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An on-line delay estimation algorithm based upon an analogue quadrature phase detector that is suitable for determining the subsample delay between two noisy sinusoidal signals is introduced. The new estimator uses a discrete-time quadrature technique to directly update the delay estimate, which is in turn used to adapt the coefficients of a simple fractional delay filter (FDF). The estimator is insensitive to variations in the amplitudes of the input signals, is simple to implement and has a reduced complexity compared to other adaptive techniques. Simulations show that in the presence of system noise, the new estimator significantly outperforms conventional fractional delay filter based estimators.