Critical Cable: Instrumentation Cables Used in the
Manufacture of Cables and Cable Assemblies
By Bill Sopchak, Product Manager for Bishop & Associates Inc.

As the Escher lithograph “Drawing Hands” depicts, to make something, sometimes you first must have the finished thing itself. Many instrumentation cable assemblies go into the manufacturing process that produces cable assemblies and wire harnesses. Starting with the wire or bulk cable being used for the assembly and concluding with the final inspection testing of the completed assembly, the instrumentation cable assemblies, the test equipment that they are interconnecting to, and the interpretation of the final test results all play a vital role in the overall quality of the finished product.

Every cable assembly specification includes both physical and electrical performance requirements. The instrumentation cables used in the manufacturing of that cable assembly can be found in the measurement of both sets of performance parameters. Instrumentation cable assemblies might be used to interconnect closed loop physical performance parameter measuring equipment in the manufacturing of bulk cable, for example, such as “laser micrometers” measuring ODs of primary conductors or jackets, or “spark testers” seeking insulation voids during the insulation extrusion process. However, this article will focus on the electrical performance requirements and the instrumentation cables associated with the measurement of these electrical performance parameters.

Electrical performance requirements, as defined on a cable assembly specification, tend to fall within two distinct groups: 1) those that are inherent in the bulk wire or cable, and 2) those that are inherent in the final assembly. As a subset to each of these groups, electrical performance requirements may be: a) a fixed value resulting from the design of the wire, bulk cable, or cable assembly itself, to be verified once upon initial production (design verification) and infrequently thereafter to verify that the materials and manufacturing processes remain consistent over time, or b) a production process variable parameter that can be directly affected by variables in the manufacturing process, requiring 100% inspection of every unit or lot coming off the production line.

Garbage In, Garbage Out!

Several of the electrical performance requirements of the finished cable assembly or wire harness are directly attributable to the bulk wire or cable and connectors that are used to make the assembly. Key requirements include current carrying capacity, voltage rating, impedance, capacitance, velocity of propagation, attenuation, and conductor DC resistance.

These parameters are measured in various ways using test equipment as simple as a handheld $15 multimeter with test probes as the “instrumentation cable,” or as complex as highly sensitive and sophisticated oscilloscopes and network analyzers that cost more than $400,000 interconnected with “gold standard” coaxial wave guide instrumentation cable test sets and baluns. The instrumentation cables used at this end of the spectrum must be of the highest quality and consistency, insuring that they themselves don’t alter or affect the test results. Further, any contribution that these instrumentation cables might add to the resulting test data of any given parameter must be carefully measured and recorded ahead of final assembly testing so the value added by the instrumentation cables to the results for that given parameter can be negated out to insure final assembly compliance within the cable assembly specification tolerances.
 

The Devil Is In the Details

Once the bulk wire or cable is selected for use, there are several variables in the wire harness or cable assembly manufacturing process that can have adverse effects on the final assembly’s performance to specification. In fact, some electrical parameters, such as crosstalk and signal skew, can only be ultimately controlled in the manufacturing process of the final assembly. A bulk cable, for example, may meet these parameters in its unterminated state as it comes off the production line. However, several variables in the cutting, preparation, and termination of the cable to the connector and back shell can cause those parameters to fall out of spec.

In the case of crosstalk: While the bulk cable might be designed with twisted pairs to reduce crosstalk, and possibly even twisted pairs with varying twist lengths within the same cable bundle, such as with Cat5E and Cat6 cables, the manner in which the cable and its twisted pairs are prepared for termination to the connector, and how “tight” or “loose” the pairs are kept and managed up to the termination point, will have an effect on the crosstalk results for the finished assembly at certain frequencies.

In the case of skew: Again, the bulk cable may meet all design parameters as manufactured, but the “electrical length” of each conductor within the final assembly must be matched using an oscilloscope during termination and final assembly to insure that data sent down one pair at a given transmission speed reaches the other end of the cable assembly at precisely the same time its complimentary data is sent down an adjacent pair at the same transmission speed. It is not just a matter of physical length, although exacting physical length tolerance control is a first step. (Just envision cutting a 25 pair cable off its reel at a 458 angle to itself. Even if the other end was cut straight, there could be as much as a ¼” to a ½” difference in the physical lengths of the pairs within that piece of cable that you are starting to build your finished assembly from. It’s almost sure to fail a final skew test after you have already sunk the cost of the connector and backshell and all the termination and test labor into it!)

If the velocity of propagation (VP) on the first pair is slightly higher than the VP on the adjacent pair, to the point where the bits of data cannot be reassembled into complete bytes, data corruption and data loss can occur. As you can imagine, the higher the transmission and system speeds, the bigger the issue skew in the final assembly becomes. Additionally, the higher the pair count in a given cable, the more difficult it is to meet tight skew tolerances in the final assembly.

Application Determines Everything

The quality of the instrumentation cables used, the simplicity or complexity of the test equipment needed, and the level of care and knowledge needed to interpret the final inspection data are all governed by the application that the final assembly is being used in. With the trend toward increased speed and performance in nearly all electronics-related industry segments, understanding where design versus manufacturing process variables can help you “hit the sweet spot” of the electrical performance requirements is more critical than ever.