Adam Cooman

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Testing the Large-Signal Small-Signal
simulator in ADS

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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 0Hz0\mathrm{Hz}. The V_1Tone and I_1Tone components are shown below, together with the HB statement that performs the LSSS simulation:

alt

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:

i(t)=ej2πftv(t)=ej2πfti(t)=e^{j2\pi ft} \qquad v(t)=e^{j2\pi ft}

where the amplitude is infinitesimally small. The frequency ff of the excitation signal is swept over the frequency range specified in the HB simulation. With the simulation settings above, the small-signal frequency ff will be swept between 1Hz1\mathrm{Hz} and 100Hz100\mathrm{Hz}.

For this example, the HB base frequency is 5GHz5\mathrm{GHz} with order 1010. The LSSS will calculate the mixing with each tone in the HB simulation, which results in 2121 contributions at the mixed frequencies f+l(5GHz)f+l\left(5\mathrm{GHz}\right) where l=10...10l=-10...10.

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.

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