Content area
Full Text
The difficult job of tuning coupled resonator filters can be simplified by using a vector network analyzer's time-domain display capability.
INCREASING wireless services are forcing more communications channels in less frequency spectrum. To avoid interference, very-stringent filtering requirements are being placed on all systems. Invariably, this means that the systems must employ coupled resonator filters to handie the power levels and provide the necessary isolation. Manufacturers of these filters are usually limited by; the ability to tune these filters quickly and accurately.
In a coupled-resonator cavity-- tuned filter, the center frequency of each resonator must be precisely tuned to achieve the proper passband response and to achieve low return loss (reflection) with small passband ripple. Tuning the resonators is essentially a trial-and-error "tweaking" process. Until now, however, there has been no alternative.
What follows is a method where this type of filter can be tuned quickly and easily by the time-domain response of its return loss. With this approach, it is possible to tune each resonator individually, since time-- domain measurements can distinguish the individual responses of each resonator and coupling aperture. Interaction caused by adjusting coupling coefficients, which may be tuned to satisfy a desired filter response, can be immediately determined and accounted for.
Perhaps the most important is the following: inexperienced filter tuners can successfully tune multiple-pole filters after only a few minutes of instruction. This rapid proficiency is impossible with previous tuning methods. The technique also appears to lend itself well to the automated production environment, which has always ranged from difficult to impossible.
While some regimented tuning methods achieve an approximate filter response, final tuning often requires the seemingly random adjustment of each element until the final desired filter shape is obtained. Experienced tuners can develop a feel for the proper adjustment, but months are usually required before a novice can be proficient at tuning more complex filters.
Another difficulty in training tuners is that each filter type may have a different tuning profile, so training may be required for each new filter design. The training process usually removes an experienced tuner from production to train the novices. Many filters do not have a generalized algorithm for the final stages of tuning (when each element interacts with its neighbors),...