This paper describes a theoretical analysis of sine-to-square conversion when noise is included in the conversion process. The analysis is done for conversion performed by a comparator. As a result of the analysis, a simple method for calculating the spectrum of clock resulting from noisy conversion process is obtained. It will also be shown that the conversion process can easily lead to poor quality of the clock signal and that special attention should be paid when the clock is intended to be used for edge sensitive digital logic. To support the derived theory, simulation results will be presented