Santa Clara, CA — In RF applications, amplifiers are considered an indispensable component and yet due to their nonlinear behavior, are often the cause of wasted frequency spectrum. Designing a power amplifier to operate only within its region of linear operation also results in an inefficient use of the available power. Amplifiers are often driven into the nonlinear region of operation and then linearized around that point. Consequently, it has become increasingly important to understand the nonlinear behaviors of active RF components such as power amplifiers and frequency doublers which, in turn, makes accurate measurement of a device's nonlinear behavior all the more crucial.
Unfortunately, making nonlinear measurements is not an easy task, especially considering that currently available tools and models are generally difficult to use and do not provide the information required.
Agilent is introducing two primary methods for measuring the nonlinear effects of a device under test (DUT): nonlinear component characterization and X-parameters. Nonlinear component characterization provides calibrated, vector-corrected waveforms of the incident, transmitted and reflected waves from the DUT. Vector calibration, power calibration and the use of a poly-harmonic phase reference and calibration removes the systematic error terms. With this measurement, all receivers must be measured simultaneously for each frequency point. The information derived from nonlinear component characterization measurements enables the engineer to better understand and more deterministically control the nonlinear behavior of the DUT.
On the other hand, X-parameters are the logical, mathematically-correct extension of S-parameters into a nonlinear, large-signal operating environment. X-parameters are to nonlinear measurements what S-parameters are to linear measurements.
X-parameter measurements require an additional source which is used to drive the DUT with both a large and small signal tone at the appropriate frequencies and phases, at the same time. Careful control of the phase and amplitude of these signals is therefore critical. Measuring the amplitudes and phases of the scattered waves under these conditions allows for the identification of X-parameters. These parameters provide the engineer with information on such things as device gain and match, while the device is operating in either a linear or nonlinear state. The X-parameters can be extracted into Agilent's Advanced Design System (ADS) or displayed like S-parameters.
Because the X-parameters relate cross-frequency dependencies, there are usually many more X-parameters than S-parameters, such as in the case of the gain of the output fundamental frequency to the input third harmonic. Here, there are eight X-parameters for this simple case with only one harmonic and no power dependency. In contrast, there can never be more than four S-parameters. X-parameters also depend explicitly on the large signal state of the device, making input power a variable. In contrast, S-parameters are assumed to be power independent.
Utilizing a solution that employs both nonlinear component characterization and X-parameters provides critical insight, essential to accurately measuring a device's nonlinear behavior. Agilent Technologies' Nonlinear Vector Network Analyzer (NVNA), shown in the picture, supports both methods in a highly integrated, powerful and simple to use instrument. With a minimum amount of external hardware, this solution effectively converts a 4-port PNA-X microwave network analyzer into a high-performance nonlinear analyzer from 10 MHz to 26.5 GHz. Because the NVNA is based on a standard PNA-X microwave network analyzer, it provides all the power, flexibility and measurement capability of the PNA-X for linear measurements. It can then easily switch into the NVNA mode for nonlinear measurement.
The NVNA's method of measuring nonlinear component behavior through the use of nonlinear scattering parameters or X-parameters provides an accurate portrayal of both nonlinear device and cascaded nonlinear device behavior using measurement-based data. Additionally, the X-parameters can be accurately cascaded from individual devices using the company's ADS to simulate and design more complex modules and systems. These parameters separate out a critical term from the measurement, XT, which enables accurate nonlinear design and simulation by taking cross-frequency mismatch properly into account for nonlinear components.
Accurately measuring and reducing a device's nonlinear behavior is crucial to creating linear high-power solutions for use in a range of applications in aerospace and defense and telecommunications, just to name a few. While conventional solutions to this challenge fail to provide the information and accuracy required, NVNA software uses component characterization and X-parameters to quickly, easily and with the highest degree of accuracy measure nonlinear behavior in a DUT. With its full match correction and accurate amplitude, as well as cross-frequency relative phase information, the NVNA is today providing a new standard in accuracy and insight into the behaviors of nonlinear components.
For further information visit www.agilent.com.
