Transmission Zeros in Filter D...

Transmission Zeros in Filter DesignTransmission Zeros in Filter Design
By Randall W. Rhea Eagleware Corporation
ore economic and effective bandpass filters can be designed by controlling the location of transmission zeros. What do we mean by transmission zeros? Consider the fifth-order lowpass filter in Figure 1(a). There is little attenuation at low frequencies and increasing attenuation above the cutoff. Only at infinite frequency is there no transmission. In fact, at infinite frequency, each inductor becomes an open and each capacitor becomes a short. This filter has five transmission zeros (TZs) at infinite frequency. The fifth order highpass filter in Figure 1(b) has five TZs at DC. It is essential that the inductors and capacitors alternate. If we were to attempt to build a fifth-order lowpass with five series inducto