Modern RF instruments have impressive measurement capabilities and accuracy that far exceed those of their predecessors. However, these instruments cannot reach their full potential without providing a high-quality signal. Complete measurement methods and precautions can ensure that you can fully obtain the benefits of your investment in RF instruments. Obtaining Reliable RF Measurements RF measurements are often simple in theory but difficult to put into practice. You can easily obtain core RF measurements such as power, frequency, and noise from the wide range of measurements available on contemporary RF instruments. But there's a huge difference between getting results and getting them right. By implementing best practices throughout your RF measurement process, you can ensure reliable, accurate, and repeatable results.
Understand terminology
Terms such as accuracy, repeatability, resolution, and uncertainty are often confused and misused in a wide variety of RF applications. Adds confusion and reduces measurement reliability. It is necessary to understand some key terminology and the environment in which they apply when making RF measurements. For example, when you want to identify the correct reading on a similar scale, a digital display on the instrument is easier to use than an analog measuring instrument. However, even if a digital display gives three digits after the decimal point for a measurement, you cannot infer the resolution and accuracy of the instrument and its measurements. Just because a display gives power in thousandths of a decibel or frequency in fractions of hertz does not mean that the instrument has the ability to measure these small changes. Often the number of digits displayed on these displays far exceeds the instrument's measurement capabilities at this level. To fully understand the measurement capabilities of an RF instrument, it is often necessary to refer to the instruction manual or data sheet. Consistent definitions reduce possible confusion in your measurements. Here are some key terms you'll see often in use: Resolution - the smallest change that an instrument can reliably detect Repeatability - the ability to obtain the same result by taking the same measurement multiple times under the same conditions Uncertainty - the change in the measured value The exact value of a measurement lacks the cognitive component of quantitative precision - there is always uncertainty in an instrument's ability to measure the actual/absolute value of a parameter within a certain error range, and an estimate of the source of the error can help determine the uncertainty of the measurement. In addition to the above, there are related terms that are useful when describing performance based on documentation from the National Institute of Standards and Technology (NIST) or other standards bodies. Descriptability is necessary to ensure that all measuring devices have the same absolute reference. By "specification" we mean guaranteed performance produced by test equipment calibrated traceable to NIST. "Typical" often means that performance is 100 percent tested, but does not include measurement uncertainties. "Token" 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 range. 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 an error specification of 5 percent is of little help. So is it plus or minus five percent, or plus three percent and minus two percent? To be accurate, the accuracy should be specified like this, for example 34 V/- 1 V, 34 V/- 1, or 34 V 2/-1 V. Please take the time to understand RF measurement terminology and become familiar with their meaning. The more precise you are about your measurements, the more understandable and trustworthy your results will be.
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 devices 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 (e.g., air conditioning cycles on and off, coverings and panels removed or added, being outdoors or indoors, and proximity to heat sources) can have a big impact. Attention needs to be paid to appropriate warm-up time, cooling needs of the device under test, and the surrounding environment to ensure temperature stability.
In active devices, excessive power can cause heating. For example, when testing high-power amplifiers, the device under test itself can remain temperature stable, but what happens to the downstream components? See if any switches or attenuators are getting heated due to the amplifier's output. Look for unusual signals produced by the amplifier, such as harmonics. Power lines are susceptible to ambient noise that can be superimposed directly onto the output. Furthermore, it is frustrating to measure an amplifier's linear parameters (gain and phase) only to later find that the amplifier is compressed at the same time. All of these affect the accuracy of RF measurements. Understanding the device itself, how it operates, and its impact on 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 description of the RF test equipment to the test requirements of the device under test. This is especially evident in higher frequency or mismatched RF measurements where, among other things, there are amplification uncertainties. Error introduced at each step of the measurement can skew the overall results. When an incorrect measurement result occurs, you should first suspect a measurement error before questioning that there is a problem with the device being measured.
Please understand the key operating specifications of the instrumentation and the device under test during the measurement process. Among other specifications, learn about matching, frequency response, noise figure, and power. At the same time, it is also necessary to understand the allowable errors of these parameters. Keep in mind factors such as: RF switching repeatability, aging and power handling coupler directivity, cable phase stability and adapter insertion loss and return loss impedance quality of circuit board traces, device sockets and circuit board switching transitions Electromagnetic Interference (EMI) Radiation and Coupling in Measurements Items that are not usually taken seriously, such as cooling, harmonics, excitation, and other nonlinear behavior, can also add errors in measurements. Look at the entire test equipment and determine the error distribution for each piece to get a realistic* estimate of the expected measurement uncertainty. Find out the causes of errors and their impact on accuracy, repeatability and uncertainty. This results in better results, allowing you to allocate budget and resources more efficiently and meaningfully.
Pay attention to all the 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 meaningful because it demonstrates that the product meets or exceeds key technical requirements tied to market share. Furthermore 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 the device under test that performs equally well. The quality and repeatability of the ancillary connections and components in the device under test test system should also be given. equal attention. Perhaps a tenth or two-tenth improvement on a key technical indicator is a competitive advantage. A match (standing wave ratio) of 1:1.5 between source and load is good by most standards, but this level of matching will introduce a false mismatch uncertainty of (approximately) /-0.35 dB. A key technical indicator of 0.2 decibels cannot be proven under the influence of so many uncertainties. Easily overlooked items such as cables, switches, attenuators, connectors, sockets, adapters and accessories can detract from the overall measurement. Start with the accuracy required and then select components suitable for the measurement. A good rule of thumb would be to 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 clean and store cables, connectors, and adapters properly. Even the best cables and connectors should be discarded if they fail; they are consumables in the testing process. Take steps to minimize the use of adapters and ensure that torque wrenches and connection specifications are used routinely to minimize hot switching. Remember proper electrostatic discharge (ESD) practices. Cascading even the highest quality components and devices under test between test systems can introduce measurement errors.
Choosing the Right Tool for the Job What parameters need to be measured and to what level of accuracy is required largely determines the choice of RF equipment needed to test the device under test. The best option is a safe strategy, but it wastes budget resources you could use to measure other things. If RF power is the only quantity to be measured, an RF power meter may be a more ideal choice than a vector signal analyzer. Scalar instruments measure only amplitude (amplitude), while vector instruments measure both amplitude and phase. Even if phase measurement is not required, consider that vector tools 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 purchasing RF instruments. A high-quality swept-tuned spectrum analyzer will cost you a lot of your budget. Although they are excellent measuring instruments for existing measurement purposes, with typical measurement accuracy of ±1 dB or less, they struggle to measure absolute RF power. Likewise, if the instrument used has a noise floor of -140 dBm/Hz, it will be difficult to measure the noise floor of -155 dBm/Hz 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 that can be spent on other aspects such as cables and switches that are more beneficial to measurement quality. Developing a Process Once you have identified and implemented your best practices, evolve them into routines or processes that are clearly understood and communicated across the group. This results in better repeatability and consistency of RF measurements.
For example, a common question about the process is "How often should I calibrate?" Many RF instruments are often sensitive to environmental changes. If this is the case, more frequent calibration may be necessary. High accuracy requirements may also require more frequent calibrations. Regardless, the requirements for RF calibration must always be kept in mind and made a strictly followed process. Processes throughout the design, verification, test, and manufacturing phases can impact RF measurement performance. Consider which operating parameters require validation and which should be tested in manufacturing, as well as the upstream and downstream processes (e.g., rework, soldering, assembly, and shielding) that affect RF measurement accuracy, repeatability, and uncertainty. Process is important in obtaining and executing a good RF approach. This facilitates routine learning and standardization of good methods. Consistently following established procedures throughout the product life cycle 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 more challenging. Using sound methods and identifying them during the measurement process can improve the quality of RF measurements. There are many ways to determine and execute the best approach. Improving RF measurements is a continuous process of gaining awareness and putting it into practice. This article describes the steps to help establish a system for improving your RF measurement techniques and getting the most out of your RF test instruments.