Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.

In order to maintain the performance of wireless communications systems, technicians and engineers are increasingly being asked to head into the field to make RF/microwave measurements—often in less-than-hospitable conditions. While the term “precise microwave measurements” may suggest images of benchtop instrumentation in a laboratory setting, handheld instrumentation has in fact recently become available with the precision necessary to match the performance of these more expensive laboratory counterparts.

Modern  all-in-one portable instruments now offer technicians and engineers the capabilities to make highly accurate microwave measurements of network, spectrum, power, and frequency with results that correlate to within hundredths of a decibel to benchtop instruments. But not all modern handheld analyzers are created equal. To ensure that a particular handheld instrument of interest can provide the required level of performance, it is necessary to compare or correlate its performance to that of a comparable benchtop instrument.

Examining the specifications found on an instrument vendor’s datasheets is a natural place to start, although it may be difficult to directly compare the two instruments since specifications are often listed under unique operating conditions. A handheld instrument, for example, may be specified for harsh operating conditions, while its benchtop counterpart may be specified for temperature-stabilized environments. Even examining technical literature with cited examples comparing benchtop instruments to handheld instruments is difficult since such material is often very limited.

Bridging this gap requires the actual correlation of measurements recorded using several benchtop instruments to measurements recorded using a handheld instrument. Doing so is the only way to ensure the handheld instrument offers the precision microwave measurements necessary for accurate RF/microwave testing in the field.

Why Correlate?

Correlation describes the relative agreement for a set of data measured from the same device under test (DUT), but recorded from different instruments—for instance, a high-performance benchtop instrument and a handheld instrument. The stronger the agreement, the greater the probability that one measurement technique or instrument can replace another in a given application. In this case, it can be used to determine whether or not a given handheld instrument can effectively provide the same level of measurement accuracy as a benchtop instrument. But why is this correlation so important to ensuring the successful operation of the DUT?

At each stage of a product lifecycle, a DUT (or product) is measured with a variety of different instruments, typically calling for characterization of a unique set of performance requirements. During the early product stages, for example—which include design validation, product development, and manufacturing test—measurements are usually obtained with benchtop instrumentation in controlled laboratory environments. For research and development (R&D) testing, the benchtop instrumentation is selected based on its performance and features.

For production test, important parameters include fast measurement speed and lower instrument cost. Once a new electronic component or system is installed in the field, many of the laboratory measurements must be repeated in the field to validate the performance of the product. Additional field testing may also be required during periodic maintenance and occasional repair operations. Often these field tests are made with a handheld instrument under the most challenging test conditions, including harsh outdoor weather.

In all field testing, correlating the data to laboratory measurements is critical to the successful operation of a device and/or system under test. If the field test data doesn’t correlate well to the laboratory data, components might possibly be failed that are actually good. Likewise, bad components might actually pass tests because of lack of correlation. Strong correlation also ensures agreement by all parties involved that the DUT is actually performing to design standards.

Correlate Handheld And Benchtop Measurements, Fig. 1

To determine how well measurement data from a given handheld test instrument correlates with data measured using high-performance benchtop instruments, it may be necessary to examine a number of different measurement types (e.g., spectrum analysis, network analysis, and RF power measurements), especially when the handheld instrument can be configured as different instruments [e.g., as a spectrum analyzer, vector network analyzer (VNA), or power meter].

As an example, Fig. 1 shows two spectrum measurements of a 10-GHz multitone signal. The signal has side tones that are equally spaced in frequency, with amplitudes that are 10-dB lower relative to the adjacent tone. The spectrum measurement on the left was captured using a high-performance benchtop signal analyzer, while the measurement on the right was captured using a handheld instrument operating in spectrum-analyzer mode.

To compare the two measurements, delta markers are employed. Note that on the benchtop instrument, the delta marker measures the fourth tone from the center at -40.37 dB, whereas on the handheld instrument, it reads -40.07 dB. This is a resulting difference between the two instruments of only 0.3 dB, which indicates good correlation. The marker results for the other tones also show an excellent correlation between the two instruments. Consequently, although the handheld instrument in this example is not a direct replacement for the benchtop analyzer based on sweep speed and dynamic range, it is well suited both for field testing and general-purpose laboratory testing.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.