LTspiceNoiseSources

White and Flicker Noise Sources for LTspice

Introduction

With the .noise command LTspice is able to perform noise analyses while considering e.g. thermal noise of resistors and noise of models provided by semiconductor manufacturers. However, LTspice's library does not contain models of simple voltage and current noise sources. Based on a blog entry from Axotron, an extension for the LTspice library is offered here that includes an easy to include and easy to use voltage and current noise source. In both cases white noise ($f^0$) as well as flicker noise ($f^{-1}$) can be defined.

The voltage noise source (Vn) has three parameters where white defines the ($f^0$) white voltage noise and flicker defines the ($f^{-1}$) flicker voltage noise, both in units of $\mathrm{V}/\sqrt{\mathrm{Hz}}$. Due to the frequency-dependent nature of flicker noise, a third parameter flickerfreq exists that defines the frequency (in Hz) at which the flicker voltage noise defined according to flicker applies. For frequencies below and above flickerfreq, respectively, the flicker voltage noise progresses with $-10~\mathrm{dB}/\mathrm{decade}$.

The current noise source (In) has three parameters where white defines the ($f^0$) white current noise and flicker defines the ($f^{-1}$) flicker current noise, both in units of $\mathrm{A}/\sqrt{\mathrm{Hz}}$. Due to the frequency-dependent nature of flicker noise, a third parameter flickerfreq exists that defines the frequency (in Hz) at which the flicker current noise defined according to flicker applies. For frequencies below and above flickerfreq, respectively, the flicker current noise progresses with $-10~\mathrm{dB}/\mathrm{decade}$.

Examples

The following two screenshots demonstrate the operation of an embedded voltage noise source (top) and an embedded current noise source (bottom).

Usage

To be able to use the noise sources, only the four files

• vnoise.asy (symbol)
• vnoise.sub (netlist)
• inoise.asy (symbol)
• inoise.sub (netlist)

from the lib subdirectory have to be copied into the directory of the actual simulation. You can simply download the examples.zip archive which contains two simulation examples together with the required lib files.

Implementation Details

The actual implementation of the two noise sources can be seen in the two figures below. Further details can be found in a blog entry by Axotron.

Voltage noise source:

Current noise source: