clock an n order fir digital filtering means to selectively band limit the frequency spectrum of the recovered input signal a second digital filtering means is coupled to the output of the first fir digital filtering means the second digital filtering means operates at a clock speed less than or equal to the second clock speed the second digital filtering means is used to further selectively band limit the frequency spectrum of the recovered input signal application_number US 86195886 A description referring now to fig1 a digital zero if selectivity section is depicted in block diagram form in accordance with the present invention digital data is routed to a mixer latch 10 from a digital mixer not shown the digital mixer would typically be mixing a local oscillator signal with the output of an a d converter operating on the incoming signal the mixer thereby provides the input zero if signal 12 the zero if signal is bussed to an n order decimating filter 14 the digital data is typically routed on an l bit data bus representing an l bit digital word the length of the word depends upon factors including the dynamic range required and the amount of roundoff and quantization noise that can be tolerated in the given application for example a word length of 16 bits may be considered to have an acceptable performance in receiving a typical radio signal sampled at 20 mhz the n order decimating filter 14 is necessary to accommodate the high sampling rate of the received.