Abstract: In digital measurement of the amplitude and phase of a sinusoidal signal of known frequency by the least square error method, the sampling frequency and sample size are carefully selected to satisfy a certain relationship. Under these conditions, the measurements are simplified and accuracy improved. This technique of measuring a sinusoidal signal is applied to the detection of a load coil in a telephone subscriber's loop circuit. The magnitude and phase of the driving point impedance of the loop are measured through the use of a series of discrete sinusoidal stimulus signals of predetermined frequencies and a digital signal processing technique.