S-parameters are so yesterday

-July 01, 2008

Textbook amplifiers operate in linear mode and are easy to analyze. Unfortunately, it’s often impossible or undesirable to operate real-world amplifiers in a linear mode. Fortunately, if you can characterize nonlinear behavior, you might be able to take advantage of it. Vendors including Agilent and NMDG can help you do that. Each at last month’s MTT-S International Microwave Symposium highlighted approaches for characterizing the nonlinear performance of amplifiers and other active devices.

Why not just design amplifiers to operate in the linear region? In “Heads and tails: Design RF amplifiers for linearity and efficiency,” EDN technical editor Paul Rako points out, “You can achieve…linearity by underdriving [an] RF amplifier and leaving head room between the output signal and the power-supply voltage. The problem with this approach is that it directly decreases the amplifier’s efficiency.”

Rako further notes that designers have always pushed amplifiers to operate nonlinearly, and those designers didn’t bother much with characterizing nonlinear behavior. That’s because nonlinearity often didn’t cause a problem. As Rako points out, “In FM transmissions, the zero crossings of the waveform contain all the information in the signal. Even if the peaks of the waveform become distorted, the fidelity of the demodulated signal does not. Overdriven FM-radio signals create frequency harmonics of the carrier frequency, and those harmonics may be objectionable from an interference standpoint, but a receiver tuned to an overdriven-FM-radio signal still operates successfully” In contrast, he writes, “To work properly, new modulation schemes, such as EDGE (enhanced data for GSM evolution), require linear amplifiers.”

Designers cope with nonlinearity using techniques such as predistortion of I and Q signals to compensate for the deterministic nonlinearity of a system. But to compensate for nonlinearity, you need to be able to measure it, and that’s where Agilent and NMDG come in, with products and techniques that they presented at the IMS.

For its part, Agilent has introduced NVNA (nonlinear-vector-network-analyzer) capability for its PNA-X microwave-network analyzer, which operates from 10 MHz to 26.5 GHz. Requiring minimal external hardware, the Agilent NVNA software effectively converts a four-port PNA-X into a high-performance nonlinear analyzer that measures what Agilent calls X-parameters—nonlinear extensions to S-parameters. X-parameters, as Agilent states in an application note, take into account cross-frequency (harmonic) effects as well as power effects. (For more information, read “Understanding Nonlinear Vector Analysis.”)

NMDG is also leaving linear S-parameters behind, as evidenced by the company’s tag line: “Leading beyond S-parameters.” At the MTT-S show, the company highlighted its NM300 extension kits for Rohde & Schwarz ZVA and ZVT vector network analyzers. NMDG director Marc Vanden Bossche explained that the kit is a combination of hardware and software that enables a VNA to perform time- and frequency-domain characterization of the harmonic behavior of components such as diodes, transistors, and power amplifiers. NMDG also offers a large-signal network analyzer in conjunction with Maury Microwave.

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