A third order in-line pseudo-elliptic filter with a transmission zero extracted at its center

The paper presents a new approach to implement a real transmission zero in a third order in-line pseudo-elliptic filter. The transmission zero is extracted at an internal node in the in-line structure and not at the input or the output as in the well-known extracted pole technique. It is shown that the coupling coefficient that is directly responsible for the real transmission zero becomes too large and physically unrealizable if the extraction is performed at the input or the output as in the extracted pole technique. On the other hand, all coupling coefficient remain realizable if the extraction is performed at internal nodes. Furthermore, internal "impedance zones" can be independently scaled without affecting the overall response of the filter. Example filters are designed to demonstrate the validity of the approach.