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.
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.