Simple RF Circuit Design

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).   

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.

  

 

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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.
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