Abstract: Using sub-sampling theorem of multiple banpass signals and digital frequency converting techniques, multi-band-pass signals can be directly sampled and processed using no aliasing sampling rate. If sampling rate is lower than the two times of the maximum frequency of the signal, the signal-to-noise ratio will become worse. The relationships of signal-to-noise ratio of sampled signal, analog filter before analog-to-digital converter and sampling rate have been discussed, the results of the discussion have instructiveness for the design of digital receivers.