7.1 Linear-phase FIR transfer function Summary A Type 2 FIR filter cannot be used to design a highpass filter since it always has a zero z=-1 A Type 3 FIR filter has zeros at both z=1 and z=-1 and hence cannot be used to design either a lowpass or a highpass or a bandstop filter A Type 4 FIR filter is not appropriate to design a lowpass filter due to the presence of a zero at z=1 Type l FIR filter has no such restrictions and can be used to design almost any type of filter7.1 Linear-phase FIR transfer function