The present invention relates to the design and development of a tunable bandpass filter.
Tunable bandpass filter is one of the vital components of frequency reconfigurable (or frequency agile) wireless systems which facilitate effective utilization of allotted frequency spectrum. Furthermore, frequency reconfigurable wireless systems can be a cost effective solution for wireless base-stations as well as for satellite & aero-space applications. In satellite application, on orbit flexible payload (or programmable payload) is one such encouraging development on the horizon. These systems inevitably require high Q (Quality factor) tunable bandpass filters with a constant absolute bandwidth over the tuning range.
One of the important requirements for tunable filters in most applications is to maintain constant absolute bandwidth over the tuning range. The data rate is bandwidth dependent thus maintaining the same date rate over the tuning range requires maintaining the same bandwidth. In addition, most of communication system applications require maintaining certain isolation requirements outside the band, which cannot be satisfied if the bandwidth is changed. Thus by maintaining a constant bandwidth over the tuning range, the achievable data rate and the filter isolation requirements remain the same over the entire tuning range, which is highly desirable.
Co-axial filters (or Combline or Evanescent mode or inter-digital filters) which are mechanically tuned are capable of achieving high Q (and hence lower loss). Typically, such filters require a tuning mechanism for each resonator to achieve the desired constant absolute bandwidth over the tuning range. In other words, the number of tuning mechanisms utilized in such filters is equal to the filter order, thus making them bulky and expensive. Hence it is highly desirable to achieve filter tuning with a single tuning mechanism yet maintaining constant absolute bandwidth. Ideally, a tunable bandpass filter is desired to maintain constant absolute bandwidth over a reasonably large tuning range with low insertion loss, where the filter is tuned using a single mechanism. A single tuning mechanism not only reduces the complexity of the closed loop control system but also results in enhanced reliability for aero-space applications.
Over the years significant inventions have been developed to realize tunable bandpass filters which have low loss (i.e. high-Quality Factor—high Q), however as will be explored below in detail these inventions utilize multiple tuning elements.
With respect to tunable co-axial filters, US patent application 2016/0049710 disclosed a high Q (lower loss) and constant absolute bandwidth over the tuning range. The invention utilizes mechanism to change the gap between resonator post and tuning disk, thus changing the frequency response of the filter. EP 2 690 702 A1 discloses a frequency tuning by changing the orientation of elliptic tuning disk. This invention does maintain a constant absolute bandwidth however the tuning range is extremely narrow (less than 2%). Similarly, U.S. Pat. No. 7,705,694 B2, discloses a frequency tuning by rotating elliptical dielectric resonators. The invention has considerable bandwidth variation over the tuning range of the filter. U.S. Pat. No. 6,147,577A discloses a tunable ceramic (dielectric resonator) filter which utilizes mechanism to vary the gap at the resonator. Similarly, US 2014/0028415 discloses a tunable bandpass filter which maintains a constant absolute bandwidth over the tuning range. The tuning is achieved by rotating each resonator within the filter. U.S. Pat. No. 7,352,263B2 discloses a variety of method and mechanisms to tune the frequency of a resonator by changing gaps and rotating the resonators. However, all the above inventions require a tuning mechanism for each resonator. Thus, the number of tuning mechanisms required is equal to the filter order.
US 2015/0180105 and U.S. Pat. No. 9,620,836 B2 disclose a waveguide cavity filter with dielectric insert in each cavity. The cavity utilizes two orthogonal modes and has two tuning states which are achieved by rotating the tuning rod either in vertical position or in horizontal position. As a result, the filter cannot be continuously tuned between these two states. Moreover, the tuning range between the two states is also quite low (less than 6%). Furthermore, such a filter at lower frequency spectrum (example around 2.5 GHz) would be extremely bulky. In-addition the filter can tune between only two fixed frequencies and cannot be tuned for frequencies in-between.
The majority of the reported inventions use multiple tuning mechanisms (at-least equal to the filter order) and do not present means to realize tunable filters with a constant absolute bandwidth. In this invention a prototype of a high Q tunable filter is disclosed which maintains a constant absolute bandwidth and insertion loss over the tuning range. Furthermore, the filter can be tuned by a single tuning mechanism. The tuning range of the filter is over at least 30%. The tunability is achieved by rotating a single tuning rod over which all the resonator posts are placed. Furthermore, the invention can be extended to dielectric resonator filters utilizing TM modes.
The present invention is a filter that achieves a constant absolute bandwidth and insertion loss over the tuning range using only one tuning mechanism. This invention finds utility in wireless communication applications requiring frequency agile (or frequency reconfigurable) systems. The filter is especially suitable in RF, microwave and millimeter-wave wireless communication applications.
The present tunable filter comprises of a plurality of tunable resonators that are coaxially aligned on a common filter axis. Each of the tunable resonators comprises of a casing having an inner wall and a cavity. The shape of the cavity is predetermined for filter tuning. In one embodiment of the present invention, the cross sectional shape of the cavity is elliptical. However, other cross-sectional shapes can also be designed. The resonators are connected through inter-resonator coupling structure to operably couple the tunable resonators to provide a balanced electromagnetic coupling with a constant normalized value. The inter-resonator couplings are iris's that have special shapes. In one embodiment, an elliptical iris is used for the elliptical resonators. The resonators are tuned using a single rotating rod that is located along the axis of all resonators. In each resonator, there is a tuning post that is attached to the rotating rod. The shape of the post is designed for the desired tuning. The shape of the posts are selected to improve the spurious performance of the tunable filter. As the posts are rotated by the rotating rod, a gap between each post and the inner walls of each tunable resonator changes and hence the frequency of the resonator also changes. Therefore, rotating the post tunes the frequency of the resonator and hence the filter. And rotating the rotating rod, tunes all resonators in the filter. A pair of end plates each having a SMA connector are attached to the first and the last resonator, and probes are mounted on to the SMA connector on each end plate. The filter also has input/output ports to connect the tunable filter to an external device. A set of tuning screws mounted in the casing of each tunable resonator are provided for fine tuning. In addition, the end plates hold the rotating rod using a ball-bearing or any other suitable bearing, for easy rotation.
The present tunable filter is tuned by a single rotational mechanism irrespective of the filter order. By rotating the rotating tube, the filter center frequency is tuned, while maintaining a constant absolute bandwidth and insertion loss over the tuning range. As the resonator post is rotated, the suitably shaped probe provides the required IO coupling as per design criteria for achieving constant absolute BW over the tuning range.
The principal objective of the present invention is the provision of a novel configuration for a tunable filter that is capable of realizing constant absolute bandwidth and insertion loss over a wide tuning range using a single tuning mechanism.
One objective of the present invention is to provide a filter that can be tuned by a single tuning element with minimum variations in absolute bandwidth and insertion loss over the tuning range.
Another objective of the present invention is to reduce the production cost of communication systems.
Another objective of the present invention is to reduce the delivery schedule of the communication systems.
Another objective of the present invention is to have less number of filter that can be easily reconfigured during production phase to fit the required frequency plan.
Another object of the present invention is to allow building of the filters ahead of time to offer a competitive delivery schedule.
Embodiments herein will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the scope of the claims, wherein like designations denote like elements, and in which:
In the present invention, the requirement of constant absolute bandwidth is taken into account right at the beginning of the design. In general, bandpass filters can be designed for constant absolute bandwidth using Coupling Matrix model. In this model, the entire filter design can be divided into two major steps. One is to design appropriate coupling between the resonators (i.e. inter-resonator coupling), and the other step is to design input/output coupling where the filter is connected to other external components/sub-system in an application. From the [ref 9 —text book], inter-resonator coupling and input/output couplings can be expressed using equation 1 and equation 2, respectively.
k
ij
×f
r
=M
ij
×BW (1)
τs11_max=4/(2π BW×Ms12) (2)
where, kij is the physical coupling co-efficient between the resonators, fr is the centre frequency, Mij is the normalized coupling co-efficient between the resonators, BW is the absolute bandwidth, Ms1 is the normalized coupling co-efficient at input (or output) and τs11_max is the peak input (or output) reflection group delay. The normalized coupling co-efficient (Mij and Ms1) depends only on the filter type and its order, and not on center frequency and bandwidth. As a result, from the model based on coupling co-efficient, the two key requirements to design a filter for constant absolute bandwidth are:
The next step is to realize the physical inter-resonator coupling and input/output coupling to match the above requirements.
Although the present invention has been fully described by way of example in connection with a preferred embodiment thereof, it should be noted that various changes and modifications will be apparent to those skilled in the art. By way of example, the techniques described above are not restricted to the shapes of the metallic or non-metallic elements illustrated in this application, other shapes of the metal (or di-electric i.e. ceramic) parts can be utilized to enhance the tuning range performance. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.
The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.
With respect to the above description, it is to be realized that the optimum relationships for the parts of the invention regarding size, shape, form, materials, function and manner of operation, assembly and use are deemed readily apparent and obvious to those skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention.
Number | Date | Country | |
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62779873 | Dec 2018 | US |