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Getting Rough with VNA Test Cables

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by Fairview Microwave

Introduction

The wireless revolution is now in full swing, where innovators and even slow-to-move businesses are making the leap toward wirelessly connecting systems that have traditionally been wired or mechanical. This wireless renaissance, along with creating innumerable new business opportunities and applications, is also risking creating spectrum congestion and a landscape rife with electromagnetic interference (EMI). In order to preemptively combat spectrum congestion and EMI, wireless standards bodies and regulatory agencies are allotting swaths of spectrum for these new applications. Some of this “new” spectrum is being repurposed from legacy applications that are being phased out, while other formerly protected areas of spectrum are now open for licensed and unlicensed use.

The main wireless networks standards driving the need for new spectrum are 5G and Wi-Fi 6 extended (Wi-Fi 6e). For 5G cellular networks, wireless bands are being/have been approved in the sub-6 GHz, 26GHz, 28GHz, 37GHz, 39 GHz, 47 GHz, 70 GHz, and even above 95 GHz. Table 1 lists currently targeted spectrum for 5G which includes a mix of existing, licensed, and unlicensed spectrum. Wi-Fi 6e and future Wi-Fi technologies will also be taking advantage of the spectrum beyond 6 GHz, which already includes 6.0 GHz Wi-Fi ISM band which may be extended to cover the full unlicensed spectrum from 5.7 GHz to 7.1 GHz. 

This means that there is a current and growing need for reliable conducted test hardware and methods to 70 GHz, with a future looking beyond that. There are a variety of methods of over-the-air (OTA) testing being developed and used for 5G and Wi-Fi technology, however, OTA testing isn’t suited to all forms of testing. Conducted RF testing is necessary for the highest precision, reliability, and repeatability types of tests, such as handover/fading, wireless network transceiver testing, laboratory prototype testing, and even field testing of critical wireless systems. In many cases, a conductive interconnect is needed to connect various components within wireless hardware assemblies, and to connect wireless assemblies within a wireless system. Additionally, conducted testing is especially useful for millimeter-wave RF testing, as the RF losses at millimeter-waves require OTA testing distances that are extremely short and that may otherwise suffer from limited dynamic range from weak millimeter-wave signals.

Frequency Limitations of Coaxial Cables

The challenge with pushing the boundaries of spectrum into the millimeter-wave (beyond 6 GHz), is that the hardware specifications become increasingly stringent as EM wavelengths shrink. This is because EM signals interact with physical materials, such as conductors and dielectrics, in a frequency dependent fashion, and there is a geometric relationship between transmission lines, antennas, and other passive RF hardware and the wavelength of EM signals being passed. Coaxial transmission lines are limited in the maximum frequency they may contain as a transverse-electromagnetic (TEM) wave within the coaxial structure that is directly related to the physical dimensions of the coaxial cable/connector. Hence, coaxial cables and connectors of set size have a maximum frequency cut-off that is a product of the coaxial cable/connector diameter and metal thickness(Table 2). Accordingly, higher frequency coaxial connectors are proportionally smaller than their lower frequency counterparts. For instance, 2.4mm coaxial connectors are needed to reach 50 GHz and 1.85mm coaxial connectors are needed to reach 67 GHz. 

Table 1: 5G Frequency Bands and Frequencies

There are also frequency dependent phenomena, such as skin effect and loss tangent of dielectrics, that have significant impact on the performance of transmission lines, antennas, and other passive components.

For these reasons, millimeter-wave RF hardware has traditionally been used for military, aerospace, space, and research/scientific applications and has come with the high price tag, custom design, and long lead times typical to those industries. With millimeter-wave technology being developed for commercial, industrial, and even consumer applications, there is a burgeoning need for high performance and reliable RF hardware in the mainstream. This means that even high performance millimeter-wave RF hardware needs to become increasingly accessible with fast shipping and a seamless e-commerce experience, without the need for quotes and extended lead times.

These conclusions are especially true for RF hardware that are considered accessories or “expendables”, such as coaxial cable assemblies and adapters. Coaxial cables and connectors are key components of virtually all wireless network hardware throughout the development and deployment cycle. From prototype testing to qualification and even testing deployed networks, RF coaxial cables provide high performance and reliable interconnect that also presents shielding from external interference.

Hazards Faced by High Frequency Test Cables

Not all RF coaxial cables are created equal, however. There are an expansive range of coaxial cable materials, assembly methods, and technologies to keep in mind when selecting a coaxial cable for a given application. For millimeter-wave applications, on the other hand, there are generally far less options available due to the stringent manufacturing requirements of producing millimeter-wave coaxial cable assemblies. Also, the new testing environments, from prototype laboratory testing to high volume quality control testing, require more rugged interconnect solutions. The results of these requirements is that more rugged coaxial test cables are needed for many applications.

Table 2: mmWave Coax Connector Types & Approximate Cutoff Frequency for 50 Ohm Coax

Vector Network Analyzer (VNA) test cables are coaxial cable assemblies that are specifically designed with high precision network analyzer test requirements in mind. These cables are often amplitude and phase stable even under flexure, exhibit very low voltage standing wave ratio (VSWR) figures, and are often assembled using many layers of shielding, dielectric, and jacketing to ensure high performance while being subjected to the rigors of a wide range of test applications. Amplitude and phase stability are especially important for test cables subject to flexure, as any drift in amplitude or phase degrades the calibration to the test plane of the DUT. Any degradation to the calibration reduces the calibration lifetime and the repeatability of the test. 

Another factor to consider is the RF shielding capability of the test cable assembly. The relatively weak millimeter-wave signals compared to lower frequency signals result in heightened importance for high levels of RF shielding to prevent EMI and noise intrusion into the coaxial transmission line, which may overwhelm sensitive millimeter-wave receivers and reduce the dynamic range of millimeter-wave test equipment. Moreover, many modern millimeter-wave systems, such as 5G beamforming antennas, have very dense interconnects with many cables closely spaced. High levels of RF shielding also prevent leakage of RF signals from one cable to another, as RF shielding is symmetric.

How to Protect Millimeter-wave Test Cables

The protective features of VNA test cables are especially important for millimeter-wave test cables, as the very thin and delicate conductor and dielectric layers need to maintain very tight tolerances on positions and stretch within the coaxial cable assembly during bending/flexure and over wide temperature ranges. Tight bending radius and repeated flexure can degrade and even destroy high precision test cables, so additional armoring and protective jacketing are desirable features for millimeter-wave VNA test cables. These extremely tight tolerances also mean that the calibration for millimeter-wave cables is much more sensitive than larger cables, which results in these cables being more susceptible to calibration degradation from flexure-induced amplitude and phase instability.

The physically small and mechanically weak structures within a millimeter-wave coaxial cable and connector need control features that limit the torsional forces to the delicate structures within the coax. These controls are especially necessary for applications that may require hundreds or thousands of mating cycles. In this way, millimeter-wave VNA test cables with torsion control features are also useful for applications where the end use of the millimeter-wave transmission line may be varied or is unknown, such as in a product development laboratory or industrial facility, as a single highly reliable cable may be purchased as opposed to several cables for different purposes. Considering the relative cost of multiple millimeter-wave test cables compared to a single ruggedized cable, the more rugged cable may also be cost effective in the long run as it may serve multiple purposes.

Though these additional protections ensure high electrical performance and physical ruggedness, they come at the cost of greater weight, larger dimensions, and increased rigidity. It is important in the design and manufacturing of rugged VNA test cables to also ensure that the insertion loss and VSWR performance of these cables isn’t sacrificed for ruggedness. This is what separates ruggedized VNA test cables from typical rugged coaxial cables. Essentially, ruggedized VNA coaxial cables fulfill the same requirements as semi-rigid and hand-formable coaxial assemblies without the need to predetermine the precise length of the assembly ahead of time, and high quality ruggedized VNA cables can even undergo tens of thousands of flexure cycles. However, not all ruggedized VNA test cables are able to undergo repeated flexure, which is why it is important to investigate the flex life, or maximum specified number of flexure cycles, for a cable to be confident it will work for an application that subjects coaxial cables to repetitive flexing.

Material & Performance Considerations for Rugged and Flexible Coaxial Cables

To ensure amplitude and phase stability, ruggedized VNA test cables are composed of a selection of materials specifically chosen to behave consistently over a wide temperature range and while subject to physical deformation. This includes precise analysis of the effects of each material’s coefficient of thermal expansion (CTE) over the operating temperature range to ensure the changing size/shapes of the cable and connector materials don’t exceed tolerance requirements (Table 3). 

Moreover, these materials are chosen for their ability to be pre-conditioned to temperature variations and mechanical strain, which ultimately reduces the range the material’s electrical and physical properties change while under environmental stresses. These cables are also designed with various techniques to further limit physical changes in the coaxiality of the conductors and dielectrics and the expansion and contraction of the length of the coaxial cables. Materials, such as stainless steel are used for connectors and for tight bend radius ends, as stainless is corrosion resistant, mechanically strong, and readily machinable to high tolerances. This is why stainless steel is also used as armoring for the coaxial cable, as it is still ductile allowing for flexibility, yet strong enough to provide crush and puncture resistance. Since the armoring and additional stability layers within a rugged VNA test cable reduces its flexibility, having connector ends with tight bend radii can minimize needing to flex the cable while enabling the cable to accommodate varied interface geometries. This is especially useful for high density interconnect scenarios, such as with millimeter-wave probe stations, multi-input/multi-output (MIMO), beamforming, wireless network transceiver, and handover testing for high channel count situations.

Table 3: Temperature Coefficient for Thermal Expansion and Dielectric Constant of Coax Materials

Lastly, the mechanized assembly and quality control of these cables is a primary consideration. Machine tolerances on length and other critical dimensions of the coaxial assemblies is essential. Given that coaxial cables are assembled of many layers and coaxial connectors have many components, the careful selection/machining of components and assembly practices dictates the reliability and performance of these cables. Considering these factors, manufacturing quality control that consists of testing each individual cable electrically is needed.

Conclusion

Be it for high-mix laboratory test facilities, robotics/automated manufacturing facilities, or wireless networking applications, high precision, test quality coaxial cables are now seeing increased demand by users whose business requirements suffer from the long lead times of traditional coaxial cable industry. For emerging applications, and even legacy applications plagued with high performance coaxial cable failure due to repeated flexure, the latest generation of ruggedized VNA test cables designed for high flex life can deliver longer lived better performing coaxial assemblies. Cables like these should also now be available at the speed of ecommerce to fit with the new paradigms of the RF, microwave, and millimeter-wave industries.

References

1. Fairview Microwave Debuts New VNA Test Cables with Flex Life Exceeding 100,000 Cycles

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