Aperture (NA) is the “light-gathering ability” of a fiber. It is especially important to match the NA of the transmitter to the NA of the fiber so that all the light emitted by the transmitter is coupled into the fiber and propagated. Mismatches in NA are sources of loss when light is coupled from a lower NA to a higher one.

In most cases, the cable chosen should match the respective T7 or T8 fiber-optic cable in all of the above attributes.  This will ensure the highest degree of compatibility with your GPIB-140 series GPIB extender.  If a different core diameter is chosen, you should ensure that your chosen fiber-optica cable matches the NA of the transmitter, as defined in the section below.

Fiber-Optic Component Specifications

GPIB-140 series extenders use fiber-optic transmitters and receivers to extend the GPIB signals.  The GPIB-140(A) provides a maximum distance of 1km, while the GPIB-140(A)/2 provides for up to a 2km extension.

Fiber-Optic Transmitter

Fiber-Optic Cable Assemblies

The T7 and T8 cable assemblies offered by National Instruments are for use with the GPIB-140 Series GPIB extenders.  It is possible to utilize a fiber-optic cable of your choosing if you require properties not attainable through the T7 or T8 cable assemblies.  The specifications supplied in this document will provide you with the information you need to choose an appropriate cable for use with your GPIB-140 Series GPIB extender.

T7 Cable Assembly

• For use with GPIB-140(A) Extenders
• Available in 10m increments to 1000 m

T8 Cable Assembly

• For use with GPIB-140(A)/2 Extenders
• Available in 10m increments to 2000 m

 

Cable Specifications

The specifications in this section apply to both T7 and T8 cable assemblies, except where otherwise noted.

Type

Duplex, riser-rated, Ultra-Fox™ series
62.5/125 μm core/cladding diameter
Graded index
Tight buffered

Material
Optic Fiber -- Glass
Outer Jacket -- Flame-retardant, fungus-resistant, soft, resilient PVC

International and National Standards
ISO 9001-1987
ANSI/ASQC Q91-1987
Meet NEC sections 770-51(b) and 770-53(b) for use in vertical runs

Dimensions
Cable diameter -- 3.0 by 6.5 mm (0.12 by 0.26 in.)
Buffer diameter -- 900 μm (0.035 in.)
Weight -- 16 kg/km (11 lb/1000 ft)

Performance Characteristics

	T7 Cable Assembly	T8 Cable Assembly
NA (numerical aperture)	0.275
Operating Wavelength	850 nm	1300 nm
Attenuation (at Operating Wavelength)	3 dB/km	1 dB/km
Bandwidth (at Operating Wavelength)	160 MHz-km	500 MHz-km
UL Rating	OFNR riser-rated (can also be used as general-purpose or residential cable)
Tensile Load Rating	1000 N/220 lbf (short term, i.e. during installation) 500 N/110 lbf (long term)
Minimum Bend Radius	Under Load: ~ 5 cm Without Load: ~ 3 cm
Crush Resistance	750 N/cm
Impact Resistance	1000 Impacts
Flex Resistance	7500 Cycles
Operating Temperature	–40° to +85° C
Storage Temperature	–55° to +85° C

Fiber Buffer Construction Details
Ultra-Fox™ cable features:

• 100 kpsi proof-tested fiber
• A primary buffer of UV-cured acrylate material to a diameter of 250 μm
• A secondary buffer of high performance PVC to 900 μm
• The versatile buffer system above permits mechanical stripping in short length (about 1 cm) to remove
• The PVC secondary buffer and leave the 250 μm primary buffer intact for splicing into similar buffered
• Fibers from loose tube cable or for Local Injection and Detection (LID) splicing techniques
• The 250 μm buffer may further be mechanically stripped to the 125 μm glass diameter for splicing
• Alternately, the primary and secondary composite buffer may also be stripped in one step for direct termination with connectors
  

Connectors

Type
ST style (Methode 908 series)

Materials
Nut -- Zinc Alloy, Nickel Plated
Body Reinforced Composite Polymer

Features

• Connectors mate with AT&T ST® products and all other functional equivalents, optically and mechanically.
• One-piece nickel plated zinc body construction for easy field or factory termination in less time
• Coupling nut with radial track to minimize mating effort
• Accommodates tight and loose tube buffer cables with jacket outer diameters of up to 3.1 mm
• Polymer ferrule pedestal supports fiber during polishing – no epoxy bead
• Standard 0.155 in. hex crimp, same tool as SC and FC
• Universal dust cap with lanyard

Performance Characteristics
Fiber Size -- 62.5/125 μm core/cladding diameter
Attenuation Loss -- <0.1 dB typical <0.5 dB maximum
Insertion Loss:
<0.3 dB typical
<0.1 dB max increase 500 cycles

References

For more information, see the following documents and related links:

• ANSI/IEEE Standard 488.1-1987, IEEE Standard Digital Interface for Programmable Instrumentation.
• Amphenol Fiber-Optic Designer's Handbook, 1983, Allied Corporation.
• Low-Cost Fiber-Optic Links for Digital Applications up to 155 MBd, 1993, Hewlett Packard Co.
• Technician's Guide to Fiber-Optics, 1987, Delmar Publishers Incorporated.
• Designer's Guide to Fiber-Optics, 1982, AMP Incorporated.

Related Links:
GPIB-140A User Manual
GPIB-140 User Manual
DevZone Tutorial: Extending the IEEE 488 Bus

Reader Comments | Submit a comment »

Add links to specific cables/wallthrough and other manufacturers hereof
Add links to specific cables/wallthrough and other manufacturers hereof. Cables for different stiffness and environment / EMC proof wallthrough etc.
- Bjoern Ahrensbach, NATO. b.ahrensbach@get2net.dk - Jun 8, 2006