Assessing Options for RF Circuit-Level Design

Designing printed circuits for RF/microwave frequencies requires many choices, including the type of transmission-line technology, suppliers for active and passive components, type of printed-circuit-board (PCB) material, and even the thermal-management approach. Fortunately, modern circuit-level software simulation tools can help, by eliminating the cycle of building and measuring prototype circuits and then doing it again until the performance levels approach target goals. The software can use a PC to show what might happen following a series of choices on an RF/microwave circuit design. Still, every choice along the way contributed to the final outcome when creating a high-frequency circuit.

Target performance specifications usually drive most of the choices required for designing an RF/microwave circuit. Specifications such as frequency, bandwidth, signal power levels, and operating conditions (such as temperature range) can narrow the number of choices for a high-frequency circuit design. For example, transmission-line technologies such as microstrip, stripline, and even coplanar waveguide (CPW) are typically used for circuits operating at RF and microwave frequencies. But for higher, millimeter-wave frequencies, such as 66 and 70 GHz, the conductive and radiation losses from microstrip and stripline transmission lines can render them unusable at millimeter-wave frequencies and more practical choices for transmission lines might include grounded CW (GCPW) or even more novel technologies such as substrate-integrated-waveguide (SIW) transmission lines.

Armed with a choice of transmission line, a high-frequency circuit designer still has to decide on a choice of PCB material, and there are many available in the industry, including “hard” circuit-boards based on ceramic materials and “soft” circuit boards that use polytetrafluoroethylene (PTFE) and other flexible dielectric materials. Again, each material choice must be evaluated in terms of the requirements of an application, such as frequency range, power-handling capabilities, and even whether a circuit will be intended for linear or nonlinear operation. Each PCB material choice will be made based on its own set of parameters, such as relative dielectric constant and its consistency with frequency. Such material parameters will determine the dimensions of the transmission lines for a target frequency range and impedance.  

Where to Begin

The number of choices involved in creating a practical high-frequency PCB design is enormous, and underscores the contributions that effective circuit-level simulation software programs have made to the circuit design process. Computer-aided-engineering (CAE) software for circuit design comes in many forms, even as free-of-charge software tools such as CircuitMaker from Informer Technologies. This is a schematic-capture (working with schematic diagrams of circuits) software tool with surprisingly advanced simulation capabilities for a no-charge program. The software can be used to design and simulate analog, digital, RF, and mixed-signal circuits and create circuit netlists that can serve as input files for autorouters and tools such as the circuit-board plotters from LPKS Lasers & Electronics. The software even includes a version of a Berkeley SPICE simulator for enhanced simulation of digital circuit designs. Admittedly, this software is geared toward general-purpose, lower-frequency designs rather than RF/microwave circuits and their needs for special transmission-line structures and component models.

For RF/microwave circuit design, one of the better-known software tools is Microwave Office from AWR Corp., now part of National Instruments. As with a number of high-frequency circuit-design programs, Microwave Office is an integrated design environment that includes a number of different software programs (including its Visual System Simulator system simulator) and even links to test equipment that allow designers to incorporate test data on prototype circuits when refining and optimizing a circuit design. Microwave Office supports schematic circuit layout and design, electromagnetic (EM) simulation of circuit designs, circuit optimization, and analysis of yield for a given circuit design based on the difficulty of fabricating that circuit with a given circuit or semiconductor assembly process.

This suite of software tools has been a mainstay of high-frequency circuit designers for many years and has been used in the design and fabrication of RF/microwave circuits from larger, discrete-component PCBs to miniature monolithic microwave integrated circuits (MMICs) on a variety of substrate materials, including silicon, gallium arsenide (GaAs), and gallium nitride (GaN). On its website, the company provides practical examples of active and passive circuit designs, such as amplifiers and filters, respectively, created with its code. The firm also offers a free, trial version of Microwave Office. It provides a good look at the many circuit-design and optimization capabilities within Microwave Office before having to make a financial commitment.

Another comprehensive suite of software tools that provides fast and effective RF/microwave circuit design is the Advanced Design System (ADS) from Keysight Technologies. This is a software suite with high-frequency circuit design built upon the use of device X-parameters and EM simulation which also incorporates full system-level design capabilities and extensive links to the firm’s test-and-measurement equipment for verification of software models through testing. It provides a straightforward schematic-capture and layout environment for the design of RF/microwave and high-speed digital circuits and includes a large number of process design kits (PDKs) that are meant to simplify the transfer of a circuit design to a commercial semiconductor foundry for fabrication of MMIC-based designs. The company also aids circuit designers choosing to focus on EM simulation through its EMPro 3D EM simulation software and Momentum planar EM simulation software.

Going 3D

Even software programs dedicated to circuit design are now multifaceted tools that go well beyond the capabilities of earlier circuit simulators and often incorporate three-dimensional (3D) EM simulation capabilities to analyze and predict the performance of transmission lines and components as well as their radiation characteristics. As an example, CST Microwave Studio software from Computer Simulation Technology is a powerful circuit-design software program that builds upon 3D EM simulation capabilities. It is particularly effective for the design of passive circuits, such as antennas, attenuators, filters, and couplers, on single-layer and multilayer circuits using a wide range of PCB and MMIC substrate materials. The software employs frequency-domain and time-domain analysis techniques, and can extract SPICE parameters for time-domain analysis. It includes numerous solver modules based on particular applications, such as filters, to help speed the design of a user’s own circuits.

The use of EM simulation for single- and multilayer RF/microwave circuits is growing, as these computing-intensive software tools become more practical with increasing PC power. Designers are discovering the effectiveness of analyzing the EM behavior of different high-frequency transmission lines and circuit structures and are avoiding extra prototype stages as part of more efficient circuit design processes. Numerous EM simulators are available to ease the design process, including HyperLynx 3D EM design software from Mentor Graphics, IE3D EM simulation software from Zeland Software (which was acquired by Mentor Graphics), and various EM-based software simulators from Remcom.