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An online 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 technique is based on the recently proposed discrete-time quadrature-delay estimator (QDE). The algorithm uses the in-phase and quadrature-phase components of the received signals to produce a bias-free estimate of the delay between the input signals. The technique directly minimizes the delay error estimate, which is in turn used to adapt the coefficients of a simple fractional-delay filter (FDF). The algorithm is insensitive to variations in the amplitudes of the input signals, and does not require an accurate prior estimate of the frequency of the input sinusoids. The algorithm directly updates the delay error estimate, which is in turn used directly as the coefficient input to an adaptive FDF. The technique 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 outperforms conventional FDF-based estimators.