Rapidly Advancing Technologies Create New Challenges for RF Test and Measurement
Test leads are used in just about every manufacturing space that deals with electronics, including avionics, wireless infrastructure, semiconductors, and more. The typical RF testing process involves a device under test connected to a vector network analyzer (VNA), oscilloscope or spectrum analyzer. The signal path from the instrument to the circuit board is critical, and the user needs to make sure the test setup does not introduce unwanted variables. This includes the test cable assembly, cable, and connectors.
As a result, RF testing requires unique coaxial cable and connector solutions. Cable assemblies must be durable enough to withstand extensive handling and continuous movement from frequent connecting and disconnecting, while maintaining precise repeatability of measurement and reliable electrical performance.
Additionally, as technology is rapidly advancing, the complexity of test setups has increased, requiring more test leads and connection points than ever before. This results in new RF testing challenges and requirements, creating the need to revisit how connection points and test leads are built as well as the different types of connectors that are available—while ensuring that the latest test assemblies work in concert with the changes made by test equipment manufacturers.
Evolving Requirements for RF Test Assemblies
Higher Frequency Ranges
Most RF systems used to work in the lower GHz range, but new technologies such as 5G are pushing that up to higher frequencies, as well as spanning different spectrum bands. Cables and connectors that may have been previously used will no longer perform in these higher frequencies.
For example, many RF test labs working in 4G LTE are outfitted with sub-10 GHz VNAs and associated test leads and accessories using Type N or 7/16 connector interfaces. To push the bandwidth into the full 5G spectrum, they will be required to invest in not only expensive RF instruments, but also millimeterWave test cables, adapters and board-level interconnects that can accommodate higher bandwidths without compromising signal integrity.
Furthermore, 5G applications are often tested in a production environment, where testing often moves from module to module. In high frequencies, this could translate to recalibration every time a module or cable is moved. However, using a cable that can bend and flex, either in R&D or in a production environment, will greatly reduce the amount of recalibration required while maintaining 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 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 cable assembly utilizing Times Microwave’s TF4™, coupled with a helically wound metalized interlayer, is recommended to maintain a flexible, phase stable test assembly.
When a signal transitions from the circuit board to the connector, it is imperative to minimize reflections as much as possible. At higher frequencies, imperfections in the transition from a coaxial connector to a circuit board structure become more apparent.
These imperfections, if not designed properly, can cause parasitic and spurious signal responses. They manifest in either return loss or insertion loss, spikes, and magnitude increases, both of which are undesirable. If the signal integrity is off and there is noise in the measurement, the test will not produce an accurate device reading. To ensure a high fidelity measurement, a very repeatable, low insertion loss cable that functions throughout the desired frequency range should be used.
Frequency, time delay, and physical properties such as length, dielectric constant and propagation velocity all affect electrical length. Coaxial cables must contain a consistent dielectric material throughout the length of the cable to create a constant velocity factor. Even if the material is consistent, environmental and handling factors such as temperature fluctuations, flexure, twisting, pulling, and crushing can still alter the cables’ electrical properties.
Materials will expand or contract as temperature oscillates. As that happens, the dielectric constant changes, altering a signal’s velocity of propagation. Therefore, materials used to ensure phase stability and amplitude include Times Microwave’s TF4™ dielectric, which allows good signal transmission to take place.
Times Microwave Systems Clarity test cables are designed with precision and stability in mind. Utilizing the flexible TF4 dielectric allows for accurate S parameter measurements even when movement occurs in the production environment; the proven solutions cover a wide frequency range from 18 GHz to 70 GHz.
The Clarity line includes highly stable RF cables with flex in a very robust package for accurate measurement and features excellent phase stability, extremely low loss, an ergonomic molded boot and a large connector selection. The newest addition to the proven Clarity line is Clarity 70, a high end, yet cost effective product for testing up to 70 GHz.
Unlike competing solutions that take months to deliver, Clarity products arrive in just weeks. Furthermore, as a manufacturer with fully integrated design, production, assembly and testing capabilities, Times Microwave can deliver RF products that meet the most demanding requirements, including customized solutions to meet any specific need.
Clarity 70 in Action: Automotive Components Testing
As the complexity and advancement of automotive systems increase, the amount of radar systems has increased as well. These include visual and radar-based sensor systems such as autonomous vehicle systems, collision avoidance, automated braking and more.
There are primarily two bands that these automotive systems operate in: 24 GHz and 67 GH. Being able to test these components and systems, not only their fundamental frequencies, but also how they behave at their harmonics, can be a challenge. In the case of components operating at fundamental frequencies of 18 GHz, 22-28 GHz, and 33 GHz, it is necessary to characterize them at the second and third harmonic frequencies. This often lands at the 67 GHz range. Having the necessary equipment to accurately test at this frequency range is paramount (Figure 3).
The test equipment is generally a vector network analyzer and test cable assemblies. Having a test cable assembly that does not introduce errors, VSWR spikes, or excessive insertion loss is required. Stability is also key as the frequency range of the test is increased. This further requires precision connectors as any type of inconstancy can introduce errors in the measurement.
For the test setup, Clarity 70 becomes a key component in producing accurate measurements due to its ability to make precision connections to both the VNA and DUT, and excellent stability with flexure. This is demonstrated in Figure 5.
The Clarity comes stock with 1.85 mm male connectors and hex nuts, allowing clean and easy attachment of connections up to 70 GHz.
In performing tests, the phase stability of the Clarity 70 became apparent as measurements are solid and the need for calibration between DUT tests is minimized.
Figure 5 shows a typical setup for testing automotive sensors at various frequencies. The tests were performed at the fundamental as well as the harmonics, giving the designer a clear indication of how the device will perform in the vehicle.