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The Rog Blog is contributed by John Coonrod and various other experts from Rogers Corporation, providing technical advice and information about RF/microwave materials.

Calculators Enhance Circuit Materials

January 26, 2014
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Choice of printed-circuit-board (PCB) material can be critical to the performance of an RF/microwave circuit design, since so many different circuit-material parameters must be considered. As those who follow this Blog series know, many parameters can impact the selection of a PCB material for a particular design, including dielectric constant (Dk), dissipation factor (Df), and a variety of thermal traits. To ease the process of reviewing these many circuit-material parameters, Rogers Corp. offers a number of practical calculators on their web site’s Technology Support Hub. One of the more useful of these personal-computer-based calculators is the ROG Calculator Online.

The ROG Calculator Online is actually a collection of different calculators related to PCB materials. These calculators can perform conversions among different conductive copper weights and thicknesses, estimate thermal coefficient of Dk and coefficient of thermal expansion (CTE) for different circuit materials, and even run conversions between return loss (in dB) and voltage standing wave ratio (VSWR).

The ROG Calculator Online is essentially a number of tools aimed at designers and users of high-frequency PCBs. The different calculators are available from a top menu bar as drop-down menu choices with thermal, measurement, and electrical calculators. The thermal utilities, for example, include a temperature coefficient of Dk (TcDk) calculator and a CTE estimator. Based on user-supplied values for a PCB’s CTE and the current temperature, the CTE estimator finds the percent change in dimensions of the PCB and its approximate dimensions based on the material’s CTE at that temperature. Similarly, the TcDk calculator computes the approximate Dk of a circuit material based on its nominal Dk, its TcDk, and the temperature entered by the user. The software also provides temperature conversions from/to Fahrenheit, Celsius, and Kelvin temperatures, which can be very handy.

Among the many computational tools in the ROG Calculator Online are several Measurement submenu selections, including one item that provides conversions between standard copper foil weights, such as 0.5 and 1-oz. copper, and foil thickness in different units of measure, such as centimeters, inches, and microns. Also, the Electrical submenu includes utilities to calculate return loss, resonance, characteristics of composite metals, and composite Dk and Df. As many as five materials can be formed into a composite PCB material for analysis. The return loss calculator provides quick conversions from among VSWR, return loss, and reflection coefficient. The resonance calculator, based on user-supplied values of inductance (in nH) and capacitance (in pF), calculates a resonance frequency (in GHz) for the combination of electrical values.

The composite materials calculator allows a user to form a multilayer material structure with different metals, different layers, and different metal skin depths, and find a usable frequency of the structure. The composite Df and Dk calculator can also be used to make calculations on multilayer structures, defining as many as five material layers and different layer thicknesses in a circuit structure. This calculator is based on specific Rogers’ material choices (and their associated Dk values) and requires that at least two materials layers are selected. The software, which performs computations based on layered parallel-plate dielectrics, can calculate the composite Dk for the multilayer structure and provide the overall thickness of the composite structure.

Among the other computational tools available from the Rogers web site are the Rogers’ Dk calculator and the MWI-2014 Microwave Impedance Calculator (with a comprehensive online user manual). The Dk calculator software determines the effective Dk of a circuit material from phase measurements with a vector network analyzer (VNA) of microstrip circuits fabricated on the material with different electrical lengths. The microstrip differential phase-length measurement method makes it possible to compare the phase of two different microstrip circuits with different electrical lengths, with analysis of the difference in phase between the two circuits helping to determine the effective dielectric constant of the PCB material. The two microstrip circuits should be formed in close proximity on the same PCB material, identical except in electrical length. The software’s documentation recommends the use of a circuit length ratio of 1:3 or more to provide a clear difference in phase between the two circuits. The shorter circuit will limit the low-frequency accuracy of the dielectric-constant computations. The documentation also recommends the use of a test fixure to ensure the same signal launch for both circuits. The Dk calculator software can provide outputs as raw Dk data or as averaged output data points. It can also produce a comma-delimited file for modeling and programming purposes, such as for use in circuit model development.

The MWI-2014 Microwave Impedance Calculator software is essentially a “manual on a disk” for circuit materials. It lists the names of different Rogers’ circuit materials, along with many of their key circuit parameters, including Dk, Df, thermal coefficient of Dk, and thermal conductivity, with additional material parameters revealed when the computer mouse cursor is floated over the name of a material of interest. Standard thicknesses are shown for selected dielectric and copper materials, but additional thicknesses can be selected to find the material parameters for those thicknesses. This software supports calculations for microstrip or stripline transmission lines, and includes references upon which the models and calculations for the different transmission lines are based. Calculations can be made based on VNA test data, on user-defined test points, and on a user-defined range of frequencies.

Among its many material values, the MWI-2014 software includes values for Design Dk, which are well suited for use with commercial computer-aided-engineering (CAE) software programs such as RF circuit simulators and electromagnetic (EM) simulators. The MWI-2014 software includes default z-axis bulk Dk values for each material, for simple impedance calculations, RF Design Dk values (for working within a specific or narrow frequency range), and digital Dk values, for executing more accurate impedance calculations.

The MWI-2014 software can compare different circuit models and how they will fare on a particular circuit material, although standard models are based on conventional microstrip and stripline transmission-line circuits. However, the different microstrip and stripline transmission-line models can be edited as needed. Once models are defined in the database, they can be readily compared with the click of a button versus standard models. As many as five models can be compared at one time, at specific test frequencies. Different circuit models and materials can be compared in terms of insertion loss versus frequency, provided that the models have been defined previously by generating a table of loss as a function of frequency. These loss-versus-frequency plots can be turned on and off with the click of an on-screen button.

The MWI-2014 software includes multiple transmission-line models for microstrip (conventional and edge coupled) and stripline (conventional, edge coupled, broadside coupled, and offset) circuit formats. Data can also be transferred from MWI-2014 into Microsoft® Excel®spreadsheet programs, allowing users to manipulate the data in Excel spreadsheets and generate graphs. With comma-delimited files, it is a straightforward exercise to generate graphs showing different performance aspects, such as dielectric loss, conductor loss, and total loss for a particular circuit material as a function of frequency. Information in MWI-2014 can also be highlighted and cut and paste into other Microsoft Windows® programs, such as Microsoft Word, for reuse.

Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.