Planar Multifrequency Wideband Bandpass Filters With Constant and Frequency Mappings

A new planar filter design technique with constant and frequency mappings has been proposed for multifrequency wideband bandpass filters (BPFs). The constant mapping treats components in the LC circuit (e.g., frequency invariant admittances, capacitors, and ideal inverters) as frequency dependent. It is consistent with practical planar circuits and caters to a wide frequency range. Following the classic single-to-multiband transformation, a new frequency mapping function is proposed by incorporating this constant mapping idea. With these two mapping functions, a direct relation between the LC circuit and its microstrip counterpart is established. Therefore, the multifrequency wideband BPF is readily designed from a low-passing LC circuit to the transmission line circuit. A ladder-type Chebyshev filter and a trisection filter with the general Chebyshev response have been designed as examples. The first one is a dual-band case exhibiting a wide bandwidth for each passband. The second filter is a triple-band one showing a large frequency ratio between the first and third passband. Both examples experimentally validate the proposed constant and frequency mapping technique.