Linear Noise Simulation Description

Linear noise simulation is an option available with the AC and S-parameter simulators. The frequency at which the noise is analyzed is the same as the AC simulation frequency. Noise voltages and currents are saved in the dataset with the keyword Noise included in the parameter name to identify the type of simulation.

The simulator performs an arbitrary-topology, multiport, network noise simulation. The following noise contributions are included in this simulation:

• Temperature-dependent thermal noise from lossy passive elements, including those specified by data files
• Temperature and bias-dependent noise from nonlinear devices
• Noise from linear active devices specified by 2-port data files that include noise parameters
• Noise from noise source elements

The noise simulation computes the noise generated by each element, and then determines how that noise affects the noise properties of the network. In most cases, the noise generated by circuit elements is calculated automatically. Lossy passive elements, for example, contribute noise according to their ability to deliver thermal noise power. The noise contributions from nonlinear devices are computed by models that include temperature and bias dependence; those models are similar to those used by SPICE. The computation of network-level noise properties from the component elements is performed by means of noise-correlation matrices. Most noise measurements are based on either noise figure or noise parameter calculations, which are defined for 2-port networks only. For networks with more than two ports, the noise figure can be measured between two user-specified ports using Input Port and Output Port; the other ports are treated as resistors for the noise simulation.

Note The temperature of lossy passive elements is used to calculate their noise contributions. Since a lossy passive element at a physical temperature of 0 K does not generate any thermal noise, you may want to disable the noise contribution of any such element by setting its physical temperature to -273.15°C (0 K). Do not use this method for nonlinear devices such as transistors. Use the Noise parameter for resistors and nonlinear devices to enable (Noise = YES) or disable (Noise = NO) noise generation.

The program's nonlinear device models include one or more of the following noise effects:

• Thermal noise generated by the resistances that exist within the nonlinear device models. This noise is proportional to the device temperature and is independent of bias.
• Channel noise for JFET, MESFET, HEMT and similar devices. This noise may be due to thermal noise, high-field diffusion noise, or other effects. This noise is generally a function of device temperature and bias.
• Shot noise is caused by the quantized and random nature of current flow across junctions and is modeled for diodes and BJTs. This noise is proportional to the device bias current and is independent of temperature.
• Flicker (1/f) noise is modeled in most nonlinear devices.
• Burst (or popcorn) noise is another low-frequency, bias-dependent noise effect modeled in bipolar transistors.

Noise Parameter Definitions

Noise parameters are used to define the noise electrical properties of an n-port electrical element at a given frequency. The noise parameters over a range of frequencies define the element's performance for all noise-power spectral density and tones that define an incident noise source.

Definitions of noise parameters can be found in standard textbooks covering electrical circuit theory. Noise parameters are used by the program to define the noise properties of any electrical element. The following discussion is for a 2-port element, but may be generalized for any n-port element.

A 2-port element noise-wave representation may use two noise waves at the element input. Otherwise, it may use one noise wave at the element input and one at the element output. A multiport-element noise-wave representation has one noise wave at each element port.

In the following noise discussions, the noise is assumed to be spot noise with a bandwidth of 1 Hz.

Note The spot noise figure is the ratio of the output noise power per unit bandwidth to the portion of output noise power that is attributable to the thermal noise in the input termination per unit bandwidth. The noise temperature of the input termination is assumed to be 290 K.

The noise correlation matrix, [N], is defined as follows:

where * represents the complex conjugate.

Noise Entries in a Dataset

vout.noise	Total noise voltage at node Vout
vnc	Noise contributors (noise voltage contribution from each noise generator in the circuit)
name	Instance name of noise contributor

These entries can be viewed in the data display. For example, in a simple circuit with two noise generators, resistors R1 and R2, plotting name and vnc on a list, generates the following table:

name	vnc
_total	641.6 pV
R1	453.7 pV
R2	453.7 pV

These results show that R1 and R2 are each contributing an equal amount of noise to the total. Notice that the sum of the individual contributors does not add up to the value for _total. The sum of the squares of the individual noise contributors is equal to the total noise voltage squared.

Note vout.noise reports that same noise voltage as the value of vnc associated with the name _total.