Tuning a switching power supply, the power supply including a switching transistor; a filter circuit; a pulse generator that drives the switching transistor; a programmable filter connected to the output of the filter circuit; a digital signal processor (‘DSP’) connected to the output of the filter circuit, the DSP configured to program the programmable filter; and a tuning control circuit connected to the output of the filter circuit, to the pulse generator, and to the DSP; including calculating by the DSP, from sampled voltage values of a tuning pulse driven through the filter circuit by the pulse generator, the actual impedance of the filter circuit; and programming, by the DSP, the programmable filter, setting the combined impedance of the filter circuit and the programmable filter to the design impedance of the filter circuit.