The use of power conditioning equipment, such as the shunt active filter, is becoming more popular to remedy power quality problems associated with non-linear rectifier loads. Typically in the shunt active filter situation the non-linear load is considered to be insensitive to the terminal voltage variation. This assumption is made tacitly by modelling the non-linear load as a constant current source. In fact all loads, including non-linear rectifier type loads, are sensitive to voltage variation at their terminals. Since a shunt active filter changes the terminal voltage when it operates by changing the current flowing in the AC system, the shunt active filter can change the current in the non-linear load. The interaction of the non-linear load with the shunt active filter has implications for the power quality delivered by and to that load. In order to determine the interaction of the non-linear load with the shunt active filter and the AC system a control system approach is taken. This shows that the non-linear load contributes to the control behaviour of the shunt active filter because it forms part of the forward control transfer. A suitable model for the non-linear load is required because classical linear system cannot accurately represent the non-linearity. A small-signal frequency domain model is used to represent the non-linear load. This model completely and accurately includes the modulation with a frequency transfer matrix by including the phase dependent nature of the modulation results. The frequency domain model ensures that modulation is represented in a linear way. This means the evaluation of the non-linear load response is achieved by the solution of the linear equation set given by its frequency transfer matrix. The very common single-phase rectifier is analysed using this frequency domain approach. The two rectifier small signal transfer mechanisms, by which it connects its AC and DC sides, are the base switching and the switching instant modulation. Partitioning the rectifier into appropriate partial transfers and then using the small signal approach allows both these mechanisms to be analysed. The effect of the switching instant modulation is found to be second order and so is ignored in this analytic model. The component transfers and the total transfer are validated by time domain simulation. The model shows excellent accuracy. Experimental measurements of the single-phase rectifier are made by injecting AC current perturbations with a DSP controlled converter and measuring the small signal load current response. These measurements show good correlation to those predicted by the analytic model. The analytic model also allows the calculation of the effect of shunt active filter operation on the single-phase rectifier current. Measurements of a threephase rectifier are made and the first order sequence coupling nature of this device for both continuous and discontinuous DC side current conduction is demonstrated. This shows that the three-phase rectifier behaviour can be modelled and analysed using the same frequency transfer matrix approach as used for the single-phase rectifier.