A new coupling technique for cross-coupled bandpass filters with arbitrarily located transmission zeros

This paper presents a new technique of cross-coupling for compact folded multilayer bandpass filters. A transmission line is used for coupling the shorted ends of the first and last resonators around four nodes. This in turn produces two transmission zeros and allows them to be arbitrarily and independently located around the passband. An example of fourth order folded interdigital filter is presented. It is shown how the impedance of the cross coupling transmission line affects the location of the higher band transmission zero, while the tapping point position affect the location of the lower band transmission zero. Experimental results show excellent agreement with the simulations. This design technique can be very effective for multilayer substrate technologies.