Tunable and bandwidth programmable multi-element filter system

Abstract

A method and apparatus which realize an RF bandpass filter having a tunable bandwidth and center frequency over a large range of the radio frequency spectrum. The filter is comprised of a cascade of filter elements. Each element is comprised of a signal splitter which simultaneously feeds a signal to a delay line and a phase shifter in a parallel path. The delayed and phase shifted signals are summed. Each filter element exhibits periodic passbands and alternating transmission zeroes. Each successive filter element has a decreased delay by an incremental factor (e.g., typically 1.2), and exhibits an increase in the frequency intervals between stopband zeroes. The bandwidth is controlled by selecting a number of 'activated' ones of the successive elements. A center frequency tuning is performed by adjusting a phase shift coefficient of at least one filter element. Such adjustments are performed by applying a test signal at each desired stopband zero frequency and adjusting the phase shift quantity of the element that is required to create the desired transmission zero. The residual power at the test frequency and transmission zero is measured at the filter output at the test signal frequency and the element's phase shifter is adjusted until a minimum output power is observed. Per the continuous sequential applications of the test signal at each of the stopband zero frequencies, and readjustments of each of the corresponding phase shifts, the filter parameters can be maintained as desired. The test sequencing and parameter controls are performed by digital computations.