How to Improve Accuracy of RF Measurements

Radio Frequency measurements are often challenging, complex, and require higher precision in most applications. Since there are many components and factors involved in RF measurements, careful evaluation of each stage is necessary to carry out proper RF tests. What are the factors affecting the accuracy of RF measurements?

Test Equipment

One of the most significant factors in every RF measurement is the test equipment used for specific applications. Proper RF equipment ensures the accuracy of each test according to the device specification. Since RF technology is advancing each year, RF test equipment has to be upgraded to perform complex tasks.

The test instrument specification from the manufacturer will give an overview of its capabilities and limitations. High-end test equipment is usually recommended for better results and repeatability.

Calibration: periodic calibration of test equipment performed by certified labs or manufacturers ensures stability and accuracy of test instruments. Calibration of equipment before measurement is necessary for sensitive devices with tighter specifications.

Instrument warm-up: all electronic test equipment must be switched on for a minimum of 30 minutes before use for stability of test parameters.

Ambient temperature: Most of the RF test equipment has an operating temperature range recommended by manufacturers for optimum performance. Test labs will have to maintain the temperature within a specified range.

RF Cables

RF test cables are also an important component of radio frequency measurement. Test cables are used to make a connection between the DUT (device under test) and the measuring equipment. RF cables are available in many specifications, sizes, and shapes to fit various applications.

The selection of proper RF test cables is significant to ensure measurement accuracy and repeatability.

Find more about the specifications of RF test cables and applications

RF Test Accessories

Multiple RF accessories are required to construct complex test environments. Most common RF test accessories are RF Isolators, Circulators, attenuators, directional couplers, power combiners,  power dividers/splitters, termination loads, etc.

RF accessories are designed to perform within a specific range of parameters like frequency, power handling, connector types, and physical dimensions. Regular verification of parameters according to specifications of each accessory and visual inspection for damage are recommended for test accuracy.

Sensitive RF accessories must be kept apart from other devices, which could contribute coupling effect or magnetic flex difference. For example, sore RF Isolators have a magnetic core inside. Other ferromagnetic materials within its close proximity could change the performance during operation.

RF accessories used in high-power applications produce heat. Proper air circulation is necessary to protect those accessories from damage (thermal damage).

External Noise

Low-power signals are extremely vulnerable to noise from various external sources. The device under test (DUT) is often required to be placed inside a shielding box or chamber to avoid external interference.

Major sources of external RF noise are transmitting devices like mobile base stations, Wi-Fi routers, mobile devices, and other high-power TV and radio transmitting stations.

At outdoor RF test locations, engineers will have to analyze the external noise level and its impact on their test system (like intermodulation).

Proper Connection

Poor connection between instruments, devices, accessories, and cables in the test system contributes to instability and measurement accuracy. In order to make sure a stable connection, cables and signal joints must be tightened with the recommended torque.

Periodic maintenance

All active electronic devices and passive components may degrade their performance over time. Regular visual inspection of accessories and functional tests will ensure individual units are within their specifications. One faulty unit of a complex test setup could affect the performance of the whole system, and troubleshooting is time-consuming.

Connectors of all devices and accessories must be clean to avoid any intermodulation distortion. At higher frequencies, dirt and oxidized connector pins cause distortion and unexpected results.

Golden samples

In order to make sure the RF test setup is stable and measurements are repeatable, test engineers often use golden samples (reference DUT) to compare previously measured results. If the test setup is stable, the golden sample will give exact same results as previously tested.

Measurement Uncertainty and Error Budgeting

Measurement uncertainty should also be taken into consideration in advanced RF testing. Each instrument, cable, and other accessory is a source of some error, and this error is cumulative in a complicated system of tests.

Engineers usually compile an error budget, which uses parameters like instrument accuracy limits, connector repeatability, temperature drift, and calibration tolerance.

Measuring these uncertainties allows easier estimation of the confidence level of the measurement results and provides adherence to the international standards such as ISO/IEC 17025.

New Challenges at Higher Frequencies

With RF applications and applications in mmWave and 5G frequency bands, keeping inaccuracy at an even more challenging task. Cables have more signal loss at higher frequencies, connections are more vulnerable to faults, and external noise is likely to introduce noise that will disrupt the cable and corrupt data.

Special low-loss cables, phase-stable connectors, and automated calibration units are typically required to minimize the amount of errors in the test setups. Also, the automation software is increasingly gaining popularity as a means of controlling test instruments, recording measurements, and raising the repeatability of large-scale or high-frequency measurements.

Conclusion

RF measurement is a challenging task due to its complexity and the numerous factors involved in its outcome. Engineers have to ensure every step from the selection of test equipment to each evaluation stage. Complex test systems are designed with the help of automation tools for continuous monitoring and real-time reporting.