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mmWave Matls

Calculator Predicts PCB Performance

February 8, 2019

The characteristics of circuit materials have a great deal to do with how well a printed circuit board (PCB) performs, especially at RF/microwave frequencies. Circuit material parameters such as dielectric constant (Dk), dissipation factor (Df), even material thickness can affect the way that different transmission lines, such as microstrip, stripline, and coplanar waveguide, perform in high-frequency circuits. Fortunately, there is a simple and pain-free way to predict how different high-frequency transmission lines will behave when fabricated on different circuit materials: the Microwave Impedance (MWI) Calculator software.

This transmission-line performance calculator and simulator software is available as an executable (.exe) file for free download from the Rogers’ Technology Support Hub, or it can be run from the site by means of access with any personal computer (PC) capable of running the Microsoft Windows 10 operating system (OS). The latest version of the software, MWI-2019, uses closed-form equations to compute the performance of transmission lines that are formed on different circuit materials according to differences in basic circuit material parameters, such as Dk, thickness, and other material parameters. The software can calculate different performance and operating parameters for the transmission lines when formed on each material, including characteristic impedance, loss, and quality factor (Q). The software allows users to select from a wide range of circuit materials and perform calculations based on a wide range of high-frequency transmission-line technologies, including microstrip, stripline, grounded coplanar waveguide (GCPW), edge-coupled microstrip, edge-coupled stripline (or differential-pair stripline), and broadside coupled stripline.

A user’s manual is also available for free download from the Rogers’ Technology Support Hub to help users get started with the MWI-2019 software. The software’s closed-form equations provide very accurate calculations, backed by many measurements on transmission lines fabricated on the circuit materials of interest. In most cases, the predictions for a given circuit material and given transmission-line technology match closely with vector-network-analyzer (VNA) measurements performed on transmission lines fabricated on the actual circuit materials, especially for some of the most popular transmission-line types, including microstrip, stripline, and GCPW.

Speedy Solutions

In addition to its high accuracy on run-of-the-mill PCs, it offers fast operating speeds. In comparison to many commercial electromagnetic (EM) field solvers that are often used for predicting transmission-line behavior for different circuit materials, where computer predictions may require hours at a time even for the most powerful and up-to-date PCs, the MWI-2019 software is written to perform well on PCs with fairly routine capabilities, and to provide accurate results almost instantly once the “calculate” button is pressed.

Of course, the long processing times of EM field solvers can deliver considerably higher accuracy than a handy, free software solver like MWI-2019 and is not meant to replace an EM simulator. The software provides very useful approximations, and the speedy calculations permit a circuit designer to predict performance possibilities for any number of different transmission-line circuits on circuit materials with different design Dk values and thicknesses among other circuit material characteristics. The speed of MWI-2019 allows a designer to investigate a wide range of different circuit materials when considering what might be the optimum set of circuit parameters for a given high-frequency transmission-line technology and circuit design, and the designer can always turn to an EM simulator when higher simulation accuracy is needed and more processing time is available.

The MWI-2019 software is written for quick comparisons of how a transmission line will behave on different circuit materials, allowing users to jump from one material to the next to compare, for example, how transmission line loss for a microstrip circuit is affected at specified frequencies for each circuit material studied. When a circuit material is changed, the software will use the nearest standard thickness for each material, along with an optimum copper type for each material to simplify comparisons.  These automatic standard values can also be over-ridden by users who are interested in making comparisons for different materials with very specific values, such as Dk and thickness.

Impressive Accuracy

Despite the fast calculation speeds, MWI-2019 provides extremely accurate results, applying closed-form equations and models tailored to each transmission-line technology. Users can perform predictions on transmission-line circuits and materials using standard material parameters built into the software or by fine-tuning circuit material parameters including design Dk, thickness, even definitions for the copper conductors (several models for copper surface roughness are contained within MWI-2019) before performing calculations for a transmission-line technology on a circuit material. The software allows users to over-ride standard values and enter their own values for customized predictions at frequencies of interest to quickly explore how the behavior of different transmission lines will change with choice of material and frequency.

When a user selects a circuit material from within MWI-2019, along with the thickness of the circuit material and an operating frequency, the software sorts through a large database of specific Dk values (through the thickness or z-axis of the material) saved from measurements made as a function of frequency for the different materials. This database of Dk values for each material as a function of frequency is built from measurements performed on hundreds of test circuits at different frequencies and for different thicknesses of the circuit materials listed within the tool. These values of design Dk are also available over wide frequency ranges: this latest version of the software features design Dk values for frequencies from 100 MHz through 110 GHz in support of the growing number of millimeter-wave applications at 60 GHz and higher for emerging 5G cellular communications networks and automotive radar safety systems. In comparison to earlier versions of the software, MWI-2019 also contains more circuit materials to choose from; all supported by the large design Dk database and transmission-line models that have been fine-tuned for each material.

Two of the most widely used high-frequency transmission lines, microstrip and stripline, may also be two of the most accurate models in MWI-2019. The software provides predictions that are extremely close to values that have been measured with RF/microwave VNAs for transmission lines that have been fabricated on real circuit materials. Models and closed-form equations also provide very accurate results for the other transmission-line types, such as edge-coupled microstrip and edge-coupling stripline, although without the large amount of measured data to reinforce the repeatability and accuracy of the predictions.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.