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Millimeter waves are electromagnetic signals with frequencies ranging from 30 to 300 GHz that correspond to wavelengths of 10 to 1 mm in free space. Electromagnetic waves in the millimeter-wave band have attractive characteristics. One of their features is the wider usable frequency band compared with waves in the microwave band or lower bands. Another feature of using the millimeter-wave band is the fact that it becomes possible to design smaller and lighter equipment that utilizes that band. So it is useful to adapt millimeter waves for short-range broadband communication systems, high-resolution sensing systems, and radio astronomy.
Fifth Generation (5G) is looking to millimeter-wave frequencies for sufficient bandwidth for high speed, short-range data links. And automakers are relying on Advanced Driver Assistance System (ADAS) equipment including short-range radars operating at millimeter-wave frequencies for safer vehicles. In transitioning from 4G to 5G, frequencies have increased 40x—700 to 2600 MHz and 28 to 40 GHz—while automotive radar frequency bands have increased from 24 to 77 GHz.
To meet the demand for increased data rates and capacity, communications, aerospace/defense, and automotive applications are utilizing more bandwidth, millimeter-wave (mmWave) spectrum, and spatial efficiency vis-a-vis multiple-in-multiple-out (MIMO) and beam steering phased arrays. Along with a need for small form-factor, cost-sensitive devices, RF to mmWave front-end components continue to evolve in order to keep pace with these new system requirements, the demands of enabling technologies, and space/cost considerations.
Engineering teams addressing these performance, integration, and cost/space goals face time-to-market pressures while striving to meet these increasingly complex product requirements. Engineers trying to bring these products to market need best-in-class simulation technology and design automation to accurately predict the performance of larger, densely integrated circuits/ subsystems designed for broadband and mmWave spectrum. In addition, since these products are developed by diverse engineering teams across multiple design tools, RF design software must provide interoperability with the broader class of EDA used in the development of mixed-signal electronic systems.
Engineering design and analysis (EDA) software
The production of microwave and mmWave components involves the design, fabrication, test, and tuning. Software packages are an essential requirement for the design of microwave components. Their use prevents the costly process of redesigning and rebuilding prototypes and often ensures a final design that gives a close-to-optimal performance.
Engineers trying to bring these products to market need best-in-class simulation technology and design automation to accurately predict the performance of larger, densely integrated circuits and subsystems designed for broadband and millimeter-wave (mmWave) spectrum.
Engineering design and analysis (EDA) software has evolved into an essential tool for wireless circuit and system design. High-frequency engineers are now using RF/microwave EDA software tools to achieve effective and efficient design flows for a wide range of circuits and systems, in commercial, industrial, and military markets. In addition, engineers now rely on EDA software tools to shorten the product design cycles significantly for many different circuits and systems.
The use of design software replaces the design and tuning stages of the production by computational design and optimization thus drastically reducing the labour time and cost of design and re-tuning in the production stage. The traditional non-computational methods of design-build, test and tuning do not enable the optimization process to be thorough and complete. Computational optimization allows a complete sensitivity analysis to be carried out and leads to more accurate designs and a final tuned design in the first build; known as ‘first pass design success. As it takes far less time to complete a final design, it is more cost-effective than the traditional methods.
In the early days of simulation software, computer programs such as SPICE and ECAP were the only available EDA tools. SPICE was developed at the University of California at Berkeley and made available to the public in the mid-1970s. SPICE included nonlinear analysis in the time domain, but not transmission lines or scattering (S) parameters. It was not very useful for RF or microwave work. The latest version of SPICE, Spice3, is still supported and available along with user manuals from the University of California at Berkeley website. The ECAP software contained no transistor models (the user had to generate their own models and create a netlist for them), and it could not handle noise or nonlinearity.
High Frequency Challenges
Circuit designers face numerous changes when making a transition from microwave to millimeter-wave frequencies and develop millimeter-wave printed circuit boards (PCBs). Wavelengths continue to shrink as frequencies rise, requiring finer circuit structures. Also, signal power—generating it and maintaining it—is typically less at millimeter-wave frequencies than at microwave frequencies, so that low circuit loss is a key circuit design goal.
Higher frequency PCBs require a thoughtful choice of circuit materials well suited for millimeter-wave frequencies and circuit fabrication processes that support those higher frequencies.
Material Selection
Without considering the type of circuit structure, such as microstrip, stripline, substrate integrated waveguide (SIW), or grounded-coplanar-waveguide (GCPW) transmission lines at millimeter-wave frequencies, circuit materials for millimeter-wave circuits should be considered for whether they have optimal basic qualities, such as dielectric constant (Dk) and dissipation factor (Df), for circuits and applications making a transition to higher frequencies.
A circuit material’s Dk is related to the real component of the material’s complex permittivity while its Df or loss tangent is related to the imaginary component of the material’s complex permittivity. These and other essential circuit material qualities can provide invaluable insight into how a circuit laminate will perform at millimeter-wave frequencies compared to microwave frequencies.
Another basic circuit material quality to be considered when specifying a circuit material for the millimeter-wave frequency range is Df or loss tangent, often referred to as just its loss. Low Df values are usually associated with low Dk values and, as with Dk, low Df values are preferred for circuit materials to be used for millimeter-wave circuits. Similarly, as Dk variations can degrade a circuit’s phase response at millimeter-wave frequencies, a circuit material’s Df variations can impact a circuit’s amplitude response as a function of frequency, with loss increasing with increasing frequency.
Copper conductor surface roughness impacts millimeter-wave circuits especially as they are used for higher frequencies since the skin depth of an electromagnetic (EM) wave decreases with increasing frequency.
Electronic design automation (EDA) challenges for designers of RF and microwave circuits, modules and systems are:
Trends toward higher frequencies have significant consequences for RF and microwave component and system design. Increasing RF frequencies, RF coupling and integration densities create challenges for designers. They must correctly lay out 3D avoidance-route, multi-technology RF modules to connect RFICs, MMICs, wafer-level packages, laminates, antennas and PCBs.
Some of the software tools for microwave circuit design do not have integrated capability. These programs exclusively adopt a particular type of routine, e.g. a time domain solver or an electromagnetic analysis. Time-domain models generally use a SPICE routine; a popular choice for microwave design is HSPICE. Electromagnetic analysis software for the use of microwave circuit design either simulates a 3D planar model, where a method of moments applied to Maxwell’s Equations is used, or a fully 3D EM model is simulated, which can handle more complex 3D structures. There is a whole range of EM simulators, such as MAFIA and HFSS, for modelling complex structures.
A number of different simulation system characteristics contribute to the effectiveness of a modern design flow, including accurate EM models, tight integration between EM and circuit simulators, tight integration with system simulators, layout and circuit interoperability, and advanced graphics capabilities. EM simulators, as invaluable as they can be, still have a strong impact on the time needed to complete a design. The use of even a high-speed EM simulator for for every microstrip step and T-junction is a waste of time. It makes more sense to analyze such structures beforehand, store the results in tables of data, and interpolate them when needed.
EM simulation is not just for microwave circuits. For example, in a cellular handset power amplifier, the load impedance is only a few ohms. Even at 1800 MHz, the inductance of the chip’s output bond wires cannot be ignored, as it is a large fraction of this value. EM simulation is really the only accurate way to predict that inductance at design time. EM simulation is also useful for finding areas of dangerously high current density or, in high-power circuits, high-field regions that could lead to arcing.
In a modern EDA system, each circuit element has a single representation in the EM, layout, system, and circuit simulators. Because it has only one instance in the simulation system, changes in its circuit parameters are immediately reflected in the layout. Similarly, changes in the layout, such as a change in microstrip line length or width, are immediately seen in the circuit simulator. No “back-annotation” or other such processes are needed, and the need for layout-versus-schematic (LVS) checking is greatly reduced. In this way, a designer sees the consequences of a change immediately and can modify the circuit if something is not practically realizable. And indeed, this is how it should be: Layout is part of the engineering design and should not be off-loaded onto a layout technician.
Increasing RF frequencies, RF coupling and integration densities create challenges for designers. They must correctly lay out 3D avoidance-route, multi-technology RF modules to connect RFICs, MMICs, wafer-level packages, laminates, antennas and PCBs.
There must be a mapping between the elements of the different substrates when placing a chip on the top-level module design; then, the layout tool must compute the proper 3D position of objects in the multi-technology design for 3D visualization and EM simulation. A good layout tool can work for both package and IC designs, and an efficient layout environment ultimately facilitates easier assembly of technologies, regardless of the design origin.
Other new approaches provide circuit designers with the ability to electromagnetically evaluate any portion of their RFIC, MMIC, RF module and RF PCB design without layout modifications using component and net extraction techniques, cutting EM simulation time. Mesh domain optimization can also provide orders-of-magnitude faster simulations of selected (extracted) components and nets.
Integration densities and layout infill requirements introduce increased RF coupling effects and longer simulation processes. To reduce simulation time and effort, the leading RF EDA software suites use a technique called EM and circuit “co-simulation.” With co-simulation, circuit designers do not need specialists to set up EM tools since correct port types are automatically assigned. Circuit simulations and EM interactions unite in one integrated workflow.
Today, engineers are simulating much more complex circuits and designing multichip RF modules, complex ICs, and integrated microwave assemblies (IMAs) often involves many different topologies and interconnect levels. In response, EDA software developers are creating new tools to help simplify such complicated designs.
Agilent EEsof EDA recently added capabilities to its Advance Design System (ADS) to model and simulate circuits that bridge multiple substrates — and reconcile the multilayer interconnect on a single schematic. This improved schematic functionality, combined with the integrated Momentum 3D planar solver and layout capabilities, gives engineers the ability to simulate complex circuits in their entirety to detect and solve issues before manufacturing and assembly, allowing them to design closer to the margins.
The lack of integration between EDA products contributed to a poor design flow, the process of moving a design from concept to realization. Generally, the design flow consisted of an electrical design, which was turned over to a layout technician, who then created an IC or hybrid layout in graphics software. At that point, a problem in the design was often discovered (such as a component or circuit structure that didn’t fit the layout) and redesign was necessary. It was not unusual to have several design iterations through layout and circuit simulation before a design was complete. This process was slow, error-prone, and expensive.
The fundamental problem was that most EDA products were “point tools”: individual software products that performed a single type of analysis, such as circuit simulation, electromagnetic (EM) analysis, system simulation, or layout. Much of an engineer’s design time was spent operating these disparate tools and shuffling data back and forth between them, and it was difficult to identify, for example, a layout problem or an EM simulation error early in the design cycle. The obvious solution was to integrate point tools so that layout, EM analysis, and system simulation could be performed concurrently with the electrical design. The goal is to perform electrical design, EM design, and layout concurrently, so that iterations between all of these functions can be avoided. This capability smooths the design flow enormously and decreases costs dramatically.
Mainstream fully integrated design software tools for microwave circuit design
EDA tools have come a long way in the past few decades. They have evolved from individual tools with limited functions to systems that provide more accurate designs, smooth the design flow, and greatly reduce design costs. These advantages come from the use of modern software technology as well as improved technical capabilities. Evolving products, such as Virtual System Simulator (VSS) from AWR, make it easy to move from system-level design and behavioral modeling to detailed component-level design and ultimately performance evaluation. Today’s EDA systems are powerful and versatile, and a huge advantage over those of even the recent past.
Some of the mainstream fully integrated design software tools for microwave circuit design are by Cadence AWR, Agilent and Ansoft. These are the well-known professional design packages with the complete range of solvers i.e. linear, non-linear, time domain and electromagnetic solvers. H-SPICE, the latest and favoured time-domain routine is incorporated into both the AWR and Ansoft design packages. In order to simulate the nonlinear behaviour of microwave components a harmonic balance routine is used.
Cadence AWR Design Environment
To optimize product development throughput and successful designs, this software must offer automation that supports engineering productivity alongside seamlessly integrated circuit, system, and electromagnetic (EM) simulation, synthesis, and optimization. These capabilities are built into Cadence® AWR Design Environment® software, inclusive of AWR® Microwave Office®, AWR Visual System Simulator™ (VSS), AWR AXIEM®, and AWR Analyst™ software, providing a robust and complete platform for RF/microwave engineers to
develop next-generation communication and radar front-end electronics.
This is a user-friendly software with all of the capabilities necessary for the accurate modelling and design of microwave components. MW Office contains a linear, harmonic-balance, time domain, EM simulation and physical layout. It includes linear and nonlinear noise analysis and can model nonlinear behaviour existing in microwave devices e.g. amplifiers in compression, mixers and oscillators. The layout that is generated can be used to represent the structure analysed by the software’s 2.5D EM solver. MW Office cannot model fully 3D physical structures but is a powerful tool for MW circuits.
An efficient layout design capability, known as iNet (intelligent Net) enables the design of, say, multi-layer PCBs to be drawn quickly and accurately. System-on-chip designs are also possible as well as a time delay neural network in order to capture memory effects. An accurate and extensive list (library) of circuit elements are also provided, which is an important requirement of accurate modelling.
An additional option, known as ACE (Automatic Circuit Extraction) produces a netlist from the layout providing the most advanced elements to simulate the most accurate model (accounting for coupled line models, capacitive strays and other effects) often missed from standard designs.
AWR’s sister program: Analogue Office uses HSPICE for its time domain modelling but does not feature as extensive linear and nonlinear modelling capability required for microwave component design.
Simulation, analysis, and circuit layout is only the beginning of a new design. The real work starts in design verification in the laboratory, where component tolerances, parasitics, interactions, and layout decisions make the design cycle interesting.
Since its acquisition by National Instruments (NI), AWR has added refinements to its Microwave Office 2011 software that cohesively integrate Visual System Simulator (VSS) with NI’s LabVIEW. This union combines VSS’s environment for circuit modeling, input signal generation, and result measurement with LabVIEW’s DSP (digital signal processing) functions, signal generation, and toolkits for telecommunication standards as well as RF instrument control and RF measurement. VSS can co-simulate with LabVIEW, with LabVIEW’s virtual instrument (VI) capturing measurements of real-life components for more realistic results when combined with simulated circuits.
The V15 Advantage
AWR Design Environment V15 offers key new and improved technologies that provide greater design efficiency and first-pass success to engineering teams developing and integrating III-V and silicon (Si) integrated circuits (ICs), multi-technology modules, and PCB assemblies.
Engineering productivity is improved with new analyses, faster and higher capacity simulation technologies, time-saving design automation, and 5G New Radio (NR)-compliant testbenches that support power amplifier (PA) and antenna/array design, EM modeling, and RF/
microwave integration across heterogenous technologies.
Design Environment and Automation
New design environment and automation features help individual engineers and engineering teams be more efficient in their design entry, data display, and project management. Designers can adjust optimization goals directly from response plots, route design rule-compliant
intelligent nets (iNets) in real-time, import Gerber-based layout designs into AWR Design Environment for EM analysis, and provide more user capabilities for the design task at hand.
Addressing the increasing size and complexity of RF/ mixed-signal electronic systems, the new, faster layout and 3D viewer rendering capability in V15 enables users to zoom in and out and rotate large structures to inspect the physical design from any and all angles without any lag time. Large boards imported as IPC-2581 or ODB++ files from Cadence Allegro® PCB Designer or other layout tools can be readily inspected visually before editing and preparation for EM analysis using the PCB editing wizard.
In addition, V15 of AWR software expands support for RF/ microwave front-end component integration within multi-technology systems. The layer process definition file (LPF) in AWR Microwave Office defines the processing layers and parameters for the physical layout design. For analysis of heterogeneous substrates and multi-chip modules utilizing different semiconductor processes and laminates, AWR Design Environment software supports multiple process definitions within a single hierarchical project. The software’s “per-process technology” native LPF units, allow different processes to specify units that are most appropriate for a given technology (e.g., mils or microns).
IC and electronic system companies, particularly those incorporating IC packaging and/or multi-technology modules, face tremendous thermal challenges that can cause late-stage design modifications and derail project schedules. The Celsius Thermal Solver embedded within the Microwave Office software offers a solution for RF PA and MMIC designs, RF PCBs, and modules, supporting electrothermal analysis through model information sourced from
Microwave Office sofware’s project information, including existing MMIC (die or packaged device) design data and geometries such as layout, material properties, and powersource values from RF simulation.
PA Simulation and Design Support
To meet bandwidth requirements, 5G systems aggregate contiguous and discontiguous spectrum at sub-6GHz and take advantage of the available bandwidth at mmWave frequencies, both of which put pressure on PA designers who need to address linearity and efficiency concerns in the presence of high peak-to-average power ratios (PAPRs). In wideband PAs, baseband impedance variations over bandwidth can impact device linearity, resulting in intermodulation distortion (IMD) levels that vary asymmetrically with instantaneous signal bandwidth. This issue, which is associated with memory effects, is commonly addressed by using video bypass capacitors to terminate the baseband impedance with short circuits.
However, performance can be improved by considering alternative baseband impedance conditions. For instance, PA developers have shown significant improvements in linearity when active, baseband injection architectures such as envelope tracking (ET) are employed. V15 of AWR software allows designers to optimize PA linearity performance through video band load-pull analysis of PAs operating under two-tone excitations. Designers can plot IMD and third-order intercept point (IP3) results as a function of (F2-F1) impedance, directly investigating intermodulation products over swept input power. Load-pull analysis also supports impedance tuning at the 4th and 5th harmonics as well as the ability to generate contours on rectangular plots for enhanced visualization of performance versus load impedance.
Transistors designed for mmWave amplifier designs have high gain at lower frequencies, making them more prone to potential spurious oscillations. Stability analysis is critical to amplifier design and optimization, especially for these high-gain devices. The commonly used K- and μ-factors, derived from linear circuit simulation, can accurately predict whether a 2-port network is unconditionally stable, yet they cannot detect instabilities for multi-stage amplifiers or devices connected in parallel. Other stability analyses such as NDF and loop gain techniques, can overcome these limitations but do so at the cost of extensive computations, making them too slow for performing optimization.
The loop gain envelope technique for evaluating the envelope of traditional loop gain stability circles has been shown to cut the simulation time of this more rigorous approach to stability analysis from hours down to seconds, making it ideal for stability optimization. V15 of AWR software supports this method with a loop gain envelope code module and equation block that can be easily applied to new amplifier designs.
The support for loop gain envelope stability analysis offers designers the following benefits: f The stability and margin of stability of each device within an amplifier is quantified.
f The input and output terminations are analytically applied, increasing analysis speed.
f Fewer N phase combination evaluations are required, increasing analysis speed.
f Due to the speed enhancements, optimization of phase margin for each device within an amplifier is now possible.
Agilent Genesys
This is a good Windows based, user friendly design software interface and has reasonable capabilities for the design of microwave devices, but it may not be to the fullest extent of the other mainstream integrated design packages. Genesys does contain fully linear and nonlinear capabilities, a harmonic optimisation tool using harmonic balance (Harbec), a time domain simulation SPICE routine (Ceyenne) and momentum GX/GXF (a 3D planar EM simulation). The capabilities of Genesys do include those listed in “the summary of capabilities for MW Office” with the exception of the full-wave planar EM solver. Instead, Genesys has an integrated 3D planar EM simulator with efficient meshing (Momentum GXF), which is the same as that used in Agilent ADS. Genesys also has an advanced optimisation routine with multi-dimensional parameter sweep and a Monte Carlo yield. Overall, this software does provide reasonable accuracy for the modelling of
microwave elements.
Ansoft DesignerRF
has all of the capabilities for the design of microwave devices. Simulation results of an example filter using Ansoft DesignerRF have been compared with those from AWR Office and showed fairly good agreement. The interface is extensive, but user-friendly with a similar menu layout as Genesys and Microwave Office. The output plots and charts are also intuitively straight forward to use.
DesignerRF features: linear/nonlinear circuit simulation with real-time tuning, optimisation and sensitivity analysis, frequency domain and transient analysis, digital and analogue systems simulation, IC and PCB layout with Java and Visual Basic and a full-wave 3D electromagnetic simulation.
Meshing and simulating the full layout is a straight forward procedure. The software benefits from a wide range of parts in its built-in library models, good continuous support is also available from an Ansoft engineer.
DesignerRF, however, has some drawbacks: (1) the manufacturer’s parts contain data files for Agilent ADS and MW Office only, which means that an equivalent circuit would have to be constructed to model each device, otherwise a Spice or S-parameter file would have to be
used which can limit its accuracy, (2) the cost is high e.g. a commercial licence is, roughly, twice that of AWR MW Office and Agilent ADS for the full range of features.
ADS Agilent
ADS Agilent is a highly extensive and sophisticated piece of software. It contains a variety of bundles, including PLL (phase locked loops) design, time domain and EM modelling (momentum simulator). The range of examples is extensive and also comes with a design guide. However, the interface of ADS was found to be the least user-friendly of the software packages and would, therefore, require plenty of training and practice in order to use it to its full potential. For those used who have mastered the interface of ADS it is a tool with access to an extensive list of vendor libraries, X-parameter and other models providing a detailed and accurate tool for microwave component design.
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