October 30, 2020
Most high-frequency electronic circuits in today’s designs are built on dielectric materials and optimal operation depends on their dielectric properties. In general, these materials can be categorized into three segments – insulators, semiconductors, and conductors.
The materials, as shown in figure 1, can be further described as:
- Insulators (dielectrics), in which the energy gap between the valence and conduction band is huge. The result is no free electrons are available.
- Semiconductors have a smaller energy band gap, so some electron movement from the valence band to the conduction band can occur.
- Conductors allow electrons to move freely between the valence and conduction bands because they overlap.
Engineers who design these circuits must gain a vital understanding of the properties of the dielectric materials. It is especially true with respect to the dielectric constant and loss tangent at the operating conditions. In today’s post, we will discuss how to best gain this understanding using a vector network analyzer (VNA).
Material Measurement Methods and Techniques
Dielectric measurement is becoming an increasingly important tool to understand material behavior, because of the dramatic growth in high-frequency designs. This type of measurement can provide the electrical or magnetic characteristics of the materials, which is a critical parameter when using them in emerging applications such as those operating in the millimeter wave (mmWave) frequencies. A number of methods have been developed to measure the complex permittivity/permeability of materials in the time/frequency domain based on reflection and reflection-transmission measurements.
For the dielectric measurement, engineers cannot rely on a single technique to characterize all the materials over a wide range of frequencies. Resonant and non-resonant methods are two prominent approaches.
Highly accurate and precise, resonant approaches generally include the resonator and the perturbation method. Five main families of resonant techniques - micro strip; cavity resonator; dielectric resonators; open resonators; and split post resonators (SPDR) – comprise the resonant method. Two measurement techniques fall under this category:
- Resonator method is founded on the fact that the resonant frequency and quality factor of a dielectric resonator with given dimensions are determined by its permittivity and permeability. This method is often used to measure low loss dielectrics whose permeability is μ0.
- Perturbation method is centered on the resonant perturbation theory. For a resonator with given electromagnetic boundaries, when part of the electromagnetic boundary condition is changed by introducing a sample, its resonant frequency and quality factor will also be altered. From the changes of the resonant frequency and Q, the properties of the sample can be derived. This method is suitable for lower and moderate loss samples.
In non-resonant methods, the material properties are derived by the impedance and the wave velocities in the material. When a wave travels from one medium to another, both its impedance and velocity change. Useful information can be deduced from the reflections and reflection transmission of the wave in the medium. Both permittivity and permeability can be calculated using this procedure.
Non-resonant methods are mainly comprised of two processes:
- Reflection method directs electromagnetic waves to a sample under study and the properties of the material sample are deduced from the reflection coefficient. Usually a reflection method measures either permittivity or permeability. Two types of reflections are often used in materials – property characterization and open-reflection. As coaxial lines can cover broad frequency bands, coaxial lines are often used in developing measurement fixtures for reflection methods.
- Transmission/reflection method inserts a piece of transmission line/waveguide into the material under test (MUT). The material properties are deduced on the basis of the reflection from the material and the transmission through the MUT. This method is widely used in the measurement of the permittivity and permeability of low conductivity materials.
The principle behind transmission/reflection method is that the characteristic impedance of the piece of transmission line loaded with the sample is different from that of the transmission line without the sample. Therefore, such difference results in special transmission and reflection properties at the interfaces. The permittivity and permeability of the sample are derived from the reflection and transmission coefficients of the sample-loaded cell.
Free Space Material Measurement Technique
Free space material measurements are used for characterizing materials that remain in an unchanged state. There are many advantages associated with the free space material measurement technique, since the MUT can be placed under a variety of conditions (i.e. temperature, pressure, and tensile variations) and the response can be monitored using the same setup for a broad range of frequencies. It must be noted that the MUT must be large and flat for this technique to be used.
A test station for free space material measurements usually utilizes two antennas. One is connected to port 1 and other to port 2 of a VNA. They are aligned in the test setup so they face each other. Focus beam antennas are typically used, though simple horn antennas can also be integrated into the system in some test environments.
VNAs are typically calibrated before making this type of measurement. The most common calibration techniques are the through-reflect-line (TRL), through-reflect-match (T/LRM), and the line-reflect-line (LRL). The LRL calibration method, having the highest calibration quality, is often chosen.
Figure 3 highlights the importance of time domain gating when this technique is used. It ensures that the multiple reflections from the DUT and edges of the antennas are not used by the VNA in the final measurement calculations. The dielectric properties are then determined by post-processing the measured reflection and transmission data using a program. Focal/lens antennas in the free space material measurement further enhance system accuracy, as they take care of the diffraction from the edges of the samples.
Anritsu has published an application note, Material Measurements with Vector Network Analyzers. The downloadable document provides more details on testing this aspect of high-speed designs.