Design Method for Tunable Planar Bandpass Filters With Single-Bias Control and Wide Tunable Frequency Range

A design method for a frequency-tunable planar bandpass filter is demonstrated. With the method, all reasonators can be designed with identical capacitances, allowing the filter to be tuned with identical varactors using a single bias control. This allows not only low-cost fabrication, but also a simple structure and design procedure. Moreover, the filter is based on miniaturized transmission lines. Therefore, a wide tuning range is achieved with a small size and outstanding out-of-band response. Most importantly, the method facilitates the design of a filter to vary the fractional bandwidth in a predetermined way as the center frequency is tuned. Thus, a tunable filter can be designed not only to maintain the fractional bandwidth, but also to expand or to reduce the bandwidth as the frequency is tuned. The experimental results of a compact third-order bandpass filter based on CPW lines with a 0.11λg×0.8λgsize show a very wide frequency tuning range of 62.8% with a 3-dB fraction bandwidth that remains within 28.65±1.55% and an outstanding spurious suppression of under -40 dB up to as high as 3f0.