With the rapid integration level of circuits and systems, how to test those complex systems in an efficient manner, in terms of test time, cost, and proficiency, has become a vital task. Among them, signal spectral analysis or testing is a dominating way to characterize the dynamic performance of analog signals. To obtain coherent sampling, especially when signals have large distortion-to-noise ratios, has been a challenge for many years. This brief presents two new methods to resolve this issue. In comparison to previous methods, these new methods are capable of achieving accuracy as well as computational efficiency. All these methods are verified via theoretical analysis, simulations results, and measurement results.