Vector Network Analyzer PNA-X Goes Beyond S-Parameter Testing

　　Engineers face many major challenges when testing RF components, both in R&D and manufacturing. In R&D, it is critical to solve design challenges faster and with less rework. In manufacturing, it is necessary to reduce test time and cost while maintaining accuracy and maximum yield.

　　One way to ease the pressure is to use a flexible, highly integrated test solution - such as the Agilent N5242A PNA-X microwave network analyzer. Due to the advanced architecture of the PNA-X, it not only provides excellent performance and accuracy, but can also be configured for a variety of measurements beyond the traditional scattering parameters (S parameters) associated with network analyzers. Some built-in components (such as a second signal source and a broadband combiner) can very accurately characterize the nonlinear characteristics of RF and microwave devices, especially amplifiers, mixers and frequency converters, giving you a more comprehensive understanding of the performance of these devices.

　　Ensure accurate system simulation

　　Accurate amplitude and phase measurements are critical to devices used in modern wireless and aerospace/defense system equipment. During the design phase, system simulation requires highly accurate component characterization to ensure that the system meets its performance requirements. In manufacturing, accurate measurements verify that each component meets its published specifications.

　　S parameters are most widely used in the measurement of RF components (such as filters, amplifiers, mixers, antennas, isolators, and transmission lines). The measurement results can determine the reflection and transmission performance of RF devices when transmitting signals in the forward and reverse directions, expressed as complex values ​​(amplitude and phase). They fully describe the linear characteristics of RF components, which is a necessary part of the full system simulation. However, when a more complete simulation of the full system is required, it is not enough to perform S parameter testing alone. Deviations such as the uneven amplitude response or the inconstancy of the phase response slope presented by the device characteristics as the frequency changes can cause serious system performance degradation.

　　Nonlinear characteristics of devices can also cause degradation in system performance. For example, if the amplifier is driven beyond its linear operating range, it will experience gain compression, amplitude modulation to phase modulation (AM to PM) conversion, and intermodulation distortion (IMD).

　　Core Measurement Overview

　　The vector network analyzer (VNA) is the most commonly used instrument for determining component characteristics. Traditional VNAs consist of an RF signal generator that provides excitation to the device under test (DUT) and multiple measurement receivers to measure the incident, reflected, and transmitted signals in both forward and reverse transmission directions (Figure 1). The signal source is swept at a fixed power level to measure S parameters, while its power is swept at a fixed frequency to measure gain compression and AM-PM conversion of amplifiers. These measurements can determine the performance of linear and simple nonlinear devices.

 

　　Figure 1. Traditional two-port VNA block diagram.

　　For basic S-parameter and compression testing, the source and receiver are tuned to the same frequency. However, by offsetting the source and receiver frequencies and tuning the receiver to integer multiples of the stimulus frequency, the harmonic performance of the amplifier can also be measured. The ability to offset the source and receiver frequencies also allows the amplitude, phase, and group delay performance of frequency conversion devices such as mixers and frequency converters to be measured.

　　These measurements are typically made using continuous wave (CW) excitation, but many devices require pulsed RF testing, where the test signal must be gated with a specific pulse width and repetition rate.

　　Traditional VNAs have two test ports, which is sufficient when most RF devices have only one or two ports. With the rapid growth of wireless communications, devices with three or four ports have become very common, so four-port network analyzers are as commonly used as two-port network analyzers.

　　Simplifying amplifier and mixer measurements

　　When using two or four ports, PNA-X has four major improvements over traditional VNA architectures:

　　* Two signal sources: The second internal signal source is independent of the frequency and power level settings of the first signal source. The second signal source can be used for nonlinear amplifier testing such as intermodulation distortion (IMD), or as a fast local oscillator (LO) for testing mixers and frequency converters.

　　* Broadband signal combiner: An internal signal combiner can combine two sources together before the instrument's associated test port coupler. This simplifies amplifier test setups that require two signal sources.

　　* Signal Switching and Access Points: Auxiliary switches and RF access points enable flexible signal path selection and the addition of external signal conditioning hardware (such as a boost amplifier) ​​or external test equipment (such as a digital signal generator or vector signal analyzer).

　　* Pulse test capability: internal pulse modulator and pulse generator provide a fully integrated pulse S-parameter solution. [page]

　　These improvements simplify the test setup process and reduce test time when measuring amplifiers, mixers, and frequency converters. These new features combine to greatly expand the range of measurements that can be made with a single connection to the device under test (DUT). Figure 2 shows an example of simultaneous measurement of amplifier S-parameters, gain compression and phase compression, and fixed-signal IMD.

 

　　Figure 2. Example of a PNA-X displaying simultaneous measurements of an amplifier's S-parameters, compression, and IMD.

　　The enhanced performance of the two built-in signal sources also simplifies amplifier and mixer measurements. For example, the maximum signal power available at the test port is typically +13 to +20 dBm (depending on the model and frequency). This is helpful for driving amplifiers into their nonlinear regions, which is often required when using the signal sources as LO signals for testing mixers. The harmonic content of the two built-in signal sources is also very low (typically –60 dBc or less), which improves the accuracy of harmonic and IMD measurements. In addition, the power sweep range, which is typically set to 40 dB, makes it easy to transition an amplifier from its linear to its nonlinear operating range when characterizing its characteristics.

　　Solve various measurement problems

　　Although a VNA can measure component S-parameters, compression, and harmonics with just one RF source, adding a second internal signal source allows for more complex nonlinear characteristics such as IMD, especially when the two sources are used in conjunction with the signal combiner inside the network analyzer.

 

　　Figure 3. Block diagram of a two-port PNA-X configured for IMD measurements.

　　For IMD measurements, a signal combiner is used to combine the two signals and then send them to the input of the amplifier under test (AUT). Figure 3 shows how the PNA-X uses internal signal sources and combiners to accomplish this process.

　　The nonlinearity of the AUT causes intermodulation products to appear along with the amplified input signal. In communication systems, these unwanted components will enter the operating band and cannot be removed by filtering. In practice, only third-order components are measured because they are the most important factor that degrades system performance.

　　Figure 4 shows an example of a swept IMD measurement done with the PNA-X. The two center traces show the stimulus signal, and the two lower traces show the IMD components. The top trace is the third-order intercept point (IP3) calculated and displayed using the PNA-X’s particularly advantageous formula editing feature.

 

　　Figure 4. PNA-X example of swept frequency IMD measurement.

　　A very useful change in performing IMD testing in the swept state is to sweep the power level instead of the frequency, which helps R&D engineers to model the nonlinear behavior of transistors and amplifiers. In the measurement results shown in Figure 5, you can see how the amplitude and phase of the fundamental signal and the third-order, fifth-order, and seventh-order intermodulation products change with input power.

 

　　Figure 5. Example of PNA-X performing power swept IMD testing.

　　There are three advantages to using a VNA to perform the above measurements compared to other methods. First, you can measure all parameters with just one test instrument and one connection: S parameters, gain compression, output harmonics, IMD, etc. Second, the VNA can be calibrated with a power meter to achieve higher measurement accuracy than a spectrum analyzer. Finally, the same test using a spectrum analyzer and two independent signal sources would take several minutes to complete, but it only takes 0.6 seconds with the PNA-X. [page]

　　Phase vs. drive is another common dual-source test that is easily accomplished with the PNA-X. This test parameter characterizes the amplifier's ability to handle small signals when large signals are present in adjacent channels or out-of-band. The test is done by combining a large signal and a small signal at different frequencies and sending them to the amplifier under test (AUT), then measuring the S21 phase of the small signal while varying the power of the large signal (using a power sweep).

　　Another parameter used in modeling transistor and amplifier nonlinear behavior using dual-source techniques is the "hot S-parameters," which characterize the small-signal S-parameters of an amplifier when there is a large input signal that deviates from the S-parameter test signal at a given frequency and the output of the amplifier under test is compressed by the presence of this large signal. When performing hot S-parameter testing, care must be taken not to allow the "hot signal" at the output of the amplifier under test to exceed the damage level of the vector network analyzer test receiver.

　　Measuring balancing elements

　　Balanced circuits can reduce both the sensitivity to and generation of electromagnetic interference. Balanced components can be balanced-single-ended devices with three RF ports or balanced-balanced devices with four ports. These components can be easily tested with a four-port VNA, which can measure differential and common mode responses as well as mode conversion terms.

　　These tests can be done with single-ended or true-mode excitation. The single-ended method tests only one DUT port at a time (requiring only one RF source) and mathematically calculates the differential and common-mode responses as well as the cross-mode characteristics. This is the fastest and most accurate technique, provided that the applied power level keeps the AUT in the linear or moderately compressed operating region.

　　When testing the balanced performance of an amplifier under high drive level conditions, if the single-ended measurement method is still used, nonlinear characteristics will cause serious errors in the measurement results, which requires true (differential or balanced) mode excitation. This method adds two signals of equal amplitude to the amplifier input terminal pair with a phase difference of 180° (differential mode signal) or 0° (common mode signal). In theory, this is easy to do using a dual-source VNA, but accurate measurement also requires two conditions: high-resolution adjustment of the phase difference between the two signal sources; and the ability to adjust the phase and amplitude of the signal source to offset the input mismatch caused by the interaction between the source output impedance and the AUT input impedance. PNA-X can meet these two requirements.

　　Testing mixers and frequency converters

　　The second internal source can also be used to test frequency conversion devices such as mixers or converters, where an LO signal is required in addition to the input stimulus. The second source is useful for swept LO testing, where the LO signal is swept along with the RF input signal, but the frequency difference between the RF and LO signals is kept constant. This method is often used to measure the front-end components of wideband converters. Using a signal derived from the VNA's internal source as the LO signal provides a significant improvement in test speed compared to using an external signal generator (up to 35 times faster with the PNA-X than with conventional methods).

　　The setup for mixer and converter measurements using the PNA-X is very simple. To test port matching and conversion loss or conversion gain, the DUT input, output, and LO port are connected to Port 1, Port 2, and Port 3 of the PNA-X, respectively. Adding a reference mixer allows testing of the phase or group delay of a mixer or converter. The two outputs of the second signal source can be used to drive the reference mixer and the DUT mixer (Figure 6).

 

　　Figure 6. Four-port PNA-X block diagram configured for vector mixer measurements.

　　in conclusion

　　VNA-based test systems provide the power to measure RF and microwave components used in wireless communications and aerospace/defense systems. The advanced architecture of the Agilent PNA-X microwave network analyzer provides greater flexibility than traditional VNAs, allowing engineers to measure a wide range of high-performance, cutting-edge components with a single connection. The most significant additions to the PNA-X are a second signal source and an internal broadband signal combiner, which simplifies the measurement of amplifiers, mixers, and frequency converters. In addition to traditional single-source measurements of S-parameters, compression, and harmonics, two sources can be used for testing IMD, phase variation with drive, thermal S-parameters, and true excitation modes. The high power output, low harmonics, and wide power sweep range of the source on the PNA-X port fully accommodate the test requirements of current devices.