VNA Reflections

VNA Applications

February 26, 2013

Vector Network Analyzers (VNAs) are used to characterize a wide range of devices, ranging from 2-port passive devices, such as filters, to complex subsystems consisting of a combination of passive and active components. Moreover, the device under test (DUT) can vary from a connectorized and packaged format to on-wafer; even to raw materials and free-space antenna measurements.

Today’s VNAs address a wide and growing range of applications, from aerospace/defense and astronomy to high-speed serial data and homeland security. The following are some applications in which VNAs are used:

• Wafer-level characterization of active and passive devices

• Characterization of packaged or connectorized amplifiers and other active components (both linear and non-linear)

• Characterization of frequency translation components

• Characterization of passive components, such as filters, diplexers, etc.

• Characterization of high-speed serial interconnects, such as backplanes, cables and connectors

• Metrology

• Materials characterization

• Antenna characterization

In the remainder of this post, I will give a thumbnail on a few of the more common applications.

Wafer-level Device Characterization

Semiconductor device manufacturers need to comprehensively characterize their devices over the broadest range of frequencies, so that their customers can optimally incorporate them into their own designs. Current state-of-the-art VNAs, including the Anritsu VectorStar® platform, permit operation over the 40 kHz to 125 GHz band from a 1-mm connector. However, on-wafer measurement imposes even more demands.

Calibration is more involved compared to a connectorized environment, as it incorporates on-wafer calibration standards. The ability to reduce the calibration frequency from multiple times a day to a single calibration per day is desirable. Recent advances in VNA technology have improved the calibration stability of broadband VNAs to the point where S21 drift of <0.1 dB and 0.5° over 24 hours have been achieved. As evidence, figure 1 shows the measured transmission stability of the Anritsu ME7838A broadband VNA system over 24 hours.

Figure 1

The use of shockline (non-linear transmission line) technology has permitted a combination of these performance gains, as well as achieving a dynamic range of 109 dB at 110 GHz. The technology also permits a dramatic shrinking in volume of 50:1 from previous, less-capable solutions, so modules can be directly mounted to the probe tips, creating performance advantages.

High-speed Serial Data

The world’s thirst for information and entertainment over the Internet is continually pushing the need for speed in the datacomm world. For example, 100 GBit Ethernet utilizes four lanes, each with a 28 Gbit/sec physical signaling rate. As speed increases, the capability of backplanes and interconnects is stressed to the point where a nice sharp signal from the transmitter is reduced to one with a very closed eye pattern at the receiver.

VNAs are invaluable for the signal integrity designer, in that they can be used to capture the S-parameters of a backplane. S-parameters can be used in a model to simulate the resultant eye pattern. Figure 2 shows a system capable to performing a 12-port measurement up to 70 GHz.

Figure 2

Amplifier Design

Not all amplifiers are designed to operate in the linear region of the underlying devices. When operated in the non-linear region, these devices will be a rich source of harmonics. This complicates the design of matching circuitry, particularly if the designer is trying to optimize power added efficiency (PAE). To enable designers to solve this problem, VNA systems are now available that work with both passive and active tuners in the form of non-linear measurement systems.

If you want to learn more about VNAs and their applications, download an application note here.

Calibration Techniques Help Ensure Accurate VNA Measurements

February 6, 2013

Those unfamiliar with VNAs may interpret “calibration” as an instrument calibration performed at regular intervals. However, with VNAs it is necessary to perform a measurement calibration as part of the setup to measure the device under test (DUT). The reason this is important is because measurements done without calibrations will be limited due to imperfections such as:

1. Source and Load Match — The raw match of a VNA is very good but not excellent. Even a match of 20 dB can lead to errors of greater than 1 dB.

2. Directivity — While high-quality couplers are used in a VNA, there is a certain amount of source signal coupling, even when a perfect termination is connected.

3. Frequency Response — Though the internal frequency response of a VNA can be factory calibrated, cables connected externally will have a frequency response that must be addressed.

4. Isolation — Rare with modern VNAs, this may be a problem with older instruments.

Calibration corrects these imperfections. Many possible calibration algorithms are implemented within VNAs. The choice between them is largely determined by the media the engineer is working in, calibration standards available and desired accuracy/effort trade-off. These choices can be categorized in two ways: calibration type and calibration algorithm.

Types of Calibrations

There are several calibration types:

 

The full 2-port calibration with isolation calibration corrects for the following error terms:

• Transmission frequency response

• Reflection frequency response

• Source match

• Load match

• Directivity

• Isolation

Since these must be corrected for the two ports in both forward and reverse directions, these six terms are doubled to become what is commonly described as the 12–term error correction model.

Calibration Algorithms

Calibration algorithms use a variety of acronyms, which can be inconsistent in literature. To add further confusion, the letters may represent different things at different times. The following table shows the acronyms used in Anritsu documentation and represent the most common usage of algorithm acronyms.

Calibration Kits

VNA vendors provide calibration kits for a variety of algorithms and circumstances that contain calibration standards and the coefficients that describe them. Anritsu offers the following:

• SOLT Kits

• Kits With Triple-Offset Shorts

• LRL Kits

• Offset-Short Waveguide Kits

• Microstrip and Coplanar Waveguide Kits for the Universal Test Fixture

• On-Wafer Calibration Kits

Automatic Calibration

Automatic calibration techniques are often the preferred method for calibrating VNAs, such as the Anritsu VectorStar® family. Automatic calibration modules incorporate extremely accurate, repeatable solid-state switches to select a variety of impedance standards from just one connection between the VNA and a calibrator module.

Calibrations employing these modules are consistent, repeatable and provide better accuracy than traditional broadband-termination, 12-term calibrations. They are also much faster than traditional calibration kits.

Residuals and Uncertainties

Once calibrated, you will want to know the measurement quality. Information provided by the manufacturers includes residual value and uncertainties.

Whereas raw parameters describe the physical performance of the components (e.g., the directivity for the directional device), residual directivity is defined as what is left after the calibration. It depends on the quality of the calibration components, algorithm and the process.

The residuals at the time of the DUT measurement, together with other parameters, describe the measurement uncertainty, not the raw parameters. The degree of uncertainty varies by frequency, the magnitude of the measured s-parameter, whether it is transmission or reflection, and the type of calibration performed.

More information on VNA calibration is available in Anritsu Application Notes, here.