How engineers can get the most out of RF instruments

Overview

Modern amplifier RF instruments have impressive measurement capabilities and accuracy far exceeding their predecessors. However, these instruments cannot reach their full potential without providing high-quality signals. Good measurement methods and considerations can ensure that you get the most out of your RF instrument investment.

Get reliable RF measurements

RF measurements are often simple in theory, but difficult to implement. You can easily obtain core RF measurement results such as power, frequency, and noise from the wide range of measurements provided by modern RF instruments. However, there is a world of difference between getting results and getting the right results. By implementing best practices throughout your RF measurement process, you can ensure a reliable, accurate, and repeatable result.

Understanding the terminology

Terms such as accuracy, repeatability, resolution, and uncertainty are often mixed and misused in a variety of RF applications, adding confusion and reducing the confidence in measurements. It is necessary to understand some key terms and the context in which they apply when making RF measurements.

For example, a digital display on an instrument is easier to use than an analog measuring instrument when trying to discern the correct reading on a close scale. However, even if a digital display gives three decimal places for a measurement, you cannot infer the resolution and accuracy of the instrument and its measurements.

Just because a display gives a power of one thousandth of a decibel or a frequency of a fraction of a Hertz does not mean that the instrument has the ability to measure these subtle changes. Often the number of digits displayed far exceeds the instrument's ability to measure at that level. To fully understand an RF instrument's measurement capabilities, it is often necessary to refer to the instruction manual or data sheet.

Consistent definitions can reduce potential confusion in your measurements. Here are some key terms you’ll often see in use:

Resolution – The smallest change that an instrument can reliably detect

Repeatability - the ability to obtain the same result when performing the same measurement multiple times under the same conditions.

. Uncertainty – the quantification of the lack of knowledge about the exact value of a measurement

. Accuracy - the ability of an instrument to measure the actual/absolute value of a parameter within a certain error range.

Uncertainty always exists, and an assessment of the sources of error can help determine the uncertainty of the measurement. In addition to the above, there are some related terms that are useful when describing performance based on the specifications of the National Institute of Standards and Technology (NIST) or other standards organizations. Describability is necessary to ensure that all measurement devices have a common absolute benchmark. The so-called "specification" means that the performance is guaranteed to be produced by test equipment with calibration traceable to NIST. "Typical" often means that the performance is 100% tested, but does not include measurement uncertainty. "Symbol" performance is usually supplementary information and is not a universal measurement on every instrument.

Precision is the ability of an instrument to measure the absolute value of a parameter within a specified error limit. In other words, X plus or minus Y, a measurement of 34 is meaningless without error limits (and units). Likewise, an error specification of 5 is useless, and even a 5 percent error specification is barely helpful. Is that plus or minus 5 percent, or is it plus 3 percent and minus 2 percent? To be accurate, precision should be specified like 34 V +/- 1 V, 34 V +/- 1%, or 34 V +2/-1 V.

Take the time to learn the terminology of RF measurements and become familiar with their meaning. The more accurately you can express your measurements, the easier it will be to understand and trust your results.

Know your device under test

The performance of the device under test (DUT) can significantly affect RF measurements. For example, temperature affects stability and is therefore closely related to repeatability. Many RF components and RF instruments do not have internal compensation for temperature changes. Therefore, they must operate at a stable temperature to minimize measurement errors caused by temperature drift. The current environment (for example, air conditioning cycles on and off, covers and panels removed or added, outdoors or indoors, and proximity to heat sources) can have a significant impact. Attention needs to be paid to proper warm-up time, cooling requirements of the DUT, and the surrounding environment to ensure temperature stability.

In active devices, too much power can cause heating. For example, when testing a high-power amplifier, the device under test itself may be kept at a stable temperature, but what about downstream components? Look for switches or attenuators that are being heated by the amplifier’s output. Look for anomalies such as harmonics generated by the amplifier. Power lines are susceptible to environmental noise that can be directly added to the output. Also, it is frustrating to measure the linear parameters (gain and phase) of an amplifier only to find out later that the amplifier is also in compression. All of this affects the accuracy of RF measurements. Understanding the device itself, how it operates, and how it affects RF measurement parameters before it is tested will yield meaningful results.

Identify areas where uncertainty arises

It is not enough to simply match the data sheet specifications of the RF test equipment to the test requirements of the device under test. This is particularly evident in higher frequency or mismatched RF measurements, which, among other things, amplify uncertainty. Errors introduced at each step of the measurement will skew the overall result. When an incorrect measurement result occurs, you should first suspect that the measurement is wrong before questioning the device under test.

Understand the key operating specifications of the instrument and the device under test during the measurement. Among other specifications, understand matching, frequency response, noise figure, and power. Also, understand the tolerances for these parameters. Remember the following:

. RF switch repeatability, aging and power handling

. Directivity of the coupler, phase stability of the cable, and insertion loss and return loss of the adapter

. Impedance quality of circuit board traces, device sockets and circuit board switching

. Electromagnetic Interference ( EMI ) Radiation and Coupling in Measurements

Items that are often not carefully considered, such as cooling, harmonics, excitation, and other nonlinear behavior can also add errors to your measurements. Look at your entire test equipment and determine the error distribution of each piece to get a realistic estimate of the expected measurement uncertainty. Identify the causes of error and their impact on accuracy, repeatability, and uncertainty. This will lead to better results, allowing you to allocate budget and resources more effectively and meaningfully.

Pay attention to all connections and components

The cost of developing, designing, testing, and bringing a product to market is a considerable investment. A company's success or failure depends on the performance of its products. Spending on high-performance RF test equipment is significant because it demonstrates that a product meets or exceeds key technical requirements that are tied to market share. It also represents a competitive advantage and a major source of incremental revenue for a company.

However, it is not enough to have a high-performance, expensive, well-calibrated test system and an equally high-performance DUT. Equal attention should be paid to the quality and repeatability of the attached connections and components in the DUT test system. Perhaps a tenth or two improvement in a key specification is a competitive advantage. Under most standards, a source and load match (standing wave ratio) of 1:1.5 is good, but this level of matching will introduce an error of (approximately) +/-0.35 dB of mismatch uncertainty. A key specification of 0.2 dB cannot be proven with so much uncertainty.

Overlooked items such as cables, switches, attenuators, connectors, jacks, adapters, and accessories can detract from the overall measurement. Start with the accuracy you need, and then select components appropriate for the measurement. A good rule of thumb is to make the test system performance ten times the parameters of the device under test you are testing.

With a high-quality channel, the next step is to adopt good measurement methods. Make sure you properly clean and store cables, connectors, and adapters. Even the best cables and connectors fail and must be discarded; they are consumables in the test process. Take steps to minimize the use of adapters and ensure that torque wrenches and connection specifications are used regularly to minimize hot switching. Remember proper electrostatic discharge (ESD) practices. Even the highest quality components between test systems and the cascade of the device under test can introduce measurement errors.

Choose the right tool for the job

Which parameters need to be measured and what level of accuracy is required largely determines the RF equipment you choose to test your device under test. The best choice is a safe strategy, but it will waste budget resources that you can use to measure other aspects. If RF power is the only quantity to be measured, an RF power meter may be a better choice than a vector signal analyzer.

Scalar instruments measure only magnitude (amplitude), while vector instruments measure both magnitude and phase. Even if you don’t need to measure phase, consider that vector tools can provide better error correction because the phase information can be used to quantify unwanted reflections in the system.

Equating price with performance is not the best rule to follow when buying RF instruments. A high-quality swept-tuned spectrum analyzer will cost you a lot of money. While they are excellent instruments for the measurements they are used for, with typical accuracy of ± 1 dB or less, they are not up to the task of measuring absolute RF power. Likewise, if the instrument you are using has a -140 dBm/Hz noise floor, it will have difficulty measuring the -155 dBm/Hz noise floor on the device under test.

Consider the right tool for each job. Paying too much for accuracy and unnecessary measurements is not only a waste of money and resources, it limits funds for other things such as cables and switches that could be more beneficial to measurement quality.

Development Process

Once you have identified and implemented best practices, you need to evolve them into routines or processes that are clearly understood and communicated across the group. This will result in better repeatability and consistency in RF measurement results.

For example, a common question about the process is, “How often should I calibrate?” Many RF instruments tend to be sensitive to environmental changes. If this is the case, more frequent calibration may be necessary. High accuracy requirements may also require more frequent calibration. Regardless, the requirement for RF calibration must be kept in mind and made a strictly adhered to process.

Processes throughout the design, validation, testing, and manufacturing phases can impact RF measurement performance. Consider which operating parameters need to be validated, which should be tested in manufacturing, and the upstream and downstream processes (e.g., rework, soldering, assembly, and shielding) that impact RF measurement accuracy, repeatability, and uncertainty.

Process is important to acquire and execute good RF methods. This facilitates the regular learning and standardization of good methods. Consistently following established processes throughout the product lifecycle can have a significant impact on RF parameters and their correct measurement.

Improving the quality of RF measurements

Making RF measurements can be easy; however, doing them well can be challenging. Using sound methods and identifying them during the measurement process can improve the quality of your RF measurements. There are many ways to determine and implement best practices. Improving RF measurements is a continuous process of gaining knowledge and putting it into practice. The steps described in this article will help establish a system to improve RF measurement techniques and get the most out of your RF test instruments.