Microwave tests benefit from correlation for accuracy in the hand
Tom Hoppin from Agilent Technologies explains how to correlate microwave measurements between handheld and benchtop analysers to achieve higher accuracy in the field
As more and more technicians and engineers head into the field to make measurements in less than hospitable conditions, the need for accurate handheld measurement instrumentation has become all the more acute. For many, however, “precise microwave measurements” conjures up images of benchtop instrumentation in a laboratory setting.
The latest handheld instruments offer technicians and engineers the ability to make accurate microwave measurements of network, spectrum, power, and frequency with results that correlate to within hundredths of a dB.
To discover if a handheld offers comparable performance to that of a benchtop analyser, examining the specifications found on the vendor’s datasheets is a natural place to start, however, it may be difficult to directly compare the two since specifications are often listed under unique operating conditions.
The handheld, for example, may be specified for harsh operating conditions, while its benchtop counterpart may be specified for temperature-stabilised environments. Even examining technical literature with cited examples comparing benchtop instruments to handheld ones 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.
Why correlate test instruments?
Correlation describes the relative agreement between a set of data measured from the same device-under-test (DUT), but recorded from different instruments such as a high-performance benchtop instrument and a handheld.
The stronger the agreement, the greater the probability that one measurement technique can replace another. In this case, it can be used to determine whether or not a given handheld 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 the product lifecycle, a DUT is measured with a variety of different instruments and typically requires a unique set of equipment requirements.
During the early 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 testing, the benchtop instrumentation is selected based on its performance, features and dynamic range.
For production test, important parameters include high measurement speed and lower instrument cost. Once the new component or system is installed in the field, many of the laboratory measurements must be repeated to validate the device performance.
In all field testing, correlating the data to laboratory measurements is critical to the successful operation of the device and/or system under test. If the field test data doesn’t correlate well to the laboratory data, components might possibly fail that are actually good.
Likewise, bad components might actually pass testing. Strong correlation also ensures agreement by all parties involved that the DUT is actually performing to design standards.
Proving measurement correlation
To determine how well measurement data from a given handheld correlates with data measured using high-performance benchtop instruments it may be necessary to examine a number of different measurement types (e.g., spectrum, S-parameters and RF power), especially when the handheld can be configured as different instruments (e.g., a spectrum analyser, vector network analyzer or power meter).
To compare the results of an S-parameter measurement, consider the example shown in Figure 1, which uses a 3 to 12GHz broadband amplifier with a gain of 23dB as the DUT.
In this example, four S-parameters are measured from three different vector network analysers, two benchtop instruments and one handheld. All three instruments were configured with a frequency range of 100MHz to 26.5GHz, 401 measurement points and an intermediate frequency bandwidth of 10kHz. They were calibrated using a full 2-port mechanical calibration.
For comparison purposes, the three sets of recorded S-parameters were ported into one of the benchtop instruments and overlaid. Notice from the figure that the three sets of measurements are virtually identical except for a deviation in the S21 data from the benchtop instrument, which is signified by the blue trace and only at the high end of the frequency range.
There is excellent correlation between the handheld (magenta trace) and the other high-performance benchtop instrument (green trace). Here again, the handheld shows itself to be ideally suited for S-parameter measurements in the field, as well as for common laboratory applications.
When measuring RF power for continuous wave (CW) pulsed and complex waveforms, a variety of equipment configurations can be used, with the primary component being a power sensor. The power sensor can be configured with a separate power meter or connected to a PC or handheld/benchtop instrument through a USB cable. The PC or handheld/benchtop is used simply to display the power measurements. Assuming the same power sensor was used for the handheld and benchtop measurement configuration, both would yield extremely close results.
While the power meter is considered the gold standard for RF power measurement, it is possible to conduct power measurements using a handheld only—assuming of course that the handheld has the ability to directly measure signal power.
For comparison purposes, Table 1 shows the measured power of a CW test signal as a function of frequency. It compares the measured power using a power sensor and meter combination to that of a measurement taken using a handheld with a built-in power measurement capability. While using a handheld may not be as accurate as employing a power sensor, the convenience of using a single instrument to measure power in challenging environments and test conditions cannot be overstated.
The previous examples effectively demonstrate how to correlate measurement data and clearly show that a modern handheld analyser is more than capable of performing measurements that have excellent correlation to those recorded from benchtop instruments.
One reason this is possible is that modern handhelds use some of the same measurement science as their benchtop counterparts. Some high-performance RF and microwave benchtop instruments, for example, rely on custom monolithic microwave integrated circuits (MMICs). Those same MMIC chip designs may also be employed in handhelds to integrate multiple functions into compact, high-performance chipsets.
These highly integrated circuits improve the handheld’s performance and reliability, while also reducing its total power consumption. Additionally, they enable the handheld to be configured as different instruments, such as a spectrum analyzer, vector network analyser, power meter, or cable and antenna analyser, just to name a few.
In addition to these technological advances, modern handhelds now offer features like automatic alignment and built-in calibration, which enable them to maintain a high level of measurement accuracy across their full frequency range. Such capabilities are a key reason why modern handhelds now boast high-performance measurement capabilities rivalling that of benchtop instruments.
Tom Hoppin is application specialist at Agilent Technologies