What is RF Circuit Design? – How to Design RF Circuits | Synopsys
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Definition

RF circuit design is a discipline that focuses on the creation of circuits that operate in radio frequencies.

RF stands for Radio Frequency, which represents the oscillation rate of electromagnetic waves. Frequency is measured in Hertz (Hz), which is equal to the number of oscillation cycles per second (1/s). RF can refer to frequencies as high as 300 GHz, or as low as 30 KHz.   

RF applications include:

  • Radio broadcasting, e.g., AM/FM radio 
  • Wireless communications, e.g., 5G, cell phones, WiFi, Bluetooth 
  • RF remote control, e.g., garage door opener, drones
  • Remote sensing, e.g., weather or surveillance radar
  • Satellite navigation, e.g., GPS, Galileo, Glonass, Beidou
  • Imaging, e.g., body scanners for airport security

RF waves can have other names such as microwaves (as in “microwave oven”), or millimeter waves (mm-wave). Microwave often refers to radio waves with the wavelength (λ) ranging from 1cm to 10cm, corresponding frequencies (f) of 30GHz to 3GHz. Millimeter wave often refers to radio waves with the wavelength (λ) ranging from 1mm to 10mm, corresponding to frequencies (f) of 300GHz to 30GHz. The relation between wavelength (λ) and frequency (f ) is expressed  as  λ=c/f, where λ is measured in meters, c is the speed of light (3×10^8 m/s), f is measured in Hz, or 1/second).   

Figure 1: RF waves and frequency scale | Synopsys

RF Circuit Types

  • Low noise amplifier (LNA). Amplifies a faint signal from far away. LNA determines the sensitivity of a radio receiver.
  • Power amplifier (PA). Amplifies a radio signal to high power for transmission.  PA determines the range of coverage for a transmitter.
  • Local oscillator (LO). Provides the local carrier frequency for RF transmitter and receiver.
  • Mixer. Mixes two signals. In a transmitter, the mixer is an “up-converter,” which will mix a low-frequency analog signal with the LO signal to produce an RF signal. In a receiver, the mixer is a “down-converter,” which will mix an RF signal with the LO signal to produce a low-frequency analog signal.
  • Filter. Constrains the signal energy in a specific frequency band.  It plays the role of keeping different radio signals from interfering with each other.
  • Switch. Controls the signal flow paths.
  • Transceiver. Consists of a transmitter and receiver.

  

Figure 2: Simple RF circuit design | Synopsys

RF circuits are “analog” in nature, with continuous time stimulus and response. While they previously were made with vacuum tubes and discrete transistors, these materials have largely been replaced by integrated circuits (ICs), except for a few high-power applications. In the applications where high power is not needed, such as cell-phone transceivers, WiFi transceivers, Bluetooth transceivers, and satellite receivers circuits often take the form of a silicon-based IC.


How to Design RF Circuits

RF IC design typically involves a top-down design and implementation process, followed by a bottom-up verification process. There are many variations on 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 conversing 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 on all its specifications using circuit simulation. During this step, 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. The 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.

RF Circuit vs. Analog Circuit vs. Digital Circuit

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 of 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 most efficient, where 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.

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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 allows the behavior of the circuit to be analyzed 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.


RF Circuit Design and Synopsys

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.

Synopsys RF circuit design flow steps | Synopsys

Synopsys Custom Design Platform tools include:

  • Synopsys Custom Compiler™ design environment. Providing design entry, simulation management and analysis, and custom layout editing features.
  • Synopsys PrimeSim™ SPICE circuit simulator. A multi-core/multi-machine simulator that is well-suited for the simulation of large, complex RF circuits. S-parameter, Harmonic Balance, and Shooting Newton analysis engines are built in the PrimeSim SPICE solution.  Model files can be either in HSPICE or third-party netlist formats. Faster speed performance and larger circuit capacity reduce the design cycle.
  • Synopsys IC Validator™ physical verification solution. A comprehensive and high-performance signoff physical verification solution that improves productivity for customers at all process nodes.
  • Synopsys StarRC™ parasitic extraction solution. The gold standard for IC layout parasitic extraction.  For RF IC layout, the electromagnetic analysis tools from Ansys and Keysight are commonly used in combination with StarRC extraction to provide a layout extracted view netlist for signoff simulation.
  • Synopsys PrimeWave Design Environment. A comprehensive and flexible environment for simulation setup and analysis, as well as graphical waveform viewer and simulation post-processing tool for analog, RF, and mixed-signal ICs
  • Synopsys PrimeSim Reliability Analysis. A comprehensive solution that unifies production proven and foundry-certified reliability analysis technologies covering Electromigration/IR drop analysis, high sigma Monte Carlo, MOS Aging, analog fault simulation, and circuit checks (ERC) to enable full-lifecycle reliability verification.  

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