The minimization of cost, power consumption and time-to-market of DSP applications requires the development of methodologies for the automatic implementation of floating-point algorithms in fixed-point architectures. A new methodology for evaluating the quality of an implementation through the automatic determination of the signal-to-quantization-noise ratio (SQNR) is presented. The modelization of the system at the quantization noise level and the expression of the output noise power is detailed for linear systems. Then, the different phases of the methodology are explained and the ability of our approach to compute the SQNR efficiently is shown through examples.