This invention relates generally to a coaxial resonator and more particularly to a coaxial resonator having an increased electrical Q for a given size and to a method for increasing the electrical Q of a coaxial resonator.
An RF resonant cavity (or multiple interconnected cavities) can be used to create a RF filter. The filter may either pass a RF signal over a limited frequency range (a bandpass filter) or exclude an RF signal over a limited frequency range (a notch or band stop filter), depending upon how the resonator is connected to the overall system. A perfect single cavity device would operate at a single, specific frequency (the resonant frequency), however due to material and other considerations all resonant frequency devices operate over a frequency range which encompasses the resonant frequency.
An RF resonator is realized by having a conductive post within an enclosed conductive cavity. The post is connected to the bottom of the cavity and extends towards the top of the cavity. The cavity is formed within a conductive housing and enclosed by a conductive lid. The resonant frequency of the cavity is selected by adjusting the length of the post.
The electrical Q of a coaxial resonator is a measure of its performance. As mentioned previously, a perfect single resonator would operate at a single specific frequency. However, due to material and other considerations all resonant frequency devices operate over limited frequency range. The electrical Q of the resonator is determined by the width of the frequency range, see FIG. 1. The higher the Q, Trace 1, the narrower the frequency range as compared to a lower Q. As is generally known, the larger the size of a cavity, the higher its Q.
Because the frequency response of a single resonant cavity is very narrow and a practical device must operate over a wide frequency range, it is necessary to combine multiple cavities to achieve a desired frequency range. In addition, the rate at which a multi-cavity filter makes the transition between passing a signal and blocking a signal (the steepness of the filter curve flange) is a function of the number and size of the cavities in the multi-cavity filter. The greater the number of cavities the sharper the transition. An ideal multi-cavity filter would have a vertical (or near vertical) edge. U.S. Pat. No. 5,894,250 describes a cavity resonator filter employing multiple cavities. The physical size of the filter is achieved by employing a combination of larger volume high Q and smaller volume low Q cavities to significantly reduce insertion loss and provide a compact filter.
The steepness of the filter curve flange is also a function of the electrical Q of the individual resonant cavities comprising the filter. The higher the electrical Q at a given insertion loss, the steeper the filter curve flange. Therefore, if the electrical Q of the individual cavities is improved, it is possible to realize a given multi-cavity filter response using smaller cavities resulting in a reduced overall device size.
It is an object of the present invention to provide a coaxial resonant cavity having a high electrical Q.
It is another object of the present invention to provide a method of increasing the electrical Q of a cavity of a given size.
It is a further object of the present invention to provide a band pass filter including a plurality of high Q cavities incorporating the present invention.
The foregoing and other objects of the invention are achieved by a coaxial resonator of the type including a conductive cavity having bottom, top and side walls with an inner conductor or stub having one end in a short circuit connection with the bottom wall and its other end in open circuit relationship with and spaced from the top wall characterized in that the inner conductor or stub tapers outwardly at the one end.