VNAs measure more than components
Article By : Martin Rowe
Vector-network analyzers have come down in price and size to where they have become scientific measurement instruments for measuring snow and soil composition and for characterizing materials.
RF engineers use vector-network analyzers (VNAs) to characterize components such as connectors, cables, filters, amplifiers, PCB traces. VNAs have also become important tools for signal integrity engineers. But, the high-end bench VNAs that these engineers often use are expensive, running hundreds of thousands of dollars. Lower-cost handheld and USB VNAs have made scattering parameters (S-parameters) measurements available to new applications. Applications now include materials characterization, snow depth and composition, soil composition, medical imaging, and others.
A VNA is essentially a high-frequency source-measure unit. It excites an object or circuit to a range of RF frequencies and measures the response amplitude and phase that exits the circuit or material under test or reflects back to the source. Most portable and USB VNAs provide two ports, where each port can both transmit and receive RF waves. That results in four measurements, often described as a 2×2 matrix. Figure 1 shows the matrix and the four S-parameters and how they relate to a two-port device or material under test. Higher-end bench VNA may have more than two ports.
Figure 1 A two-port device, system, or object under test connects to a two-port VNA, resulting in a 2×2 matrix of S-parameters when connected to a two-port VNA (VNA not shown).
If you’re familiar with S-parameters, then jump to Page 2 for unusual VNA applications.
S11 and S21 are referred to as forward S-parameters because the incident wave enters the device or material under test at port one. S11 is defined as the ratio of the wave reflected at port one to the incident wave entering at port one (Reference 1).
S21 is defined as the ratio of the wave exiting at port two to the incident wave entering at port one.
S22 and S21 are the reverse S-parameters. S22 is the ratio of the wave reflected by port two divided by the incident wave entering at port two.
S12 is the ratio of the wave exiting port one to the incident wave entering at port two. S-parameter values (amplitude and phase) change with frequency.
The DUT in Figure 1 need not be an electrical device. Indeed, it can be just about anything. You will, though, need a way to send and receive signals to and from the VNA. Electrical devices often have connectors to connect it to the VNA ports through cables or they can attach to a PCB. Other objects or substances may need a different interface such as an antenna.
[Continue reading on EDN US: Unusual VNA measurements]
—Martin Rowe covers test and measurement for EDN and EE Times. Contact him at martin.rowe@AspenCore.com
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