adio 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.
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.
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.
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
- 2.92 mm – 40 GHz
- 2.4 mm – 50 GHz
- 1.85 mm – 67 and 70 GHz
- 1.0 mm – 110 GHz
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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.
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.
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.
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.
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.
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.