The noise analysis portion of Ngspice gives the device-generated noise for a given circuit. When provided with an input source and an output port, the analysis calculates the noise contributions of each device, and each noise generator within the device, to the output port voltage. It also calculates the equivalent input noise of the circuit, based on the output noise. This is done for every frequency point in a specified range - the calculated value of the noise corresponds to the spectral density of the circuit variable viewed as a stationary Gaussian stochastic process. After calculating the spectral densities, noise analysis integrates these values over the specified frequency range to arrive at the total noise voltage and current over this frequency range. The calculated values correspond to the variance of the circuit variables viewed as stationary Gaussian processes.