I was given the task of designing a band pass filter that required a five stage filter. I choose a simple design using five parallel L-C resonators coupled by four coupling capacitors. I used lumped components and constructed it on a FR4 PC board. The specifications required a certain pass bandwidth and stop band response. When I tested the filter using a VNA, I noticed the stop band response below the pass band was much steeper than the stop band above the pass band.
Looking at some filter design books, it stated one has to count the number of transmission zeros or points of maximum attenuation that occurs in the stop band response. Using this technique, I determined that in a shunt parallel L-C circuit, a zero occurs at a low frequency by an inductor and a zero occurs at a high frequency by a capacitor. Now a series capacitor is a zero at low frequencies while it passes all high frequencies -- that means it is not a zero.
Using this analysis, there are nine zeros. Five shunt inductors and four series capacitors at the low stop band response, while there are only five zeros at the high stop band response. I solved this problem by using four series capacitor-inductors as coupling between the five parallel L-C resonate circuits. This then made both the stop bands equal in roll off attenuation.
The moral is always count your zeros.
This entry was submitted by William J. Garner and edited by Lauren Muskett.
William J. Garner is an RF microwave engineering consultant with 47 years design experience. He has published papers in trade journals and holds seven patents.
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