VNA Reflections

Fundamentals of VNAs

January 30, 2013

Today, we will review vector network analyzer (VNA) fundamentals and how they have helped the analyzer become the instrument of choice in so many applications.

A VNA is essentially a combination of a source used to provide a stimulus to ports 1 and 2, and a set of receivers measuring both the source power (as a reference) and the transmitted or reflected signals from the DUT. The objective of a VNA is to capture S-parameter data. It acquires incident and reflected waves at every port in question and, in most cases, provides a semi-known terminating impedance at all but the driving port. Basic requirements include:

• One or more signal sources with, at minimum, controllable frequency and a sufficiently-clean spectral tone for making a measurement. Sources with controlled power are preferred.

• Directional devices for separating incident and reflected waves at the ports.

• A means of switching RF signals when there are fewer sources than ports, or either more or fewer receivers than ports.

• One or more receivers, usually with down converters, to take incident and reflected waves down to some convenient IF for processing.

• An IF section and digitizer to transform the converted wave amplitudes into a useful form for computation.

S-parameters are derived by making "ratioed" measurements on the downconverted IFs of the incident, transmitted and reflected signals. Figure 1 shows S-parameters as they appear on an Anritsu VectorStar® VNA display.

Figure 1a

Figure 1b
                       

VNA Specifications

Specifications such as dynamic range, noise floor and available power give information on DUT conditions that can be measured, and are not intrinsically “accuracy specifications.” Measurement accuracy is determined by factors such as directivity, source and load match, and isolation. These factors are often presented in the form of residual performance after calibration.

S-parameter performance of the DUT also contributes to the overall measurement accuracy. When considering measurement accuracy, it is important to take into account all parameters of the VNA’s residual performance, combined with the S-parameter performance of the DUT at each frequency point-of-interest.

Important characteristics that determine the VNA’s measurement-performance include:

• Dynamic Range

• Compression Level

• Noise Floor

• Trace Noise (High Level Noise)

• Power Range

• ALC Power Accuracy and Linearity

• Frequency Accuracy and Stability

• Harmonics

• Raw Directivity

• Raw Source and Load Match

• Residual Directivity, Source Match and Load Match

 

Reference Plane

Perhaps one of the most important, and yet poorly understood, S-parameter measurement concepts is the reference plane. When performing a RF calibration, an engineer implicitly establishes a set of planes (one per port) that serve as the reference for the calibration, as shown in figure 2.

Figure 2

Assuming that a calibration had been performed at points 1A and 2A, these would be the reference planes and, ideally, where the DUT is connected. If additional items, such as adaptors, cables, etc., are interposed between the “A” reference plane and the DUT, then 1B and 2B become the reference planes and new calibration is required. If not, the measured phases and amplitudes will be incorrect due to the distortions caused by those additions/changes. While the effects may be insignificant at low frequencies, they can be quite large above 30 or 40 GHz.

Calibration is such a central and important concept that we will devote our next post to the subject. Until then, please visit our VNA page to learn more.

New Year, Same High-speed Test Challenges

January 8, 2013

By Bob Buxton, Manager of Product Marketing, Anritsu Company

As the old saying goes, “the more things change, the more they stay the same.” That phrase rings true in the engineering community. Even though the calendar says 2013, one challenge that Father Time handed off to Baby New Year was how engineers can accurately and cost-efficiently test high-speed signals in complex designs.

Judging by the early buzz, that challenge is one key topic that will be bandied about during DesignCon 2013, which will be held January 28-31 at the Santa Clare Convention Center in Santa Clara, CA. Anritsu will be exhibiting in booth #501 and will have test solutions geared to meet the needs of accurately testing high-speed signals. Among the instruments on display will be our VectorStar® Vector Network Analyzer.

Helping to kick-off the exposition on Monday, January 28th will be a presentation that deals with high-speed designs, including those at millimeter wave frequencies. I will be part of that presentation, to be held in Ballroom G at 9:00 a.m., as will Anritsu Fellow Jon Martens, PhD, and two highly respected engineers from Wild River Technology – Director of Engineering James Bell and Chief Technologist Alfred Neves.

Entitled Methods of Improving 3D EM Model Development and Associated Time/Frequency-Domain Measurements, the three-hour practical tutorial will deal with improving the convergence of 3D EM modeling methodology and advanced high-confidence measurements. With the heightened requirements of advanced fixture complexity, an ability to adapt to different calibration scenarios and various conditions of S-parameters so they are suited for modeling is important.

We will begin with a macro understanding of S-parameter measurement confidence, starting with validating a good calibration. Next will be a discussion on testing the S-parameters for properties that render good time-domain response. The tutorial then moves on to explaining how to improve the calibration approaches suited for 3D EM modeling, especially in difficult cases where the launches are not the same, thereby requiring hybrid or partial calibration methods. A series of practical measurement cases and examples will be utilized to emphasize the key points. The final module of the training focuses on practical, but difficult, 3D EM modeling situations.

I certainly will be in good company during the discussion. Dr. Martens, who earned his PhD in electrical engineering from the University of Wisconsin, has 20 years of experience in high-speed measurement methodologies and instrumentation, and high-speed circuit/system design. He has been with Anritsu since 1995, working on measurement system architecture, measurement algorithm design, and high-speed circuit development.

James Bell is an experienced design and signal integrity engineer with 30 years’ experience in complex system design, interconnect, and signal integrity engineering. He has been a consultant to engineering organizations worldwide, with expertise in pre- and post-route signal integrity and timing validation for advanced systems. And Al Neves also has 30 years of experience in the design and application development of semiconductor products and capital equipment design focused on jitter and signal integrity analysis. He is involved with the signal integrity community as a consultant, high-speed system-level design manager, and engineer.

You can learn more about the presentation by visiting the DesignCon webpage, which can be found here. To read about other signal integrity design challenges and how VNAs such as VectorStar can help in conducting high-speed measurements, you can download a white paper here.