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Abstruct-This paper describes a perturbation source for in situ parameter estimation applications for apparatus such as motors, transformer windings, etc. The system topology comprises two basic components, namely a digital-to-analogue (D/A) converter and a controllable bi-directional current source. The D/A converter is computer controlled and features an onboard transputer which generates control signals for the current source. The current source employs a half-bridge topology with MOSFET switches and a blocking arrangement to allow for in situ applications, i.e. voltage generated by the circuit under test. The overall system is designed to generate current perturbation waveforms such as pseudo random binary sequences or step functions. Experimental and calculated results are given for a magnetic voltage transformer application, using a maximum likelihood estimation algorithm.