Getting reliable RF measurements
RF measurements are often simple in theory, but difficult to implement. You can easily obtain core RF measurements such as power, frequency, and noise from the wide range of measurements offered by contemporary RF instruments. However, getting the results and getting them right are worlds apart. By implementing optimal methods throughout your RF measurement process, you can ensure a reliable, accurate and repeatable result.
Because a display gives a thousandth of a decibel of power or a fraction of a hertz of frequency doesn't mean that the instrument has the ability to measure these small variations. Often the number of bits in these displays far exceeds the instrument's ability to measure at that level. To fully understand the measurement capability of an RF instrument, it is often necessary to refer to the manual or data sheet.
Consistent definitions can minimize 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 for the same measurement performed multiple times under the same conditions
Uncertainty - the quantification of the lack of a perceived portion of the exact value of the measurement being made
Accuracy - the ability of an instrument to measure the real/absolute value of a parameter within a certain margin of error The ability to measure the actual/absolute value of a parameter
Uncertainty is always present, and an *estimate* of the source of the error can help determine the uncertainty of the measurement. In addition to the above, there are a number of related terms that are useful when describing performance based on descriptive documents from the National Institute of Standards and Technology (NIST) or other standards bodies. Descriptivity is necessary to ensure that all measurement devices have an absolute datum that is ****identical. The term "normative" refers to the assurance that the performance is generated by test equipment whose calibration is traceable to NIST. Typical" often means that the performance is 100 percent tested, but does not include measurement uncertainties. Symbolic" performance is usually supplemental information, not a universal measurement on every instrument.
Accuracy is the ability of an instrument to measure the absolute value of a parameter within a specified margin of error. In other words, X plus or minus Y, without error limits (and units) a measurement of 34 is meaningless. Similarly, an error statement of five is useless, and even an error statement of five percent is hardly helpful. So is it plus or minus five percent, or plus three and minus two percent? To be accurate, accuracy should be specified like this, for example, 34V+/-1V,34V+/-1%, or 34V+2/-1V.
Please take the time to learn more about RF measurement terminology and familiarize yourself with its meaning. The more accurately you can articulate your measurements, the more understandable and reliable the results will be.
Know your DUT
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 devices and RF instruments do not have internal compensation for temperature variations. Therefore, they must operate at a stable temperature to minimize measurement errors caused by temperature drift. The current environment (e.g., air conditioning cycles on and off, removal or addition of covers and panels, being outdoors or indoors, and proximity to heat sources) can have a significant effect. Attention needs to be paid to proper warm-up time, the cooling needs of the unit under test, and the surrounding environment to ensure temperature stability.
In active devices, excessive power can lead to heat generation. For example, when testing a high-power amplifier, the device under test itself can remain temperature stable, but what happens to downstream components? Look to see if any switches or attenuators are being heated by the output of the amplifier. Look for unusual signals, such as harmonics, generated by the amplifier. Power lines are susceptible to ambient noise that can be directly superimposed on the output. Also, it is frustrating to measure the linear parameters of an amplifier (gain and phase) only to find out later that the amplifier is simultaneously compressed. 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.
Identifying areas where uncertainty occurs
It is not enough to simply match the RF test equipment's data sheet description with the test requirements of the device under test. This is particularly evident in higher frequency or mismatched RF measurements where, among other things, there is amplification uncertainty. Errors introduced at each step of the measurement can bias the overall result. When an incorrect measurement result occurs, you should first suspect an error in the measurement before questioning the existence of a problem with the device being measured.
Learn the key operating specifications for the instrumentation and the device under test during the measurement process. Among other specifications, understand matching, frequency response, noise figure, and power. Also, understand the tolerances for these parameters. Keep the following elements in mind:
"RF switching repeatability, aging and power handling
"Directivity of couplers, phase stability of cables, and insertion loss and return loss of adapters
"Impedance quality of circuit board wiring, device sockets and switching transitions on and off of boards
"Electromagnetic interference (EMI) radiation and coupling in measurements
Some items that are not typically considered carefully, such as cooling, harmonics, excitation, and other nonlinear behaviors, can also add to errors in measurements. Look at the entire test setup and determine the error distribution of each piece to get a realistic *price* for the expected measurement uncertainty. Identify the causes of errors and their impact on accuracy, repeatability, and uncertainty. This yields better results and allows you to allocate your budget and resources more efficiently and meaningfully.
Note all the connections and components
The expense 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 the product meets or exceeds key technical requirements tied to market share. In addition it represents a competitive advantage and a major source of increased revenue for the company.
However, it is not enough to have a high-performance, expensive, well-calibrated test system and an equally high-performing DUT, but equal attention should be paid to the quality and repeatability of the ancillary connections and components in the DUT test system. Perhaps a one-tenth or two percent improvement in a key technical specification is a competitive advantage. A source-to-load match (VSWR) of 1:1.5 is good by most standards, but this degree of match will introduce a false (approximately) mismatch uncertainty of +/-0.35 dB. It is unlikely that a 0.2 dB Critical Technical Indicator can be justified with this much uncertainty affecting it.
Easily overlooked items such as cables, switches, attenuators, connectors, sockets, adapters and accessories can be detrimental to the overall measurement. Start with the accuracy needed and then select the right components for the measurement. A good rule of thumb will result in a test system that performs ten times better than the parameters of the unit under test you are testing. With a high quality channel, the next step is to use good measurement methods. Make sure you clean and store cables, connectors, and adapters properly. Abandon the use of even the best cables and connectors when they fail; they are consumables in the testing process. Take steps to minimize the use of adapters and make sure that torque wrenches and connection specifications are used routinely to minimize thermal cutover. Remember proper electrostatic discharge (ESD) practices. Measurement errors can be introduced by cascading even the highest quality components and devices under test between test systems.
Selecting the right tool for the job
What parameters need to be measured and what level of accuracy needs to be achieved largely determines the choice of RF equipment needed to test the device under test. The best choice is a safe strategy, but it wastes budgetary resources that you can spend on other aspects of the measurement. 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 amplitude (amplitude), while vector instruments measure amplitude and phase. Even if there is no need to measure phase, it is important to consider that vector instruments provide better error correction because 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-tuning spectrum analyzer will cost you a lot of your budget. While they are excellent measuring instruments for the measurement purposes for which they are already used, with typical measurement accuracies of ±1 dB or less, they struggle to meet the requirements for measuring absolute RF power. Similarly, if an instrument is used that has a background noise of -140 dBm/Hz, it will have difficulty measuring a background noise of -155 dBm/Hz on the device under test.
Please consider the right instrument for each job. Paying too much for accuracy and unneeded measurements is not only a waste of money and resources, it limits the funds available for other things such as cables and switches that are more favorable to measurement quality.
Developing a process
Once you've identified and implemented the optimal methods, evolve them into a practice or process that can be clearly understood and communicated between groups. This can lead to better repeatability and consistency of RF measurements. Processes throughout the design, verification, test, and manufacturing phases can affect 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 affect RF measurement accuracy, repeatability, and uncertainty.
Processes are important for obtaining and executing good RF methods. This facilitates routine learning and standardization of good methods. Consistently following established processes throughout the product life cycle can have a significant impact on RF parameters and their proper measurement.
Making RF measurements can be easy; however, getting it right can be challenging. Using sound methods and identifying them during the measurement process can improve the quality of RF measurements. There are many ways to identify and implement optimal methods. Improving RF measurements is a continuous process of gaining awareness and putting it into practice. The steps presented in this article help to establish a system to improve RF measurement techniques and to get the most out of your RF test instruments.
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