# Chapter 5. AC Analysis

AC analysis calculates the linear (i.e. small signal) response of the circuit to stimuli or noise generating elements. As with port-based linear analysis, an initial DC analysis calculates the operating point of all nonlinear elements, which determines their linear equivalent for the AC analysis. Unlike the port-based linear analysis, however, AC is a nodal analysis that calculates complex voltages and currents, at all nodes and through all branches.

AC analysis can include one or more stimuli with arbitrary magnitude(s) and phase(s), but it only simulates one frequency at a time. It uses the Tone 1 frequency sweep for all sinusoidal voltage and current sources in the circuit, as well as any other sources with an ACMag parameter that is set to a non-zero value. All of these stimuli are “on” for the simulation. Non-sinusoidal signal sources are assumed to be of zero value (voltage sources are shorted, and current sources are open circuit). Regardless of type, all ports act as terminations only, not stimuli.

To perform AC analysis, add AC measurements in the Add/Modify Measurements dialog box by selecting Linear > AC as the Measurement Type. AC analysis is supported by AWR® APLAC®, so select Aplac AC Noise from Simulators. For AC voltage and current measurements, selecting the dB check box calculates 20*log10( |measurement| ). For example, if the magnitude of the input voltage is 1V, then a voltage magnitude measurement in “dB” is the voltage gain of the circuit from the input source to wherever the measurement is made. Because AC is a linear analysis, the magnitude of the input has no effect on the gain; for example, if the voltage gain is 10, then 1mV input yields 10mV out, and 1kV input yields 10kV out, the DC supply is ignored after the initial DC analysis.

## 5.1. AC Noise Analysis

AC noise analysis requires at least one sinusoidal input source and one V_NSMTR noise meter with its InpSrc parameter set to the ID of the input source. It then calculates the noise contribution of each noise generating element in the circuit to the voltage measured by the V_NSMTR. AC noise measurements and annotations include the total output noise, input-referred output noise (total output noise divided by gain from input source to output voltage), and the contributions of individual noise generators to the output node.