1. Introduction
A LSSS simulation calculates the linearised response around a periodic solution of a circuit to a small-signal excitation. A LSSS simulation is performed in two steps: First, an Harmonic Balance (HB) simulation is run to obtain the large-signal solution of the circuit to the external excitation. In a second step, the linear response around the obtained solution is obtained using the Jacobian matrix of the final solution of the HB simulation.
In ADS, the small-signal excitation is applied using a V_1Tone or I_1Tone component. Both components contain USB and LSB parameters that control the upper sideband and lower sideband small signal components. The USB and LSB components have a frequency relative to the frequency set in the 1Tone component. We will always work relative to DC, so the Freq parameter of the 1Tone components are set to . The V_1Tone and I_1Tone components are shown below, together with the HB statement that performs the LSSS simulation:
Figure 1.1 Small-signal sources and the HB statement which controls the LSSS simulation.
When the V_1Tone or I_1Tone components are configured as shown above, the LSSS will calculate the response of the periodically varying system to the following complex excitation signal:
where the amplitude is infinitesimally small. The frequency of the excitation signal is swept over the frequency range specified in the HB simulation. With the simulation settings above, the small-signal frequency will be swept between and .
For this example, the HB base frequency is with order . The LSSS will calculate the mixing with each tone in the HB simulation, which results in contributions at the mixed frequencies where .
Due to the way the response is calculated, the excitation frequency cannot be equal to any frequency in the large-signal simulation. This limitation can usually be resolved by adding a small frequency offset to the frequency grid of the small-signal simulation.