RF/microwave network analyzers have enabled the evolution of high-frequency components and how they are designed, and vice versa. The basic ability to measure the essential properties of circuits and devices—transmission, reflection and impedance—enables engineers to optimize the performance of amplifiers, frequency converters, signal-separation and filtering devices, and other components. The performance of present and future communication and defense systems depends heavily on the capabilities of these components and their test systems.
Note: Many of the instruments mentioned below were sold under the HP name until 2000 when Agilent Technologies was spun out as an independent company. On August 1, 2014, Agilent’s Electronic Measurement Group began operating as Keysight Technologies.
In the 1940s and 1950s, most high-frequency communication systems used tubes (e.g., klystrons, magnetrons) and AM or FM modulation techniques. Rudimentary signal generators, power detectors and impedance bridges were used to measure the transmission, reflection and impedance characteristics of these elements, thereby enabling successful development of improved systems. To construct a modern-day Smith Chart, hours of tedious, hand-tuned measurements were taken one frequency at a time. The “network analyzer” of the day was a swept scalar analyzer combined with tedious, point-by-point reconstruction of the relative phase characteristics of devices.
By the 1960s, semiconductor technology was just taking hold. Samplers based on semiconductor diodes became the fundamental building blocks of instrumentation. These were used to sample waveforms and enable relative amplitude and phase measurements to be made on signals. Agile signal sources based on backward wave oscillators allowed measurements to be taken across a wide frequency range. The first network analyzer capable of swept amplitude and phase measurements was the Hewlett-Packard 8407 RF network analyzer, which was based on the HP 8405 vector voltmeter. The 8407 allowed comparison of the amplitude and phase of two waveforms up to 110 MHz.
In 1967, HP introduced the 8410 network analyzer, which extended swept capability to 12 GHz. This was a benchtop system based on multiple boxes that were integrated to perform the network analysis function (Figure 1).
At the same time, the concept of S-parameters was just becoming popular. This put transmission, reflection and impedance into a single two-dimensional representation that could be rapidly measured and visualized. This revolution in high-frequency design enabled engineers to utilize the new high-frequency semiconductors that were just becoming available. These devices had marginal gain and would not have been very useful without design and measurement methods that helped designers extract maximum performance from these new devices. The interplay and bootstrapping of good measurements and optimized devices helped engineers move forward on both fronts.
By 1970, computers were emerging that could expand instrument capabilities—and this enabled creation of the 8542 automatic network analyzer (Figure 2). This large, three-rack system brought error-correction mathematics, pulsed measurements and other capabilities to circuit designers. However, the system required three racks of equipment.
In 1976 the first integrated, microprocessor-controlled network analyzer was introduced: the 8505. This included a synthesized source, two receivers, a test set and a display in a benchtop box, and it operated up to 1.3 GHz.
In the mid-1980s, the marriage of broadband solid-state sources, improved samplers and faster microprocessors led to three very important products: the 8510, 8753 and 8720 vector network analyzers (VNA). The 8510 became the metrology standard for microwave measurements and enabled many improvements in component design (Figure 3).
The 8753 came to market as manufacturing demands were growing for first-generation cell phones (Figure 4). The 8753A was the first fully error-corrected RF network analyzer, and its combination of low cost and high capability helped it quickly become the industry standard. The 8753A came to be widely used in wireless component manufacturing, just as the 8510 and 8720 became mainstays in the development and manufacturing of avionic and radar components.
This same period saw the emergence of the first commercial CAE tools for high-frequency designers. The interplay between simulation and measurement allowed for acceleration of design cycles and technological capabilities. The first commercial microwave ICs emerged at this time, greatly aided by this measurement and simulation capability. The 8720 was the first fully integrated microwave VNA and it embodied most of the capability of the three-rack 8542 in a single box.
The 1990s saw a huge boom in wireless device deployment. This was the first high-frequency consumer market, driven by the commensurate high-volume manufacturing and intense cost pressure. The network analyzer, once an R&D tool, became a mainstream manufacturing device and the ability to provide fast measurements became very important. During this time the 84000 RF IC tester was introduced (Figure 5). This was a high-speed, multifunction network analyzer. In some ways, the 84000 was similar to the 8542 automatic network analyzer of 1970: this multiple-box IC test system introduced new capabilities that began to migrate into benchtop network analyzers around 2007.
Around 2000, the integration level of RF and microwave devices began expanding rapidly, and this placed new demands on test equipment. For example, network analyzers evolved from two-port, swept-frequency instruments into devices with much broader capabilities. In the late 1990s, commercial RF components started using balanced (differential) topology to take advantage of lower power requirements and higher isolation. A key improvement for testing these devices came in 2001 with the introduction of the ENA (E5071A), the first four-port network analyzer designed for mass-production applications (Figure 6).
This provided a simulated balun and mixed-mode S-parameters, bringing balanced measurements fully into the RF world. By 2006 the test requirements for balanced measurements extended far into the microwave region, with solutions in the PNA family providing this capability up to 67 GHz.
Eventually, wireless designs combined balanced and single-ended components into package-scale integration with a large number of input/output (I/O) ports. While the overall response of these components must meet the same criteria as a design comprised of discrete components, the performance of individual elements inside an IC, particularly with respect to isolation, may be degraded. As such, it is very important to properly terminate the I/O ports and also account for mismatch effects at every port. Valid measurements were made by combining two- or four-port measurements, and this required proper termination of every port. Because the number of measurements grows as N-squared, measurements with higher port counts become more complex.
During this same time, Agilent and others introduced a new generation of test sets that extended the port count of a network analyzer (Figure 7). These N-port systems use internal switches and couplers to seamlessly integrate the test set with the analyzer, giving the N-port test set performance that is directly comparable to that of two- or four-port systems. Eight- and 12-port versions of N-port network analyzers became available in 2007 and 16- and 32-port systems soon followed.
Calibrating this type of configuration is time-consuming if done conventionally. Fortunately, new techniques were developed to greatly shorten the calibration process without compromising measurement quality. Eventually, these provided a full NxN matrix of calibrated measurements with only N connection steps; traditional mechanical calibration requires more than N-squared steps.
This was also a time when many integrated components began to include internal amplifiers, and these require characterization of noise and distortion properties. Then-available measurement solutions provided for a single connection of multiple-test equipment, but further integration of these advanced capabilities into a single platform was inevitable. The 84000 tester of the 1990s had many of these capabilities and, like the evolution of the network analyzer from the 8542 to the 8720, a new generation of benchtop instruments emerged, embodying most of the 84000 capabilities. The challenge was to provide sufficiently good measurements across a wide range of requirements: A network analyzer requires a very fast sweeping source, but this fundamentally conflicts with creating a source with good phase noise and low distortion, which are required for intermodulation measurements.
The wide dynamic range of a network analyzer receiver, which is achieved through the use of narrow receiver bandwidths, is in conflict with the wide bandwidth required for noise measurements. All this becomes more complicated because many mobile-communications systems are time-domain-duplexed, creating a need for pulsed measurements. These devices also may include frequency conversion as well as balanced inputs or outputs. All of these challenges had to be met without giving up the crucial requirement of fast measurement throughput.
Finally, each of these measurements must be calibrated to ensure accurate, repeatable and traceable results. The solution to these challenges came in the form of the PNA-X network analyzer. This innovative VNA led a transformation in enhancing the functionality of a network analyzer to go beyond traditional S-parameters and include complete linear and nonlinear characterization of components (Figure 8). Its range of available measurement applications now includes noise figure, gain compression, intermodulation and harmonic distortion, conversion gain/loss, true-differential stimulus, antenna test, and nonlinear waveform and X-parameter* characterization.
Today, the PNA-X is the most integrated and flexible single-connection microwave test engine for measuring active devices such as amplifiers, mixers and frequency converters. It also provides exceptional configurability with a built-in second source, combiner and internal signal-routing switches. With all of these capabilities in one box, the PNA-X makes it possible to reduce equipment count and replace racks and stacks of equipment.
* X-parameters is a registered trademark of Keysight Technologies. The X-parameter format and underlying equations are open and documented. For more information, visit www.keysight.com/find/eesof-x-parameters-info.
Since 2010, powerful network-analysis capabilities have migrated into the handheld and modular form factors. The two products of note are the Keysight FieldFox handheld microwave analyzers, introduced in June 2012, and the Keysight M937XA Series PXIe VNAs, launched in September 2014.
For most, the phrase “precise microwave measurements” brings to mind a lab bench in a comfortable indoor setting. These days, more and more technicians and engineers need to make accurate measurements in less hospitable conditions: in a base transceiver station during a snowstorm, aboard a ship sailing through rough seas, or at a satellite trailer in a sandstorm. Increasingly, high-performance handheld analyzers are needed to test the power and bandwidth of jamming systems, check the alignment of antennas in point-to-point microwave links, and validate antenna and cable systems in commercial and military aircraft.
Inside and out, the FieldFox family was designed with these conditions, applications and end-users firmly in mind. To provide precision virtually everywhere, FieldFox delivers Keysight-quality microwave measurements in a compact, 6.6-lb package: cable and antenna testing, vector network analysis, spectrum analysis, power measurements, interference analysis, and vector voltage measurements (Figure 9).
In VNA mode, built-in calibration engines leverage extremely accurate algorithms from high-end VNAs—and this enables precise and repeatable measurements in the field. To enhance ease of use, FieldFox includes an internal CalReady standard that enables S-parameter measurements at power on- or boot up. To further enhance instrument accuracy, the QuickCal capability extends the measurement plane to the end of the test cable.
Needs are also changing on the production line. From our vantage point as the leader in network analysis, ongoing conversations with key customers reinforce three major trends:
Within these scenarios, testing must be able to address the increasing complexity of silicon wafers, wireless devices, advanced radar systems, and more, that continue to pack more capability into less space. These intersecting requirements inspired the creation of what is one of our most remarkable achievements in vector network analysis: the M937XA Series PXIe VNAs.
These PXI vector network analyzers are full two-port VNAs that fit in just one slot (Figure 10). They perform fast, accurate measurements and reduce the cost of test by enabling simultaneous characterization of many devices—two-port or multi-port—using a single PXI chassis.
Many of our customers have implemented multi-function testers within a single PXI chassis. As the chassis fills up, fewer slots are available to incorporate VNA capability. A one-slot PXI VNA is ideal for this situation—and that’s why we endeavored to create our unique solution.
On the production line or in a wafer fab, there is a growing need to test multiple devices or multiple wafer sites at a single test station. Examples include cell phone handsets, military radios and increasingly dense silicon wafers. In these situations, one of the key needs is reducing the overall size of the test solution. The ability to install multiple two-port PXI VNAs in a single chassis provides a tremendous space reduction when compared to using multiple benchtop analyzers on the production line or as part of a probing station.
As devices become increasingly complex, there is also a need to easily characterize a full set of S-parameters on a large number of ports—8, 16, 24, or more. Examples include smart antennas and phased-array transceiver (TRx) modules. For these complex devices, the ideal solution must be both flexible and space-efficient. It should also be easy to reconfigure a set of modules—through software—to create arbitrary arrangements of N-port VNA instruments within a single chassis. For example, a single chassis containing 16 two-port VNAs could be configured as eight four-port VNAs, four eight-port VNAs or one 32-port VNA. All of these permutations are possible with the Keysight PXI VNA.
One might be tempted to conclude that the need for parametric test could disappear. Why not do a functional test on every component? In fact, while functional test will provide a convenient pass/fail test that could be used at the end of a production process, the functions that must be verified may become so complex that true functional test is not a practical way to ensure that every unit will work in all environments.
As an example, the input filter of a radio system is designed to remove interfering signals. A functional test to verify the correct operation of the system in the presence of other signals might mean creating myriad interference scenarios and measuring the associated bit error ratio (BER). A more efficient method might entail applying a swept-sine stimulus to the input of the system and determining the cutoff characteristics of the filter. Even so, as the interfaces between components become more difficult to access, new ways of validating designs and controlling manufacturing processes will be required.
Today, it’s possible to embed a Keysight logic analyzer into an FPGA design. In the future, complex stimulus/response capability, or even an entire network analyzer may be designed directly into RF circuits, providing the ultimate realization of “design for test.” As interfaces between components become more complex—and therefore more difficult to probe—it seems that integrated component test may be the only logical way to verify future generations of RF and microwave systems.
History suggests that complementary advances in components and network analyzers helped bootstrap each area and accelerate the respective rates of technological progress. Going forward, we expect this type of bootstrapping to continue between simulation and embedded test inside the chips themselves. The network analyzer is used in two ways: to determine the fundamental characteristics of the chip’s building blocks and feed simulation engines; and to verify the designs of chips and embedded test instruments.
Large opportunities for stimulus/response characterization — the forte of network analyzers — also exist beyond electronic devices. Examples include characterizing the attributes of materials, even down to the nanometer scale. The growth of these new applications and measurements will keep the network analyzer an essential tool for many years to come.
Whether testing is focused on active or passive devices, the right mix of speed and performance gives end-users an edge. In R&D, Keysight VNAs provide a level of measurement integrity that helps developers transform deeper understanding into better designs. On the production line, our cost-effective VNAs provide the throughput and repeatability needed to transform undifferentiated parts into competitive components.
Every Keysight VNA is the ultimate expression of our expertise in linear and nonlinear device characterization. On the bench, in a rack or in the field, we help our customers gain deeper confidence.
Keysight is a global electronic measurement technology and market leader helping to transform its customers’ measurement experience through innovation in wireless, modular, and software solutions. Keysight provides electronic measurement instruments and systems and related software, software design tools and services used in the design, development, manufacture, installation, deployment and operation of electronic equipment. Information about Keysight is available at www.keysight.com.