No exaggeration attends the statement that filters represent one of the most important elements in electrical and electronic technologies. Filtering plays critical roles in noise and interference suppression, and in signal conditioning and discrimination. Communication systems employ a staggering variety of filter types, from passive analog filters used for high-power signals in base station antenna systems, to digital filters used for low-power signals in demodulation and decoding circuitry. The particular challenges and complexities that arise in filter design and implementation vary with filter type and application.
For example, “tunable” filters or filters that are otherwise adjustable pose potentially significant design and implementation challenges. For example, “active” electronic filters offer significant opportunities for adjustable and dynamic reconfiguration, but it may be impractical or even impossible to implement active filters at very high power levels, and/or for higher radio frequency and microwave applications.
U.S. Pat. No. 5,987,304 to Lätt illustrates an active amplifier chain that is configured to provide a variable bandwidth filter for use in a mobile telephone repeater. The amplifier chain includes a filter device that has a series of bandpass filters that have controllable, overlapping passbands. Active mixer circuits (oscillators) provide passband variability in flexible fashion, but their use imposes practical limits on the signal frequencies and power levels at which the filter can be used.
Use of electromechanical systems and passive filter elements provides for adjustability, even with higher power levels and higher frequencies, but such implementations are not without their own disadvantages. For example, filters with tunable center frequencies may be realized by implementing series filter stages with tunable resonators. These tunable resonators may be conductive rods (quarter-wave coaxial resonators) that are capacitively loaded at their open ends with conductive, asymmetrical disks mounted on mechanically-turned shafts (or as stepper-motor actuated disks). However, the expense and complication of such electromechanical arrangements is self-evident, and it can be difficult to achieve good return loss across the desired tuning range, particularly without the addition of isolation stages between individually tunable filter stages.
In another approach, mechanical switches allow for the selection of individual filters from among a set of filters, each one of which can be optimized according to a desired filter response. So-called switched filter banks represent one implementation, where one or more filters in a set of filters are switched into electrical connectivity with input and output filter ports using respective input and output switches. The switches generally are multi-pole/single-throw switches, such that one filter at a time is selected.
Switching in one filter at a time offers the significant advantage of eliminating the filter interaction problems that otherwise might arise when two or more filters are connected together.
This separation of filters allows each filter in the set to be optimized individually without regard to the other filters in the set. On the other hand, the use of switches carries with it certain inherent disadvantages. For example, the input and output switches increase insertion loss and, as a general proposition, limit the signal power levels that can be practically accommodated. Furthermore passive intermodulation, PIM, in the switches may limit their usability in frequency division duplex (FDD) systems. Repeatability and reliability also may be compromised by the use of mechanical switches, particularly with higher signal powers and/or with frequent switch actuations.