Solving RF Measurement Challenges with Vector Network Analyzers

The need to accurately characterize the linear and nonlinear characteristics of high-frequency devices and the trend toward increasing subsystem integration in today’s electronic devices are changing the way RF and microwave devices are tested.

This article details how the next generation of vector network analyzers are better suited to modern applications by using two signal sources within the vector network analyzer and increasing the number of test ports in the test system.

Accurate linear and nonlinear measurements are key to accurate system simulations

Accurate measurement of the amplitude and phase performance of radio frequency (RF) components is critical to modern wireless communications and aerospace/defense systems. During the design phase, system simulation requires accurate component data to ensure that the final system will operate within the designed parameters. In production, accurate measurements ensure that each component meets the published specifications. Among the basic components that make up RF systems, filters, amplifiers, mixers, antennas, isolators, and transmission lines are all components that need to be tested frequently.

For RF components, the most widely used measurement parameters are scattering parameters, referred to as S parameters. These parameters characterize the reflection and transmission characteristics (complex information of amplitude and phase) of RF components during forward and reverse transmission of signals. Using S parameters to fully describe the linear behavior of RF components is essential for comprehensive system simulation, but it is not enough. Because once the deviation from the ideal linear performance, such as the unevenness of the amplitude-frequency response characteristics and the nonlinearity of the phase-frequency response characteristics, will seriously affect the system performance.

The nonlinear performance of RF components will also affect the system performance. For example, for an amplifier, if the power level is driven beyond the linear range, it will cause gain compression, amplitude modulation to phase modulation (AM-PM) conversion and intermodulation distortion (IMD). It is also important to measure these indicators of components.

The most commonly used instrument for measuring the characteristics of RF components is the vector network analyzer (VNA). The "network" here refers to the network in the concept of electronic circuits, not computer networks. Traditionally, VNA uses an RF signal source as an excitation and uses multi-channel measurement receivers to measure the incident, reflected and transmitted signals in both the forward and reverse directions. Traditional VNAs have two test ports because most early devices only had one or two ports. In order to measure multi-port devices, it is necessary to change the test cables and termination loads between the ports of the device under test (DUT) many times until all ports are measured. This article will introduce a better solution to replace this measurement method.

VNA can use a fixed power sweep method to measure S parameters; it can also use a fixed frequency power sweep method to measure the gain compression of the amplifier. In this way, the linear performance and some simple nonlinear performance of the components can be quantified. Now, the new VNA has two built-in RF signal sources inside, which can measure IMD, which was previously mainly done by two external signal sources and a spectrum analyzer. The VNA-based test method makes the instrument setup simpler, the measurement time shorter, and the accuracy higher during the test process. A typical representative of this type of instrument is Agilent's new 13.5GHz N5230APNA-L network analyzer with two built-in signal sources, which has option 146.

Multi-port measurement is becoming more and more common.

Now, many devices used in RF systems have three or four ports, and devices with up to seven or eight ports are becoming more and more common. There are two reasons for the increase in device port counts: the widespread use of balanced components and the increasing integration of subassemblies, such as the front-end modules used in current cell phones.

Balanced circuits offer considerable advantages in terms of reduced susceptibility to external EMI and reduced EMI to other systems.

Balanced components can be in the form of two-port to single-port devices with three RF ports or two-port to two-port with four RF ports. 4-port VNAs are now common, and Agilent's 4-port VNAs are very convenient for measuring any balanced device operating below 67 GHz. These VNAs can measure the differential and common-mode response and mode conversion performance of balanced devices.

The main factor in the increasing number of device ports is the increasing level of integration. In the mobile phone industry, this trend can be seen in both handsets and base stations. Multi-band phones that can operate on multiple frequency bands and may also include non-phone functions such as GPS or Wi-Fi typically use 4-port modules that include one or two antenna input ports, multiplexers, duplexers, filters and amplifiers, all integrated on a single substrate. On the base station side, duplexers and low noise amplifiers are usually integrated into combiners/splitters with multiple RF ports.

When measuring such devices, the upper limit of the test frequency is usually much higher than the designed operating frequency band, because the industry generally requires that out-of-band rejection performance must also be measured. For example, when testing mobile phones operating at frequencies below 2GHz, the maximum test frequency is as high as 12.5GHz, only in this way can it be measured whether these components will cause interference to devices in other frequency bands.

In order to meet the requirements of a large number of ports and high test frequencies at the same time, the number of ports of the VNA can be expanded by using an external test set (which usually is placed at the bottom of the VNA) (which contains more test port connectors and directional couplers) and the necessary switches (which allow the external test set to be tightly integrated with the VNA itself). In this way, a multi-port test solution with a large number of ports can be implemented, and the signal channels between any combination of port pairs can be measured, and the necessary error calibration procedures are also included to eliminate the systematic errors of all test ports and channels. Agilent N5230APNA-L network analyzer is such a device. The device uses option 145 and the Z5623AK44 test port expansion base to form an 8-port 13.5GHz test system (Figure 1). In addition, Agilent recently launched a 12-port 20GHz vector network test solution based on the N5230APNA-L network analyzer (configured with option 225) and the U3022AE10 test port expansion base. [page]

Figure 1: Agilent's 8-port, 13.5 GHz vector test system

Two built-in RF sources simplify amplifier and mixer measurements

Although a single RF source is sufficient for measuring component S-parameters, gain compression, and harmonics using a VNA, a second internal source is helpful for more complex nonlinear measurements such as IMD and for efficiently testing mixers and frequency converters.

For IMD measurements, the two signals (often called “tones” in two-tone intermodulation) are combined using a power splitter or directional coupler and then sent to the input port of the amplifier under test (AUT). Figure 2 illustrates how this test can be accomplished using a 4-port VNA.

Figure 2: IMD test connection diagram using a 4-port VNA

Due to the nonlinearity of the AUT, an intermodulation signal appears at the output port of the amplifier in addition to the two amplified input signals. In communication systems, these unwanted signals fall within the desired operating frequency band and cannot be removed by filtering. Although there is theoretically an infinite series of intermodulation signals, only third-order intermodulation signals are usually measured because they have the greatest impact on the system. The frequency difference between the two input signals determines where the third-order intermodulation signals appear. For example, if the two input signals are 1.881 GHz and 1.882 GHz, the lower IMD signal will be located at 1.880 GHz, while the higher IMD signal will be located at 1.883 GHz. Figure 3 shows an example of an IMD measurement on a VNA.

Figure 3: IMD measurement results using a VNA

The figure above shows a single sweep, just like what you would do with a spectrum analyzer.

This approach is familiar to engineers and the results are intuitive, but it can extract too much unnecessary data and increase test time. The figure below shows a better approach, where the data collected is primarily the IMD signal and two test signals.

Using a VNA for these measurements has two distinct advantages over other methods. First, you can use a single tester to make a single connection to measure all parameters, including S-parameters, gain compression, output harmonics, and IMD. Second, by taking advantage of the VNA's power meter-based calibration capabilities, the accuracy of these measurements is much higher than what you would get with a regular spectrum analyzer.

It is also desirable to have a second internal signal source in a VNA when testing frequency conversion devices such as mixers and converters, which require an additional local oscillator (LO) signal. This is especially true when performing LO sweep testing. In this test, the LO signal and the RF input signal are swept simultaneously (at a fixed frequency difference). This is very common in broadband converter testing to measure the frequency response of the converter's front-end components. Using the built-in source as the LO signal greatly improves speed. For example, the N5230A with Option 246 can increase the sweep speed of swept LO measurements by 35 times compared to using the external Agilent PSG source as the LO. Figure 4 shows the measurement of a single-stage frequency converter. [page]

Figure 4: Single-stage inverter measurement results using VNA

The top figure is a fixed LO measurement showing the frequency response of the frequency converter. The bottom figure is a swept LO measurement showing the flatness of the frequency response of the frequency converter front end.

Agilent also provides advanced error calibration routines designed specifically for mixer and converter measurements. These routines calibrate the mismatch error at the input frequency between the DUT input match and the test system source match, and the mismatch error at the output frequency between the DUT output match and the test system load match, minimizing the mismatch ripple in the conversion loss and conversion gain measurements. Agilent has also developed a similar technique that allows low ripple and absolute value measurements of mixer and converter group delay.

Multiport test systems can achieve both high speed and high accuracy

The advantage of a multiport test system is that multiple measurements can be made with a single connection to the multiport DUT, greatly increasing the test speed compared to using a traditional two-port VNA. The first VNA-based multiport test system used a simple switch matrix placed in front of the VNA test port. Although this approach is simple and economical, it does not provide the high performance typically required of modern devices at high frequencies. A better approach is to use a coupler-based test setup that has several directional couplers on each test port. In this approach, switches are needed to get the signal to the VNA for testing, and these switches are placed between the couplers and the VNA's receivers. This type of test port expansion base improves sensitivity and stability, which is particularly important for microwave frequency measurements. The

switches in the test port expansion base can be either electronic or mechanical. The advantages of electronic switches are faster switching speeds and unlimited life, but they have higher insertion losses and cannot withstand high power. When there are more than 12 test ports, using many electronic switches generally makes the test equipment more expensive and more difficult to use. Mechanical switches have the best RF characteristics: low loss and high power handling. Mechanical switches are generally cheaper than electronic switches. However, the main disadvantage of mechanical switches is that the life of the switch contacts is limited. Although high-reliability switches are usually guaranteed to have more than 5 million switching cycles, high-volume production applications usually cause these switches to fail in less than a year. Agilent offers both electronic and mechanical switch-based test port expansion bases. The choice of port extension depends on the frequency range, the number of ports required, and the specific application. Many test port extension bases have additional switches that can switch other test components (such as signal combiners) or test equipment (such as noise figure analyzers) into the test signal path. These additional switches greatly increase the flexibility of the entire test system.

For multi-port test systems, error correction is a key component of the entire solution. Basic VNA calibration procedures can calibrate all systematic errors in the test path. In a multi-port environment, load matching of test ports outside the specific test signal path may cause significant measurement errors. The more test ports there are, the greater the potential for error, and the degree of error is related to the isolation between the DUT ports. Modern VNAs can correct all the effects on overall test performance caused by poor test port performance, regardless of which specific ports are in the measurement channel. This is often called N-port calibration, where N is the number of ports on the DUT and test system. N-port calibration provides the best accuracy, but at the expense of increasing the number of sweeps and increasing test time. Devices with low isolation between ports or devices with high isolation that must be verified by measurement usually require N-port calibration, such as power splitters, hybrids, switches, and isolators/multiplexers.

A new application that requires N-port calibration is measuring crosstalk on the physical layer structure or connectors on the backplane of high-speed digital network equipment, and crosstalk between multiple connectors on interconnect cables. For example, two differential transmission lines are essentially equivalent to an 8-port device. When measuring far-end crosstalk (FEXT), we apply a differential stimulus signal at one end of a pair of differential lines and measure the differential response at the other end of the other pair of differential lines. If N-port calibration is not used, the load matching of the four test ports that is not used in the FEXT measurement process may cause considerable errors. Similar crosstalk measurements are also required for the affected differential line located between the two interfering differential line pairs. These measurements require a 12-port test system and 12-port calibration. The most demanding physical layer tests usually require test frequencies that can reach 50GHz, and sometimes even up to 67GHz.

To improve measurement time, many multi-port devices are usually divided into several M-port measurements and M-port calibrations during testing, where M-port calibration is performed. Advanced VNA-based test systems provide the core measurement engine for measuring RF and microwave components used in the physical layer of current wireless communications, military systems, and network equipment. Configuring two signal sources inside the VNA simplifies and speeds up the measurement of amplifiers, mixers, and frequency converters while ensuring high test accuracy. When testing amplifiers, the two built-in signal sources can be used to measure S parameters, gain compression and harmonics, and to generate signals required for measuring IMD. When testing mixers and frequency conversion devices, one of the signal sources can be used as the input signal of the mixer or frequency conversion device, while the other signal source can be used as the local oscillator signal, so that fixed local oscillator measurements and local oscillator sweep measurements can be completed simultaneously with one connection to the device.

Although 4-port VNAs are now very common, higher integration is increasingly requiring test systems to have more than 8 test ports. This goal can be easily achieved by combining the VNA with an external test port expansion base consisting of switches, couplers, and additional test ports. By using N-port calibration, the VNA can achieve the same high accuracy expected in a multi-port test system as when using a two-port VNA for testing. At the same time, for a specific device, different error calibration levels can be selected to optimize the overall test accuracy and test times.