How to improve your RF measurement skills and get the most out of your RF equipment

New RF instruments have excellent accuracy and measurement capabilities that have far surpassed previous products, but these instruments cannot perform their functions if the signal does not reach a certain quality. Sound measurement implementation and related factors will enable users to fully understand the RF instrument they have invested in.

Perform stable RF measurements

Ideally, RF measurements should be easy to perform, but in reality, there are many challenges. Existing RF instruments can meet the main RF measurement tasks, such as power, frequency, and noise, but "getting results" does not necessarily mean "getting the right results." If you can build best practices in RF measurement operations, you can ensure stable, accurate, and repeatable measurement results.

Understand the terminology first

Terms such as "accuracy," "repeatability," "resolution," and "uncertainty" are often used interchangeably or incorrectly in RF applications, which reduces the accuracy of measurements. Before performing RF measurements, it is important to understand the important terms and their correct equivalents.

Compared to analog meters, digital displays are much easier to discern the correct reading on an analog meter. However, if a digital display shows values ​​with 3 decimal places, the user will not be able to understand the resolution and accuracy of the instrument or the measurement.

Even if the power can be displayed in thousands of dB or the frequency can be displayed in decimal units of Hertz, it does not mean that the instrument can measure changes within a few minutes. The number of digits displayed should exceed the measurement function of the instrument. In order to fully understand the functions of RF instruments, you should always refer to the specifications or data. The correct definition of terms will reduce the user's doubts about the measurement operation.

The following are some common key terms:
Resolution – the smallest change that an instrument can detect.
Repeatability – the number of times a measurement can be repeated under the same conditions and with the same results.
Uncertainty – the quantification of the unknown absolute value.
Accuracy – the actual/absolute value of a parameter that an instrument can measure within a known error range.

If you can estimate the sources of error information, you can often determine the uncertainty of the measurement operation. In addition to the terms mentioned above, you can also find relevant specification documents from the National Institute of Standards and Technology (NIST) or other standards organizations. Traceability ensures that all measurement instruments are defined by common standards.

"Specifications" are guaranteed performance of the equipment under test and are traceable to NIST-related calibration certifications. "Typical" refers to performance that has been fully tested but does not include measurement uncertainty. "Nominal" performance is auxiliary information and not all instruments have been measured.

Precision is the actual/absolute value of a parameter that an instrument can measure within a known error range, also known as X plus or minus Y. Without some error limits and units, a measurement value of "34" would have no meaning. Likewise, an error specification of "5" alone would have no meaning; but an error specification of "5%" would also have no meaning.

“5%” can mean “±5%”, or it can mean “+3%” or “-2%”; for example, the correct way to express accuracy is “34 V +/- 1 V”, “34 V +/- 1%”, or “34 V +2/-1 V”. A closer look at RF measurement terminology will help you become more familiar with its meaning. If you want to accurately communicate your measurements to others, you should first understand the results.

Know your device under test

The device under test (DUT) can significantly affect RF measurement operations. For example, temperature can affect stability and repeatability. Many RF devices and instruments do not compensate for temperature changes on their own, so the temperature must be stabilized first to minimize drift errors in measurement operations. There are also immediate environmental effects (such as whether there is air conditioning circulation, whether it is covered and paneled, whether it is indoors or outdoors, whether it is near a heat source) that should be considered as variables, and attention should be paid to warm-up times, DUT cooling conditions, and the surrounding environment, and maintaining a stable temperature.

In active devices, excess power may cause the device to heat up. For example, in a high-power amplifier, the DUT itself can reach a stable temperature, but the subsequent components may not. The switches and attenuators that connect the amplifier output often heat up. At this time, it may be necessary to find the uncertain signals generated by the amplifier, such as harmonics.

Power supply lines may generate environmental noise and directly affect the output; when the amplifier is in compression, if you measure its linear parameters (gain and phase), you will not get relevant results. Because all factors will affect the accuracy of RF measurement operations, before measuring the device, you must first understand the DUT, the operation method, and its impact on the RF measurement parameters to obtain meaningful results.

Find the range of uncertainty

If you want to compare the specifications of the RF test equipment with the measurement requirements of the DUT, it is also slightly insufficient; if the frequency of the RF measurement operation is higher and the instrument does not meet the required specifications, the range of uncertainty will be further expanded. Then, errors may occur in each measurement step, which will affect the overall result. When making an erroneous measurement, you should first find out the possible errors in the measurement operation and then find out the DUT that may be affected.

Users should understand the important operating specifications of the instrument, as well as the devices involved in each measurement step (including DUT); other related specifications should be understood, such as pairing, power, frequency response and noise figure. They should also understand the tolerance range of all parameters and remember the following parameters:
˙RF switching repeatability, aging, and power carrying;
˙Coupler directivity, connection line phase stability, and adapter insertion loss and return loss;
˙PCB circuit impedance quality, adapter card slot, and PCB transmission switch status;
˙Measurement operation electromagnetic interference (EMI) intensity.

Not formally considered are cooling, harmonics, spurs, and other nonlinear motions that may affect the measurement. The overall setup can be looked up to find the margin of error for each component to get actual data on measurement uncertainty. The source of error should also be identified to understand its impact on precision, repeatability, and uncertainty. This will allow for more accurate measurements and efficient budget and resource decisions.

Note all components and connections

The development, design, testing, and launch of a product all require huge investments. The company's survival may depend on the performance of a single product. For high-performance RF test equipment, the investment is difficult to estimate because it must meet or exceed the important specifications required by the current market. In addition to having a competitive advantage, it may also affect the company's subsequent revenue.

However, expensive, high-performance, and accurately calibrated DUTs and test systems are not enough. The quality and repeatability of the connecting components used to connect the devices must also be considered. If the key specifications can be improved by 1/10 or 1/5 dB, a high competitive advantage may be achieved.

For most standards, it is best to achieve a VSWR of 1:1.5, but the strength of the match may also affect the error, with an uncertainty of +/-0.35dB (approximately). When too much uncertainty is caused, it is impossible to achieve the key specification of 0.2 dB.

Other neglected items (such as cables, switches, attenuators, slots, adapters, and accessories) can also affect the overall measurement results. If you want to start the measurement operation, you should first achieve the required accuracy and then choose the appropriate components. According to the currently recognized standard, the performance of the measurement system should be 10 times the spectrum of the DUT parameter being measured.

If you already have a high-quality signal path, the next step is to deploy a complete measurement implementation; users should ensure that cables, connectors, and adapters are clean and stored. Even the most advanced cables and adapters will wear out, and if the parts are aged, they should be discarded. These are all consumables for testing operations, and the use of adapters should be gradually reduced.

In addition, hot-switching can be avoided as much as possible by regularly using a wrench and a line meter for adjustment. Also, please note that electrostatic discharge (ESD) should be performed in a timely manner. Even if the highest quality components are used between the test system and the DUT, too many connected parts may cause measurement errors.

Choose the right tool for the job

Depending on the parameters to be measured and the required accuracy, the RF equipment used to measure the DUT is also different. It is of course best to invest in equipment, but if only a part of the equipment can be used, it will be a waste of budget. If only RF power needs to be measured, then an RF power meter is of course better than a vector signal analyzer (VSA).

Scalar instruments can only measure intensity (amplitude), while vector instruments can measure both intensity and phase. Even if the measurement does not require phase, the phase information of the vector instrument can be used to correct errors because it can find and quantify unwanted reflections in the system.

When purchasing RF equipment, price often does not equate to performance. A high-quality swept-tuned spectrum analyzer can often take up a large portion of the budget; in terms of the original measurement performance of this instrument, although it can reach ± ​​1 dB or worse accuracy and can be used for general measurements, it cannot meet the measurement needs of absolute RF power. Similarly, if the instrument in use can reach a noise level of -140 dBm/Hz, this instrument will have difficulty measuring a DUT with a noise level of -155 dBm/Hz.

So choose the right tool for the job; if you buy equipment that is too powerful to measure the accuracy you need, you waste money and resources, and may crowd out budget allocations elsewhere. In some cases, cables and switches can even help improve measurement quality.

Developing measurement programs

Once you have built your own best practices, you can install them into your measurement procedures, which will facilitate communication across the team and allow for better repeatability and consistency in RF measurement results. For example, one of the common questions about measurement procedures is: “How often should I calibrate?”

Many RF instruments are extremely sensitive to environmental changes, so the equipment must be calibrated frequently; the need for high-precision measurements often affects the frequency of calibration. In either case, the calibration requirements of the RF equipment should be understood and included in the measurement procedure.

All processes from design, inspection, testing, to manufacturing will affect RF measurement performance. Users also need to consider the operating parameters that should be tested and inspected during the manufacturing process. Pre- and post-processes (such as reworking, welding, assembly, and insulation) that may affect accuracy, repeatability, and uncertainty should also be taken into consideration.

If you want to build a good RF implementation, you should also consider the relevant procedures. This can also simplify the learning and standardization process. And from the construction process to the product life, "consistency" will also affect the RF parameters and measurement results.

Improve the quality of your RF measurements

It is easy to make RF measurements, but it is a little difficult to measure accurately. If a complete implementation can be built and used in the program, the quality of RF measurements will be improved.

There are many ways to find and implement best practices. You should constantly seek to improve the quality of your RF measurements so that you can truly understand the key points of your measurements and apply them to your practice. The steps mentioned in this technical article are just basic concepts, from improving your RF measurement skills to fully utilizing the performance of your RF equipment.