TCAD Distortion Analysis Based on the Harmonic Balance Method
Introduction
The dynamic range of linearity is a critical specification in most analog applications. Even if the output signal changes almost linearly with the input signal, intrinsic device distortion effects are present, and higher harmonics draw power from the fundamental signal.
Besides small-signal and noise characteristics, these distortion effects are the most important properties in analog radio frequency (RF) circuits such as power amplifiers, mixers, and drivers. Although distortion is the fundamental mechanism of mixers to generate signals at various frequencies, it must be as low as possible for most other applications.
Distortion characteristics are divided typically into two groups:
· Harmonic distortion: A sinusoidal input signal at frequency f (also called one-tone) results in an output signal with spectral components at f (first harmonic), 2f (second harmonic), and 3f (third harmonic) (see Figure 1 (left)). The extrapolated crossing point of the first and third harmonics is called the third-order interception point (IP3) and is a typical metric to characterize the linearity of a device or system (see Figure 2).
«Fig. 1.» (Left) Input signal with modulation frequency f1 (black) results in output signal with spectral components at f1 and integer multiples of f1 (harmonic distortion). (Right) Two-tone input signal at f1 and f2 also generates spectral components at 2f1-f2 and 2f2-f1 (intermodulation distortion).
· Intermodulation distortion: The input signal comprises two sinusoidal signals with fundamental frequencies f1 and f2 (also called two-tone) with equal amplitudes. The resulting, more complex, spectrum of the output signal is shown in Figure 1 (right). The typical quantity to characterize this situation is the extrapolated crossing point of the fundamental signal at f1 with the amplitude at 2f1-f2, which is called the third-order intermodulation intercept point (IIP3).
In most applications, the intercept points are not reached due to saturating signals at high input amplitudes. Nevertheless, the intercept points serve as main figures of merit for distortion.
Although distortion characteristics are relatively easy to measure, little is known about the physical origin at the device level.
«Fig. 2.» Schematic of third-order intercept point, which is defined by the extrapolated crossing point of the fundamental first and third harmonics of the output signal.
In addition, most modeling activity has focused on compact models, which inherentlymiss the important physics behind distortion andare often not scalable.