This paper deals with spectrum sensing in Cognitive Radios to enable unlicensed secondary users to opportunistically access a licensed band. The ability to detect the presence of a primary user at a low signal to noise ratio (SNR) is a challenging prerequisite to spectrum sensing and earlier proposed techniques like energy detection and cyclostationary detection have only been partially successful. This paper proposes the use of FRESH (FREquency SHift) filters  to enable spectrum sensing at low SNR by optimally estimating a cyclostationary signal using its spectral coherence properties. We establish the mean square error convergence of the adaptive FRESH filter through simulation. Subsequently, we formulate a cyclostationarity based binary hypothesis test on the filtered signal and observe the resultant detection performance. Simulation results show that the proposed approach performs better than energy detection and cyclostationary detection techniques for spectrum sensing.