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An Introduction to Test Cables

Parent Category: 2017 HFE

By Fairview Microwave

Test cables are the cornerstone of any research and development project. Because of their prolific use in the RF and microwave industry, they can often go overlooked. While many engineering labs seem to have an endless supply of basic test cables, many have been used so much that they fail or provide subpar performance. Depending upon the test environment, frequency range, and frequency of use, cables can have additional mechanical and electrical features for robustness.

Construction Types

While there are a wide variety of coaxial cables, much of these constructions can be boiled down to three basic cable types: Semi-Rigid, Hand Formable [Formable], and Flexible. Semi-Rigid cables are intentionally built as the most inflexible cables so that, once they are formed to a shape, they maintain it and so they cannot be reformed. Formable cables are moderately flexible in that they can be formed and reformed [a predetermined amount of times] to hold particular shapes. As the names suggests, flexible cables are the most flexible but do not maintain any particular shape. Each cable serves different and highly relevant test purpose and application.

Semi-Rigid

There are a several key aspects of semi-rigid cables: wide frequency range, rigidity, phase-stability, and low PIM performance. The excellent electrical performance can be attributed to the dimensional rigidity of this cable as unpredictable phase changes often occur due to bending and temperature. Bending is automatically a non-issue for a semi-rigid cable and these cables assemblies are often carefully inspected and endure thermal cycling after it has been formed to ensure robust performance. To achieve rigidity, semi-rigid cables have a solid sheath outer conductor of extruded metal with a solid metal inner conductor of copper or a copper alloy. Connectors are soldered on after carefully being cut and bent to the desired length and shape.

1708 HFE cables 01

Semi-Rigid cables are composed of solid inner and outer conductors with a 360˚ soldered connection providing reliable performance in a wide frequency range.

Formable

Hand formable coaxial cables are a more pliable form of the semi-rigid cable. The primary utility of these cables are the fact that they can be formed and reformed to hold a variety of shapes, making them an alternative to the semi-rigid cable. Still, more often than not, they do not hold up to the same electrical performance because of their flexibility. To maintain pliability, the formable cable still has a solid inner conductor but the outer conductor is made with soft (un-annealed) copper or dead soft aluminum [1].  Similar to semi-rigid cables, formable cable connectors are soldered on to maintain a reliable connection.

Flexible

Flexible cables are mostly used in test and measurement scenarios as there is often no need to shape a cable for a test, generally, the use of a coaxial cable in a test is to extend the DUT to an accessible distance from lab equipment. There are more complexities in the inner conductor and shielding of a flexible cable in order to generate reliable performance while being bent in torsionally and longitudinally. The inner conductor can also be solid or stranded to increase flexibility, still, at higher frequencies solid inner conductors perform best as they are conformal.

Shielding, or the return path of the signal going through the inner conductor, is either a thin aluminum foil tape or braided (or both) as a solid sheath would rapidly degrade and crack with bending, the material can be made of copper wire or a copper alloy, and can be silver plated. Shielding parameters such as coverage and shielding effectiveness (SE) are specific to braids since they are not dimensionally conformal and so cannot fully ‘cover’ the electrical signal from internal signal leakage and external signal interference, also known as Electromagnetic and RF Interference (EMI/RFI). SE is a measure of the immunity of the cable from EMI and RFI, ideally the shield would support currents that are the additive inverse to those in the center conductor across the cable, since this is not reality, it is necessary to understand the SE in particularly noisy environments. While coverage and SE are correlated they are not the same, a cable with high coverage can have less SE than a cable with less coverage (depending on the conductivity of the braid material).

Spec Sheet Parameters

The standard parameters of test coaxial cables are operational frequency range, insertion loss (attenuation),and return loss (or VSWR). Where insertion loss is a parameter that indicates the reduction of signal power from the input to the output of a transmission line and return loss indicates the loss of power due to reflections. Measured in decibels (dB), both of these parameters can be obtained with a Vector Network Analyzer (VNA) through the 2-port S-parameters where insertion loss is S21 and return loss is S11. Since coaxial cables are passive reciprocal transmission lines, S12 should have approximately the same values as S21 across the frequency band while S11 and S22 should also be nearly identical.

1708 HFE cables 02

Depending upon the required amount of shielding effectiveness and coverage, an assembly can utilize a combination of braids and foils.
http://www.l-com.com/content/Coaxial-Cabling-Tutorial.html

Specification sheets can also express velocity of propagation, capacitance, and power handling. Often expressed as a percentage, the velocity of propagation is a ratio of the speed of a signal in the coax over the speed of light in a vacuum. The capacitance of a cable per unit length can give insight into the characteristic impedance of the cable and the energy stored in the cable. The maximum continuous wave (CW) power handling of a cable is important to note particularly in high powered tests where calculations for padding need to be done prior to the start of a test in order to prevent any damage to expensive test equipment from power surges. Commonly listed mechanical properties of a cable include the minimum bend radius and mating cycles of the assembly to allow for proper handling during the lifespan of the coax.

Features of Test Cables and How They Are Achieved

Phase Stability

The phase stability of a cable is a factor that has become increasingly relevant in highly phase-sensitive RF and microwave systems, for instance, in systems utilizing multiple coaxes to fed energy from a common source, or to collect energy from scattered sources. In these cases, the phase variations at the end of each line depends on the evenly matched electrical lengths of the transmission lines. This parameter is affected by changes in electrical length that generally occur through temperature variations, bending and flexing, frequency, vibrations, mechanical tension, humidity, and equality of physical length [2].

1708 HFE cables 03

Phase changes can be analyzed and assessed on a VNA through repetitive flexure.

With Active Electronically Scanned Arrays (AESA) becoming more prevalent in modern radar through technologies such as switched beams arrays and phased arrays, the beamforming of these systems can depend heavily on the integrity of their transmission lines. Long cable runs from control rooms to phase sources [e.g.: LO] in the device under test (DUT), rely heavily on a stable electrical length for accurate range measurements. For instance, modern, high energy physics experiments are large installations that spread over hundreds of meters to kilometers [3]. Long-term tests (days to weeks) made in environments without stable ambient temperatures such as outdoors, or on large manufacturing floors have to account for drift rates that lessen the accuracy of a calibration. Even though there are methods to automate a calibration, access to phase stable cables can even be convenient for test engineers that perform more complex calibrations--such as VNA multi-port calibrations using switch matrices--as there is less of a need to recalibrate; saving time and effort.

Coaxial cable phase stability can be generally be achieved by one of two methods: by undergoing extensive temperature cycling until there is minimum deviation in phase, or by specially designing the mechanical properties of the dielectric and inner/outer conductor to minimize the changes that can occur in them (e.g.: change in dielectric constant) due to temperature cycling [4]. With connector heads often being the most delicate segment in the transmission chain, they are often ruggedized to ensure performance even with extensive mating cycles (1,000+). Phase stability can be measured (with a VNA) by observing the phase differences that occur in a cable while undergoing flexure.

PIM

Passive intermodulation distortion (PIM) is a phenomena that occurs in the passive components of an installation that causes nonlinearities in electrical performance that cannot be accounted for. PIM in coaxial cables often occurs at the metal-to-metal joints as they can be subject to the most torsional strain, ingress, discontinuities, and corrosion. Torsional strain can occur particularly on the connector heads and the pivot point between the cable and connector due to environmental vibrations from rain and wind, or even just from an excessively tightened or loosened connector head. Ingress such as dust or moisture can cause nonlinear behaviors. Discontinuities can exist in surfaces between the metal-to-metal junctions such as cold solder joints, or even stray flecks of metal, possibly from excessive grinding during the mating of the connectors. Semi-Rigid, Rigid, or even formable coaxes pose less of a PIM threat than flexible cables with braided shielding--the loose junctions between the strands contribute greatly to PIM failures.

The issue of PIM often arises in multi-frequency high-powered systems such as distributed antenna systems (DAS) and base stations due to the necessity for highly sensitive receiving equipment and the use of high powers where electro-thermal distortions can occur at metallic junction points. PIM power obstructs signal transmission (increase in bit error rate (BER)) leading to poor communication quality, dropped calls, or reduced data rates. PIM can emerge during times of high traffic due to the high power concentration and decreases with traffic, often appearing as an intermittent signal that is extremely difficult to troubleshoot as the source of the issue has to be precisely identified and fixed. PIM testing scenarios in dynamic environments that require a high degree of sensitivity may be subject to failures due to PIM and therefore require low PIM cable assemblies for a reliable interconnection.

Solutions vary in order to create a more uniform mating surface between metals where corrugated shields, extruded metal shields, or tin-filled braids can be employed as solid outer conductors are best to minimize PIM. Avoiding magnetic, paramagnetic materials, and braided shields are a readily evident solution. Strain relief such as injection molded boots on the connector-to-cable can mitigate the damage due to vibrational effects on the cable.

Ruggedized

While a rugged cable can mean a number of things in an industrial application such as an unmated seal, IP rated, oil resistance, and UV resistance, this is not necessarily the case for a rugged test cable. Ruggedized cable assemblies for testing scenarios can encompass crush resistance, super-flexibility, and extensive mating cycles. Often, rugged cables are also precision, phase stable and can operate with low PIM as the armoring leveraged can limit the torsional twist of the cable as they are flexed. The armoring also contributes to the crush resistance of the cable and is of particular utility in scenarios where a precision cable is employed in order to eliminate any cross sectional deviations due to mishandling. The need for a high mating cycle life is necessary in a test lab where cables are used and reused on a number of different test benches multiple times a day for an indefinite amount of time.

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A microscopic view of silver metallization reveals merged grains, the uneven surface can cause non-linearities particularly at high operating powers.
https://people.engr.ncsu.edu/mbs/Publications/vitae_theses/wilkerson_phd_2010.pdf

Oftentimes the scenario occurs where an engineer has taken the time to do a calibration on a VNA, observes a flat response at the 0 dB reference value, performs their test, and later realizes it all failed. Then, while troubleshooting discovers a faulty, highly phase unstable coax while bending it and has to redo the entire test. Ruggedized and precision cable assemblies are often necessary because of the frequency of these issues, quality components can save on time and money in the long run.

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Ruggedized cabling often features precision connectors, cable armoring, and flexible protective sleeves for highly reliable, phase stable performance.

Conclusion

Coaxial cables are the go-to transmission line that forms the backbone for all kinds of test and measurement systems. With the electrical and mechanical characteristics of these cables varying greatly to serve particular test scenarios, it is important to be able to choose the right cable parameters in order to save on cost and time. Sometimes, understanding the standard S-parameters just doesn’t cut it and more advanced features are necessary such as phase stability, low PIM performance, and a ruggedized design.

References

1.
https://books.google.com/books?id=MCj9jxSVQKIC&pg=SA7-PA5&dq=semi+rigid+coaxial+cable&hl=en&sa=X&ved=0ahUKEwjYh5_i9tHUAhUDeD4KHbNQC8QQ6AEIPjAF#v=onepage&q=semi%20rigid%20coaxial%20cable&f=false

2. http://www.dtic.mil/dtic/tr/fulltext/u2/628682.pdf

3.
http://przyrbwn.icm.edu.pl/APP/PDF/119/a119z4p17.pdf

4.
https://books.google.com/books?id=06jMBQAAQBAJ&pg=PA354&dq=why+is+phase+stability+in+coaxial+cable&hl=en&sa=X&ved=0ahUKEwiG1t7vvt7UAhWDCD4KHe2gB1oQ6AEILTAC#v=onepage&q=why%20is%20phase%20stability%20in%20coaxial%20cable&f=false

November 2017

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