What is RF Circuit Design?

RFIC design typically involves a top-down design and implementation process, followed by a bottom-up verification process. There are many variations in this overall approach. Here are the basic steps:

1. Develop a high-level specification for the design. What functions will it perform?  What are the key specifications, such as LNA gain and noise figure, PA output power, LO phase noise, and mixer conversion gain.
2. Create the device-level circuit descriptions using components such as transistors, inductors, and capacitors. This step often draws from a library of pre-defined devices in a foundry PDK.
3. Verify that the design delivers all its specifications using circuit simulation. During this step, the manufacturing process and operational variability will be modeled to ensure the device design remains robust in the face of these uncertainties.
4. Implement a physical layout of the design by assembling the pre-defined layouts of all components. Placement rules must be followed to ensure manufacturability.
5. An equivalent circuit is then extracted from the layout. Parasitic effects are now present in the design description, and the design is re-simulated to ensure it still operates as intended with these new effects added.

These traditional methods of RF circuit design and verification are insufficient to meet modern RFIC design standards. As designers seek to converge analog, digital, and RF functionality into large SoCs and operating frequencies continue to increase, it becomes increasingly difficult to design RF integrated circuits using pre-characterized device libraries and other legacy methodologies. Moreover, electromagnetic interactions at high frequencies lead to parasitic effects such as signal reflections, crosstalk, substrate leakage, and electromagnetic interference (EMI), which degrade circuit performance and should be considered as early as possible in the design cycle.

An RF circuit is a special type of analog circuit operating at the very high frequencies suitable for wireless transmission. One salient feature of an RF circuit is the use of inductive elements to tune the resonant circuit operation around a specific radio carrier frequency. The primary difference between RF design and low-frequency analog design is the type of analysis performed on the circuit.

In RF design, steady-state operation is a primary concern. The behavior of the circuit is often modeled in frequency domain with attention focused on the signal fidelity, noise, distortion, and interference. When modeling a modulated signal on an RF carrier, a hybrid time-frequency domain analysis is the most efficient. Time domain focuses on the dynamic signal changes, and frequency domain focuses on the RF carrier and its harmonics and intermodulation products. RF circuit variability, both manufacturing and design induced, must be modeled, and compensated for.

In analog design, circuit stimulus is treated as a continuously varying signal over time. In the context of wireless communications, analog design often refers to the “low frequency” or “baseband” circuit as opposed to the “RF” circuit. In the context of wireline communications, analog design often refers to the analog front end or high-speed analog transceiver circuits. The behavior of the analog circuit is modeled in the time and frequency domains with attention focused on the fidelity/precision, consistency, and performance of the resultant waveforms. Circuit variability, both manufacturing and design induced, must be modeled, and compensated for as well.

Digital design treats circuit stimulus as a series of discrete logic “ones” and logic “zeros” over time. A logic “one” is typically represented by the presence of the supply voltage for the IC and a logic “zero” is represented by the absence of this voltage (i.e., zero volts). The devices in digital circuits must spend most of their time at either logic “one” or logic “zero.” If the circuits processing these signals are consistent in their response to these logic levels, digital design works well. Analog design is responsible for delivering these qualities. This enables the analysis of circuit behavior using combinatorial and sequential models, only considering two voltages (“one” and “zero”), which substantially simplifies the design and verification process.

To put RF circuits, analog circuits, and digital circuits together in a radio system, an analog-to-digital converter (ADC) acts as a bridge between analog circuits and digital circuits. A mixer acts as a bridge between analog circuits and RF circuits. An antenna acts as an interface between an RF circuit and air space.

The Synopsys Custom Design Platform is a unified suite of design and verification tools featuring a complete RF development flow. It facilitates design/layout collaboration that makes it easy to communicate design intent and achieve RF design closure, as shown below.

For RFICs, a silicon-optimized electromagnetic simulation workflow complements the Custom Design Platform. With Synopsys VeloceRF inductive device layout synthesis and modeling tool, designers achieve fast, accurate RF component synthesis and modeling. Ansys RaptorX silicon-optimized electromagnetic solver for design analysis and modeling, dedicated to on-chip designs, models the passive elements and routing of a layout for floor planning decisions. Designers input IC databases into Ansys HFSS high-frequency electromagnetic simulation software for system-level simulation through the Ansys RaptorH silicon-optimized electromagnetic solver with built-in high-frequency simulation interface, running full die, package and printed circuit board (PCB) electromagnetic simulation. For the signoff phase, Synopsys Exalto silicon-optimized electromagnetic modeling software for signoff enhances plain RC results with electromagnetic models for the electromagnetically critical parts of the RF circuit, back-annotated for fast post-layout simulation.