In radio frequency integrated circuits (RFICs), the high-frequency signals create unique phenomena that are not typically observed in regular digital and low-frequency analog ICs. Even seemingly trivial design changes to an RFIC can degrade its behavior and overall performance. As a result, rigorous simulations and verifications are essential after every modification without affecting team productivity and time-to-market.
If regular integrated circuit (IC) design itself is complex, imagine a niche that is an order of magnitude more complex. Even a tiny design change can drastically degrade their functionality and performance. The industry often uses terms like black magic and wizardry for them.
Yes, we’re talking about the esoteric art of designing radio frequency ICs (RFICs) and their even more sensitive cousins in the microwave and millimeter wave (mmWave) bands.
In the previous blog onelectronic design automation (EDA), we explained how EDA tools are used for digital IC designs and briefly mentioned analog ICs. In this post, we explain the specialized EDA tools that provide the rigorous simulations and validations required for designing RFICs, monolithic microwave ICs (MMICs), and mixed-signal ICs.
Figure 1. Stability analysis using EDA tools
EDA tools for RF and mixed-signal IC design accelerate the design of RF and mixed-signal semiconductor devices using in-depth knowledge of all the complex phenomena and effects that occur in these high-frequency analog circuits. These circuits are typically composed of wafer-level power amplifiers, oscillators, filters, mixers, modulators, demodulators, antennas, transmission lines, and impedance-matching networks.
Some of the common phenomena that engineers must design for are signal degradation, electromagnetic interference (EMI), crosstalk, parasitic effects, and antenna effects due to the high-frequency signals used in RF, microwave, and mmWave use cases.
In this context, RF conventionally ranges from tens of megahertz (MHz) up to 3 gigahertz (GHz), covering most wireless communications (like Wi-Fi, 2G/3G/4G telecom, and Bluetooth), satellite communications, and global positioning systems. The microwave band of 3-30 GHz is used by radars and Wi-Fi 5/6/7. The mmWave 30-300 GHz band is used for 5G/6G telecom, 802.11ad gigabit Wi-Fi, radars, and automotive vehicle-to-everything.
To predict high-frequency behaviors and mitigate their effects, these specialized computer-aided design and automation software provide features like:
- simulating all the high-frequency effects of various alternating and direct current waveforms
- predicting scattering parameters (S-parameters) from circuit schematics
- modeling RF-relevant aspects of the physical layouts of chips, such as the antenna effects of interconnects and bonding wires
- predicting parasitic behaviors from the physical layouts of chips
Figure 2. RFIC design flow
RF and mixed-signal IC design is very different — typically even more rigorous and cautious — compared to regular IC design, which is an already rigorous workflow. The sections below outline these differences.
Careful analysis of every design change
Everything in RFIC design is much more sensitive to even trivial changes. At high frequencies, every interconnect and bonded wire is a radiating antenna that adds noise. Every capacitor exhibits inductance, and every inductor has capacitance. Even a small change in component specification, layout, or packaging can drastically attenuate a signal.
So, predictive simulations after every change are essential, starting from the schematic stage itself through the physical layout right up to tape-out. In fact, even the post-packaging stages are simulated because the packaging as well as printed circuit board components around an RFIC can affect its RF performance.
Different metrics
Since mixed-signal ICs process digitally modulated signals, they require metrics like error vector magnitude (EVM) instead of the traditional P1dB or third-order intercept point (IP3) analog specifications. EDA tools must facilitate the tuning and optimization of EVM at the circuit level.
More complex fabrication
RFIC fabrication is different in every way.
First, the semiconductor materials are different, which requires unique device models. For example, regular digital ICs use silicon with simple complementary metal oxide semiconductor (CMOS) processes. In contrast, RFICs use silicon germanium in BiCMOS configurations, and MMICs prefer III-V materials like gallium arsenide, indium phosphide, and gallium nitride.
Second, there are no simple standardized cell libraries like in digital ICs. Miniaturization of passive components is unique to every RFIC design. Resistors are typically implemented as diffused regions in the semiconductor substrate and adjusted by changing dimensions and material properties. Capacitors are formed using overlapping metal layers with an insulating dielectric layer in between or metal-insulator-metal structures. Inductors are created using spiral metal traces on the die.
For these reasons, RFIC fabrication is offered by foundries that specialize in RFICs, MMICs, and III-Vsemiconductors.
When designing an RFIC, EDA tools must consider how these components will interact, their parasitic effects, and other high-frequency phenomena.
System design budgets
Most of the systems that RFICs and mixed-signal ICs go into often involve stringent regulations and standards. So the system-level specifications impose budgets on parameters like the noise figure, power, phase noise, harmonics, linearity, and more. These budget constraints are passed down to the RF designers.
To satisfy these complex constraints without affecting signal integrity and performance, EDA tools are essential.
EDA tools are typically used as follows:
- Circuit simulations: These are computational techniques to model and predict the behavior of electronic circuits based on their schematics. Mathematical equations or models describe the behavior of each component under different operating conditions. After modeling the circuit, simulation software is used to solve the equations and predict key characteristics of the circuit’s behavior. The increasingly complex and dense designs of modern RFICs require complex simulators capable of handling large intricate circuits.
- Stability analysis: High-frequency transistors complicate the design flow for stable circuits. Instability problems can emerge at lower frequencies due to significant increases in gain. EDA tools allow stability analysis of amplifiers.
- EM co-simulation: These simulations allow the EM characterization for every component of the design. The circuit designer can perform 3D EM analysis and EM-circuit co-simulations iteratively throughout the design phase.
Figure 3. EM visualization
The design workflows for RFICs and mixed-signal ICs are very different from digital IC workflows, as outlined below.
Synthesis
Digital circuits consist of a small number of well-defined logic gates (like NAND). The circuit schematic is converted to a gate-level netlist expressed in a hardware description language like Verilog or very high-speed integrated circuit hardware description language (VHDL). During routing and placement, these gate-level constructs are then converted to on-wafer cells defined by the cell libraries in the selected fab’s process development kit (PDK).
In contrast, RF and mixed-signal ICs are analog circuits with unique custom designs. The arrangements of resistors, inductors, capacitors, and active components into amplifiers, mixers, or other subsystems are often unique to each IC. They are not readily distillable into standard cells like digital gates are.
Instead, each subsystem is individually converted into on-wafer structures and interconnects. For example, an oscillator may utilize a complex configuration of transistors in feedback loops. The foundry PDKs for RFICs do provide process information, design rules, and models for active and passive components, but they are not as simple or standardized as digital ICs.
Simulation
In digital chip design, the digital nature of signals makes simulations relatively simpler. Digital IC simulations include various digital inputs and timing analyses.
In contrast, RFICs must contend with an infinite set of continuous high-frequency waveforms. Noise, parasitic effects, electromagnetic interference (EMI), and antenna effects emerge from the circuit arrangements as well as the physical on-wafer structures. So, realistic simulations are required at every step throughout the design cycle.
Verification
Design-rule checks (DRC) and implementation using a field-programmable gate array (FPGA) are common in digital IC design.
DRC verification tools are used in RFIC and mixed-signal semiconductor design as well. However, prototyping an RFIC with an FPGA is rare because their RF characteristics will be totally different. FPGAs are still used to verify the digital portions of mixed-signal ICs.
In addition to accurate RF and EM modeling and simulations, an essential feature is the ability to test the designs against wireless standards (like 5G and 802.11ad) right from the start.
This is possible using design tools that include virtual test benches (VTBs) for all the major wireless standards. VTBs ensure that the designs stay within the thresholds that standards place on signal power, noise, interference, and more.
What are the main challenges faced when using EDA tools in complex 3D circuit design?
Some of the main challenges in 3D RF circuit design include:
- modeling electromagnetic behaviors in 3D
- impedance matching
- noise
- linearity
- stability
- power consumption
- electromagnetic interference
- problems caused by increasing densification like integrating different materials, 3D integration, and advanced packaging
For effective design of RFICs and mixed-signal ICs, look for these key features:
- Advanced simulation algorithms developed for Keysight RF instrumentation and Keysight RF EDA software ensure equivalent results between virtual simulations and physical measurements.
- Fast-envelope techniques, compact test signals, and distortion EVM make simulation run times practical for RFIC designs in data-intensive, high-bandwidth mmWave or sub-THz applications, including 5G/6G, electric vehicles, and AI-enabled systems.
- Authentic waveforms and VTBs incorporate system context when simulating designs, enabling teams to optimize RFIC designs for their intended system applications sooner and successfully integrate first-pass into physical devices and systems.
Figure 4. Keysight RFIC EDA tools
Keysight’s ecosystem ofEDA tools streamlines RFIC and mixed-signal IC design, simulation, and verification. For large design teams looking for high productivity, the Keysight designcloud enables offloading RF and EM simulations to high-performance cloud platforms for rapid results and quick design cycles. The main design and simulation tools for RFICs and mixed-signal ICs are outlined below.
Keysight Advanced Design System (ADS)
Figure 5. Keysight ADS
The Advanced Design System is tailored for high-frequency RFIC and mixed-signal IC design and simulation. It achieves 3D heterogeneous integration (3DHI) of RFICs, MMICs, packaging, PCBs, and antennas using multi-technology modules. Key features are listed below.
- Multi-technology integration: ADS enables comprehensive 3D integration of chips, packaging, interconnects, and boards, facilitating realistic simulations of assembled products.
- Superior simulation capabilities: The platform’s advanced circuit-level RF simulations and EVM optimizations for mixed-signal ICs produce predictions that are close to real-world measurements.
- Comprehensive component libraries: The platform offers vendor component libraries and PDKs that include symbols and layout footprints, as well as high-accuracy RF and microwave models.
- Standards compliance: ADS enables verification against wireless standards like 5G, automotive radar, and 802.11ad.
- Flexible automation: ADS supports Python scripting for automation and integration with external applications like artificial intelligence (AI) and machine learning (ML) to streamline design and verification workflows.
RFIC Design (RFPro Circuit)
RFPro Circuit is a specialized software for RFIC design. It can:
- model components on silicon chips accurately
- optimize designs with sweeps and load-pull analysis
- simulate RF designs in the Cadence Virtuoso and Synopsys Custom Compiler environments
- increase performance using Monte Carlo and yield analysis
- assess error vector magnitude (EVM) for the latest communication standards early in the design phase
- use the latest foundry technology immediately
Keysight RFPro
Figure 7. RFPro
RFPro enables RFIC and MMIC designers to run interactive electromagnetic-circuit co-simulation for tuning and optimizing their circuits. It includes 3D planar and full 3D finite element method EM simulators.
PathWave ADS provides Python application programming interface (API) endpoints for integrating AI frameworks like TensorFlow and PyTorch. This enables the use of advanced artificial neural network models in the design modeling and simulation workflows.
How is Python integrated with Keysight EDA tools?
Keysight EDA tools provide Python API endpoints that expand custom features and improve usability.
For example, Python scripts can be used to control data analysis, simulators, and processes.
Python scripts enable the training of custom AI-based simulation models that are trained on measured or published data.
Streamline your RFICs with Keysight EDA solutions
This post gives you an overview of the challenges and problems of RFIC and mixed-signal ICs that EDA tools can solve.
Contact us for deeper insights on your specific RFIC design challenges.