Balanced driven transversely coupled waveguide resonator filters

Transversely coupled waveguide resonator filters are widely used as IF filters in cellular and pager applications. A lot of modern circuit concepts demand IF filters which can be driven balanced at input and/or output. Normally the transversely coupled waveguide resonator filters are not suitable for balanced application because all interdigital transducers work against a grounded coupling electrode within the waveguide. We have developed methods to design filters which can be driven balanced at input and/or output and which enable the designer to choose the matching impedance in a very wide range. Assuming filters with four or more poles most of these methods can be combined. Using this combination we have the possibility to use strongly different impedances at input and output. These methods can be used as well for standard waveguide resonator filters on the basis of first and second waveguide mode as for special designs using for instance second and fourth waveguide mode. In the paper most of the methods and practical results will be shown at 82.8 MHz, 246 MHz and 400 MHz with filters having a 3 dB bandwidth of 55 kHz, 225 kHz and 350 kHz respectively. Using a filter at 311 MHz with a 3 dB bandwidth of 280 kHz the limits of these methods and possible technological problems will be shown