The analytical model for the output relations of anti-aliasing filtering, sampling, quantization and de-spreading was established. The formulation of de-spreading output SNR (signal to noise ratio) under the influence of three factors was deduced. Analysis and simulation results show that the total SNR loss can be separated to the product of quantization, filtering and sampling loss when the quantization length up to 4 bit; besides, the optimized clipping level coefficient is only related to quantization length. When the quantization length is less than 4 bit, the total SNR loss approximates to the product of the quantization and sampling plus filtering loss. In the condition of the quantization length up to 4 bit, the filter bandwidth up to 5 times of PN (pseudo-random) code rate, the sample rate up to 4 times of PN code rate, increasing the parameters mentioned above cause the SNR loss less than 0.05 dB, thus the despreading performance cannot be increased obviously. Results can be used to optimize the design of the front-end of lowcost global navigation satellite system receiver.