This invention relates generally to the field of electromagnetic signal communication and, more specifically, to the filtering of high power signals for broadcast communications.
In the field of broadcast communications, electrical filters are required to separate a desired signal from energy in other bands. These bandpass filters are similar to bandpass filters in other fields. However, unlike most other electrical bandpass filters, filters for broadcast communication must be capable of handling a relatively high input power. For example, a signal input to a broadcast communications filter might have an average power between 5 and 100 kilowatts (kW). Many electronic filters do not have the capacity for such large signal powers.
For many years, high power electrical bandpass filtering has included the use of waveguide cavity filters. A variety of different waveguide filter types have evolved, each having its particular benefits and drawbacks. One popular class of filter in the industry is based on a pseudo-elliptical filter function. This type of filter function may be achieved in a number of different ways. Some waveguide bandpass filters make use of the “evanescent mode” to provide coupling between the separate resonators of a filter. In an evanescent mode filter, the waveguide is “below cutoff” (i.e., having a cross-sectional dimension small enough that frequencies within a desired passband cannot proceed normally from one end of the cavity to the other). In such a filter, resonances are formed between the inductance of a section of the waveguide, and the capacitance of a resonator, typically in the form of an adjustable length element projecting into the cavity, such as a tuning screw.
In accordance with the present invention, a multiple-section bandpass filter is provided for filtering broadcast communications in a predetermined frequency band. The filter operates in evanescent mode and has coupling bandwidths between adjacent filter sections that establish a frequency band for the filter between fL and fH. The filter has a waveguide that includes a first segment and a second segment adjacent to each other in a direction perpendicular to the signal propagation direction of each segment, and a connecting segment that has a perpendicular orientation to that of the first and second segments. The connecting segment connects a cavity of the first segment with a cavity of the second segment to form a continuous cavity through which a signal propagates along a substantially U-shaped path. As evanescent mode cavities, each of the waveguide segments has a predetermined groundplane spacing that creates a lower cutoff frequency fC=c/2a that is higher that fH, where “c” is the speed of light and “a” is the groundplane spacing.
Within each of the first and second waveguide segments are resonators, each of which comprises a conductor that extends into the waveguide in a direction substantially perpendicular to the direction of signal propagation. Coupling bandwidths in the filter are established by the physical separation between adjacent resonators in each of the first and second waveguide segments, without the need for a passive decoupling structure located between them. The filter also includes a cross coupling conductor, for example, a coaxial conductor, that is connected between the first and second waveguide segments and that provides capacitive coupling between a resonator of the first waveguide segment and a resonator of the second waveguide segment to create additional transmission zeroes for the filter. An inductive coupler could also be used that would provide delay equalization to the filter.
The first and second waveguide segments may have a physical separation between them, and may have a rectangular outer shape, although other shapes are also possible. The waveguide cavities may be formed by an extrusion process which provides a low-cost means of production. Adjustable coupling screws located between adjacent resonators may be provided to allow adjustment of the relative coupling between them. In addition, a decoupling structure, which may be adjustable, can be provided in the connecting segment to allow a certain amount of decoupling between resonators of the first and second waveguide segments.
The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which:
A cross-sectional, schematic side view of the filter is shown in
As shown in
The filter uses evanescent mode coupling between resonators. That is, the waveguide sections have a groundplane spacing that is small enough that they have a cutoff frequency higher than the operating frequency of the filter, so that the signal propagation through the filter is via evanescent modes. For example, for a filter operating at a frequency of 500 MHz, the waveguide cavity may have a spacing of 7.75 inches, which establishes a cutoff frequency of approximately 762 MHz. The operation of the filter “below cutoff” creates a reactance in each filter section that, together with the capacitance of an adjacent resonator 18, forms a resonant circuit having a particular resonant frequency. This resonance may be adjusted by adjusting the tuning screw attached to the plunger of the resonator.
With the spacing of the resonators along the length of the waveguide, there is coupling of the resonances from one filter section to the next. The degree of coupling between adjacent sections is controlled through the use of coupling screws 28, each of which is positioned between two adjacent resonators 18. Threads of each screw 28 mesh with a bracket on the waveguide surface, so that rotation of the screw changes the extent to which it extends into the waveguide cavity and inhibits capacitive coupling between the adjacent resonators. In this way, the relative coupling from one resonator to the next may be controlled.
Vanes separating one filter section from the next are common in the prior art for decoupling one section from the other. However, in the waveguide segments 30, 32, spacing of the resonators themselves is used to establish a default level of decoupling. That is, the physical distance from one resonator to the next is used to establish the degree of coupling between adjacent resonators. While this results in a longer waveguide for the given number of sections, the filter benefits from a substantially higher quality factor “Q” than exists in similar filters having vanes separating the sections.
The use of increased resonator spacing to establish a desired decoupling between filter sections is also notable with regard to the two resonators furthest from the input and output terminals. Referring again to
Because of the spacing between the third and fourth resonators, it is necessary to separate the two waveguide sections from each other. As shown in
In this particular filter embodiment, it is desirable to have a cross coupling between non-adjacent resonators of the first waveguide segment 30 and the second waveguide segment 32. To this end, a coupling path 38 (as shown in
The following is an example of a broadcast waveguide filter according to the present invention. Those skilled in the art will recognize that this is an example for descriptive purposes only, and should not be considered limiting of the overall scope of the invention. In this example, the filter has the form shown in
In this example, the third and fourth resonators are separated by a distance of 13 inches, with the tuning screw located between them and equidistant to each. The post for providing additional decoupling is equidistant from the third and fourth resonators, and positioned 2.625 inches from the waveguide wall 35. Like the waveguide portions 30, 32, waveguide portion 34 has a groundplane spacing of 7.75 inches. With this separation between the third and fourth resonators, the length of the cross coupling conductor 40 is 5.62 inches. At this length, there is no risk of the conductor having a resonance too close to the desired pass band. The spacing between all of the resonators also contributes to a relatively high unloaded quality factor “QU.” For this particular design, the QU of the filter is approximately 10,500.
While the invention has been shown and described with reference to a particular embodiment thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Number | Name | Date | Kind |
---|---|---|---|
4630009 | Tang | Dec 1986 | A |
5867077 | Lundquist | Feb 1999 | A |
5936490 | Hershtig | Aug 1999 | A |
6232852 | Small et al. | May 2001 | B1 |
6236292 | Hershtig | May 2001 | B1 |
6300850 | Kaegebein | Oct 2001 | B1 |
6342825 | Hershtig | Jan 2002 | B1 |