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9 Steps to Select a Test Cable for Millimeter Wave Applications

Radio signal frequencies rise to the millimeter-wave (mmWave) range as applications such as 5G networks, automotive radar systems, and RF semiconductor probing demand more bandwidth at higher frequencies. Testing is an integral aspect of developing quality new products that will operate in these mmWave frequencies, and coaxial cable assemblies play a vital role in the process. 

However, given the complexity of mmWave applications, not just any coaxial cable will do. Critical parameters such as impedance match and insertion loss must be considered to obtain repeatable, reliable test results at higher frequencies. As a result, RF testing for mmWave applications requires unique coaxial cable and connector solutions. 

Factors to Evaluate in Selecting the Optimal Coaxial Cable Test Assembly 

Typically, the RF testing process incorporates a device under test (DUT) connected to a vector network analyzer (VNA), spectrum analyzer, or oscilloscope. The signal path to the circuit board is critical, and the test setup must not introduce unwanted variables or errors, VSWR spikes, or excessive insertion loss. This includes the test cable and connectors.

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Test cable assemblies must be robust enough to withstand extensive handling and continuous movement from frequent connecting and disconnecting while also maintaining precise repeatability of measurement and reliable electrical performance. There are numerous characteristics to evaluate before selecting the optimal coaxial cable assembly for a specific application, including frequency range and cable diameter, test equipment type, connectors, measurement type/application, flexibility, phase stability, power, impedance, and allowable loss budget.

Frequency Range and Cable Diameter

When selecting a test cable, one of the first things to consider is the frequency range required to test the application. This will help to determine other factors, such as the cable type and mechanical structures needed. For example, as frequency rises, the cable’s diameter becomes smaller. It’s an inverse situation, and all of the ratios must work correctly at the different frequency levels. 

Test Equipment 

Next, evaluate the type of test to be performed and what kind of equipment will be used. For example, the test could be a standard S-curve type of measurement looking at the loss of a device under test or evaluating how it performs at specific frequencies. All variables must be considered up front when selecting a test cable assembly that will perform well for each unique testing scenario.

Connectors

Once the frequency, type of test equipment, and cable size are determined, the connector type comes next. This is a critical consideration in mmWave applications because any inconstancy in connectors can introduce errors in the measurement that will be amplified as the test frequency range increases. 

The test equipment will have a specific connector type on it, usually determined by the highest frequency that the test equipment can achieve. For example, if testing at 110 GHz, there will be a 1-millimeter connector on the test equipment; therefore, a mating connector of the same size will be required on the test cable assembly.

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Some common mmWave connector sizes include:

  • 2.92 mm – 40 GHz
  • 2.4 mm – 50 GHz
  • 1.85 mm – 67 and 70 GHz
  • 1.0 mm – 110 GHz

For applications where high signal density is needed, push-on connectors such as SMP, SMPM, and SMPS connectors are also ideal.

Measurement Type/Application 

Each application being tested will have specific environmental factors to consider, including, among others:

Most coaxial cable manufacturers provide guides to help assess these further considerations. For example, an ultra-high frequency application will require a cable that not only meets the frequency requirements but is also phase stable. By going with a high-frequency, phase stable cable, the type of connector available for use becomes limited. As the user moves through the process, options that match these additional considerations will be provided.

Flexibility 

In testing, many users are further interested in a cable’s flexibility and bend radius. Due to the nature of test environments, it is often essential to use a very flexible cable material that can be moved around on a test bench, either in a production or R&D environment.

Testing also often moves from module to module. High frequencies could require recalibration when a module or cable is moved. Using a coaxial cable that can bend and flex will significantly reduce the need for recalibration while maintaining stability.

Phase Stability

Another key aspect related to the need to constantly move the cables around is phase stability.  Movement introduces phase change, and the test assembly needs to maintain a very low rate of change to get accurate measurements. A robust cable is therefore critical to keep phase as stable as possible.

Additionally, when testing mmWave technologies such as 5G, the source and receiver might be running at two different frequencies at the same time. A phase stable assembly will further ensure that harmonics are not introduced back into the system. A phase-stable cable assembly utilizing a TF4, or microporous PTFE dielectric, coupled with a helically wound metalized interlayer, will help maintain a flexible, phase, and amplitude stable test assembly.

Power

It is also critical to understand what power levels the test cable can withstand. For example, a standard test cable is likely unable to handle a high-power application. In terms of mmWave applications, higher frequencies will push through less power because the cable diameter inversely shrinks, as previously mentioned. 

Impedance  

The standard impedance seen in test environments is 50 ohms. However, there are also 75-ohm impedances used in a video type of environment and also some lower frequency measurements. At any rate, impedance differences are essential to keep in mind when going through a test cable selection checklist.

Loss Budget

Everything related to radiofrequency (RF) characteristics involves trade-offs. Higher frequency equals smaller cable diameter, which also often results in higher losses on the cable. However, the loss can be negated using the VNA in a typical RF measurement application. A network analyzer has the ability to “null out” loss in the cable assembly, so when the device is hooked up to it, the cable’s loss will not reflect the measurements taken on the device itself. 

On the other hand, when a signal transitions from the circuit board to the connector, it is imperative to minimize reflections. At higher frequencies, these imperfections in the transition from a coaxial connector to a circuit board structure become more evident and may cause undesirable effects such as parasitic and spurious signal responses that result in return loss or insertion loss, VSWR spikes, and magnitude increases. In this case, if the signal integrity is not quite right and there is noise in the measurement, the test will not produce a correct reading. Therefore, a repeatable, low insertion loss cable that functions throughout the desired frequency range should be used to ensure high-fidelity measurement.  

Some Examples of mmWave Testing Applications

5G

The increased speed of 5G is partially achieved using higher-frequency radio waves with a much more extensive potential frequency range. This has introduced challenges for 5G testing, including repeatability, reliability, and reproducibility. 

For example, 50 GHz cables have been used in production environments for 5G modules. The stability and repeatability of this type of cable are paramount in producing reproducible results in the test.

Automotive Testing 

RF electronics technologies incorporating automotive radar have created safer, more efficient connected vehicles. Automotive radar sensors that use RF are increasingly used to detect the speed, range, and angle of objects in the automobile’s vicinity in complex and safety-critical applications such as advanced driver-assistance systems and autonomous driving. 

Unfortunately, they have also created new challenges for RF testing. Many new applications are moving away from the previous standard of 24GHz to 77 GHz plus mmWave ranges due to the wide bandwidth available in those bands. Wider bandwidth increases range resolution and accuracy by up to 20x in some applications and produces shorter wavelengths that enable smaller form factors.

This increases the complexity of test setups, requiring more test leads and connection points than ever before, along with new RF testing requirements. As a result, it’s necessary to revisit the way connection points and test leads are built and review the different types of connectors available, ensuring that the latest test assemblies work in concert with changes made by test equipment manufacturers. 

A 70 or 90 GHz cable will have the ability to test at both
the fundamental and harmonic frequencies required. 

RF Semiconductor Probing 

As the semiconductor industry continues to experience rapidly increasing demand with aggressive time-to-market goals, the ability to perform high-precision testing with the expedience of automation is critical. Highly sensitive RF testing processes are required to measure RF performance on the surface of a semiconductor wafer, requiring coaxial cable assemblies that can support the smooth, robotic movement of a probe and automatically and precisely touch down on the surface to measure performance and functionality. Because of the specific type of measurement required, the test assembly must also be able to pick up and move to repeat the process at a different location on the device surface. 

A test cable that provides a low loss, stable connection for up to 70 GHz testing is often optimal for use in probing measurement of RF circuits in wafer and semiconductor manufacturing. They can be specially designed for firm attachment to a manipulator device to enable the highly stable placement of a probe for making individual measurements at multiple points, automatically or semi-automatically, utilizing a solid tube at the attachment point. A low-profile design allows the probe heads (manipulators) to traverse their full range of motion without optical scope interference.

High-Frequency Testing

There is a new class of test cables designed specifically to accommodate the challenges addressed above for the higher frequencies needed for 5G, automotive systems, semiconductor probing, and other advanced mmWave application testing. These test leads offer very repeatable, low insertion loss cable options that function throughout the desired frequency range to ensure a high-fidelity measurement, with specific options available that cover 70 GHz to 90 GHz and up to 110 GHz if required. For precision and stability, phase-stable cable assemblies that utilize a microporous PTFE dielectric coupled with a helically wound metalized interlayer can also help maintain a flexible, phase- and amplitude-stable test assembly. 

Conclusion

When selecting any type of test cable assembly, designers should partner with a manufacturer with fully integrated design, production, assembly, and testing capabilities for customized solutions to meet the most demanding mmWave testing requirements. 

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