Patent Publication Number: US-9905898-B2

Title: Digitally tunable coaxial resonator reflective band reject (notch) filter

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a coaxial tunable band stop filter that utilizes tuning elements, such as PIN diodes and varactor diodes, for electrically tuning a coaxial resonator to change the resonance frequency of the resonators; whereby a voltage is applied to the tuning elements to change their capacitance, such that they electrically lengthen and shorten the coaxial resonator; whereby the voltage varies the center frequency of the bandwidth. 
     BACKGROUND OF THE INVENTION 
     It is generally known that a band stop filter is configured to pass most frequencies through a bandwidth unaltered, but attenuates those in a specific range to very low levels. It is also known that a band-stop filter with a narrow stopband is a notch filter. 
     The present invention is directed to a coaxial tunable band stop filter utilizes tuning elements, such as PIN diodes and varactor diodes for tuning a coaxial resonator to change the resonance frequency of the resonators. A voltage is applied to the tuning elements to change their capacitance, such that they electrically lengthen and shorten the coaxial resonator. The voltage varies the center frequency of the bandwidth. When the resonators are electrically extended or shortened in length, the center frequency in the bandwidth is changed accordingly. The bandwidth for the coaxial tunable band stop filter is tunable to increase and decrease based on the position of the center frequency. 
     Generally, band stop filters require precise transmission characteristics to attenuate a band of frequencies at a specific bandwidth and to pass frequencies outside the bandwidth at both higher and lower frequencies. The band stop filters are generally characterized by a bandwidth, center frequency, insertion loss, selectivity or rejection, ripple and return loss. In one known parameter, the band stop filter may have a center frequency of 1000 MHz and bandwidth of 100 MHz or 950-1050 MHz. 
     Often, resonators have a resonance frequency that can be electronically controlled by means of varactors diodes, PIN diodes, MEMs, etc. The varactor diodes could include a tuning band extending beyond the very narrow limits usually obtainable with conventional networks. 
     It is known in the art that, conventional coupling network between a dielectric resonator and a varactor diode, both placed on the same face of a microstrip circuit, includes a length of 90° transmission line which is terminated at one side only, by means of the varactor diode and near to which the resonator is fixed. The control voltage is applied to the varactor diode through a suitable RF decoupling network. 
     In many instances, the transmission line and varactor diode assembly is dimensioned in such a way as to resonate at about the nominal frequency of the dielectric resonator. During the circuit operation, the magnetic field lines of the resonator interlink with the transmission line. By varying the bias voltage of the varactor diode, the capacitance of the latter is modified and the change of the resonance frequency of the dielectric resonator is thus determined. 
     Thus, an unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies in non-tunable filters. Even though the above cited methods for band stop filters meets some of the needs of the market, a coaxial tunable band stop filter that is electrically tunable through tuning elements, such as PIN diodes and varactor diodes is still desired. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a coaxial tunable band stop filter that utilizes tuning elements, such as PIN diodes and varactor diodes for electrically tuning a plurality of coaxial resonators to change the resonance frequency of the coaxial resonators. A voltage is applied to the tuning elements to change their capacitance, such that they electrically lengthen and shorten the coaxial resonator. The variable voltages work to change the center frequency across a bandwidth. 
     The novelty of the present invention is that the tuning elements are electrically manipulated to tune the coaxial resonator. Additional novelty is that the tuning elements are manipulated electrically, not manually, to tune the coaxial resonator. Additional novelty is found in that instead of an inductor, a coaxial resonator is tuned. 
     The coaxial tunable band stop filter is composed of a plurality of coaxial resonators, a plurality of tuning elements, an inductive coupling, a 90° transmission lines (90° TL), and a control circuitry. The coaxial resonators can be implemented using different dielectric materials such as: Air, Teflon, high Q ceramic materials, etc. 
     The tuning element can be implemented using varactors diodes, PIN diodes, MEMS, FETs, and CMOS switches. In any case, the capacitance is changed electronically, not manually. In one embodiment, a ninety degree transmission line is used for coupling the components of the filter. A digital control is used for manipulating the tuning elements. A coupling loss member is integrated into the filter for improving the coupling return loss. 
     By varying the voltage to the tuning elements, the capacitance changes. By applying variable voltages to the tuning elements, the length of the coaxial resonators may be electrically lengthened and shortened. When the coaxial resonators are electrically extended or shortened in length, the center frequency in the bandwidth is changed accordingly. The bandwidth for the tunable band stop filter is tunable to increase and decrease based on the position of the center frequency. 
     One objective of the present invention is to change the capacitance by electrically manipulating tuning elements, such as PIN diodes and varactor diodes in a reflective high Q tunable filter. 
     Yet another objective is to apply a bias voltage on the tuning elements to alter their capacitance. 
     Yet another objective is to provide cost effective and efficient coaxial band stop frequency filters. 
     Other systems, devices, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a diagram of an exemplary coaxial band stop frequency filter, in accordance with an embodiment of the present invention; 
         FIG. 2  illustrates a first graph response of a coaxial six resonator tunable band-stop filter, showing a high pass structure, in accordance with an embodiment of the present invention; 
         FIG. 3  illustrates the first graph response of a coaxial six resonator tunable band-stop filter, showing a high pass structure, but the band of interest is shown in more detail, in accordance with an embodiment of the present invention; and 
         FIGS. 4A and 4B  illustrate an exemplary coupled line band stop frequency filter having a plurality of resonators, where  FIG. 4A  shows a diagram of the coupled line band stop frequency filter, and  FIG. 4B  shows a graph of the frequency response, in accordance with an embodiment of the present invention. 
     
    
    
     Like reference numerals refer to like parts throughout the various views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to make or use the embodiments of the disclosure and are not intended to limit the scope of the disclosure, which is defined by the claims. For purposes of description herein, the terms “first,” “second,” “left,” “rear,” “right,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
     At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions, or surfaces consistently throughout the several drawing figures, as may be further described or explained by the entire written specification of which this detailed description is an integral part. The drawings are intended to be read together with the specification and are to be construed as a portion of the entire “written description” of this invention as required by 35 U.S.C. §112. 
     In one embodiment of the present invention presented in  FIGS. 1-4B , a coaxial tunable band stop filter  100  utilizes a plurality of tuning elements  104   a - c , such as pin diodes and varactor diodes, for electrically tuning a plurality of coaxial resonators  102   a - c  to change the resonance frequency of the coaxial resonators  102   a - c . In operation, a voltage is applied to the tuning elements  104   a - c  to change their capacitance, such that they electrically lengthen and shorten the coaxial resonator  102   a - c . The variable voltages work to change the center frequency across a bandwidth. 
     The novelty of the present invention is that the tuning elements  104   a - c  are electrically manipulated to tune the coaxial resonators  102   a - c . Additional novelty is that the tuning elements  104   a - c  are manipulated electrically, not manually, to tune the coaxial resonator. Additional novelty is found in that instead of an inductor, a coaxial resonator  102   a - c  is tuned. Thus, the present invention utilizes coaxial resonators  102   a - c  rather than an inductor, and tuning elements  104   a - c , such as PIN diodes, rather than a fixed filter. 
     In one possible embodiment, the coaxial tunable band stop filter  100 , hereafter, “filter  100 ” is composed of at least two coaxial resonators  102   a - c , a plurality of tuning elements  102 , an inductive coupling  106 , a 90° transmission lines (90° TL)  108 , and a digital control  110 . 
     In one embodiment, the coupling element  106  can be implemented using a lumped element such an inductor. The center conductor of the coaxial resonator can be surrounded by air. The coaxial resonator  102   a - c  may include a circuit that is configured to reject frequencies in a specific band, while simultaneously admitting frequencies outside the band. In one embodiment, the coaxial resonators  102   a - c  can be implemented using different dielectric materials such as: Air, Teflon, high Q ceramic materials, and the like. The coaxial resonators  102   a - c  comprise a cavity containing a center conductor. 
     In some embodiments, the tuning elements  104   a - c  may be implemented using varactors diodes, PIN diodes, MEMS, FETs, and CMOS switches. In any case, the capacitance is changed electronically, not manually. In one possible embodiment, eight PIN diodes are used as tuning elements  104   a - c . Though any number of tuning elements  104   a - c  may be used, depending on filtering requirements. In another embodiment, a coupling loss member  112  is integrated into the filter  100  for improving the coupling return loss. The control circuitry sends signals to turn on and off the tuning elements. 
     In some embodiments, the inductive coupling may be implemented using a lumped element, a high impedance TL, or through a magnetic coupling. It is significant to note that a resonator, in general, requires a 90° transmission line for coupling components thereto. Thus, the 90° TL  108  of the present invention can be implemented using a coaxial 90° TL, a Micro-strip line, a Suspended Substrate, and a Waveguide, etc. IN some embodiments, the inductive coupling may be implemented using a low loss lumped inductor or a high impedance 90° transmission line. The digital control  110  provides all the necessary stimulus for activating or deactivating the tuning elements  104   a - c . The digital control  110  represents the digital control that sends signals to turn on or off the tunable elements. 
     The coaxial resonator  102   a - c  is formed by using a coaxial transmission line and a lumped capacitor. In one embodiment, the coaxial resonator  102   a - c  is configured as a high Q coaxial filter resonator. The high Q coaxial filter resonator is configured to convey frequencies across a bandwidth. By varying the voltage on the tuning elements  104   a - c , the capacitance changes. 
     The change in capacitance for the tunable elements  104   a - c  enables the length of the resonators  104   a - ca - c  to be electrically lengthened and shortened. When the resonators  104   a - ca - c  electrically extend or shorten in length, a center frequency  116  in the bandwidth  114  is changed accordingly. The bandwidth  114  for the tunable band stop filter  100  is tunable to increase and decrease based on the position of the center frequency  116 . 
       FIG. 2  shows a first graph  200  of a high pass structure.  FIG. 3  shows in more detail the notch response. 
     The result is that, by varying the voltage on the tuning elements  104   a - c , the capacitance of the coaxial resonator changes. By applying variable voltages to the tuning elements  104   a - c , the length of the coaxial resonators  102   a - c  may be electrically lengthened and shortened. When the coaxial resonators  102   a - c  are electrically extended or shortened in length, the center frequency  400  in the bandwidth  402  is changed accordingly. The bandwidth  402  for the coaxial tunable band stop filter  100  is not tunable though. 
     The present invention differentiates from the prior art by addressing the tuning of a coaxial band stop filter  100  with a very narrow band stop, which is known in the art as a notch filter. In one possible embodiment, the tunable band stop filter  100  is a notch filter having a narrow bandwidth  402  for the frequency to pass through a passband. Specifically, the tunable band stop filter  100  is obtained with a high Q coaxial resonator which is connected to the 90° TL  108  using a low loss inductive element. The notch filter can be used in a frequency agile system for suppression of unwanted signals. 
     The present invention may be applied to various types of filters and their effects on frequencies, known in the art. In one embodiment, this includes a high pass filter frequency response, a low pass filter frequency response, a band pass filter frequency response, and a band stop filter frequency responses. The attenuation and the frequencies for each frequency filter are graphed in an x-y relationship. For example, the frequency response for the high pass filter, in which only frequencies above a cutoff frequency are allowed to pass. Another example includes the frequency response for the low pass filter, in which only frequencies below a cutoff frequency are allowed to pass. 
     The coaxial resonator  102   a - c  may include a circuit that is configured to reject frequencies in a specific band, while simultaneously admitting frequencies outside the band. The rejected frequencies inside the band are the stopband. The present invention specifically addresses tuning a band stop filter with a very narrow band stop, which is known in the art as a notch filter. 
     In some embodiments, the coaxial tunable band stop filter  100  tunes a range of frequencies from a VHF (30 MHz-300 MHz) to microwave frequencies. However, in one embodiment, the tunable band stop filter is configured to tune frequencies from 10 MHz through 40 GHz or more. Nonetheless, for any frequency band, the passband for the tunable band stop filter is easily varied through both analog, and digital control. 
     The circuit shown in  FIG. 4A  uses coupled lines. In one embodiment, the coaxial resonators  102   a - c  are configured to exhibit resonance behavior, oscillating at some frequencies (resonant frequencies) with greater amplitude than at other frequencies. In one embodiment, the resonators  102   a - c  increase the tuning capacity of the coaxial tunable band stop filter  100  by about 1 octave. In another embodiment, the resonators  102   a - c  are about λ/4 long. The second embodiment graph  406  depicted in  FIG. 4B  shows the frequency response of the coaxial tunable band stop filter  100 , including the resonators  102   a - c . Here, the frequencies  400  are centered across the bandwidth  402 . 
     In conclusion, by varying the voltage to the tuning elements  104   a - c , the capacitance of the coaxial resonators  102   a - c  changes. Thus, the length of the coaxial resonators  102   a - c  may be electrically lengthened and shortened. When the resonators  102   a - c  are electrically extended or shortened in length, the center frequency  400  in the bandwidth  402  is changed accordingly. The bandwidth  402  for the coaxial tunable band stop filter  100  is tunable to increase and decrease based on the position of the center frequency  400 . 
     REFERENCES 
     
         
         [1] Ou, Yu-Chin, “High Performance Channelizers, Tunable Notch Filters, and Silicon-based Antennas for RF to Millimeter-wave Communication Systems”, PhD thesis, UC San Diego, 2011. 
         [2] Jachowski, Douglas R., “Octave Tunable Lumped-element Notch Filter”, Microwave Symposium Digest (MTT), June 2012. 
         [3] Jachowski, Douglas R., “Tunable Lumped-element Notch Filter with Constant Bandwidth”, IEEE International Conference on Wireless Information Technology and Systems, September 2010. 
         [4] Zhengzheng Wu, Yonghyun Shim, Mina Rais-Zadeh, “Miniaturized UWB Filters Integrated With Tunable Notch Filters Using a Silicon-Based Integrated Passive Device Technology”, IEEE Transactions on Microwave Theory and Techniques, vol. 60, no. 3, March 2012. 
       
    
     Since many modifications, variations, and changes in detail can be made to the described preferred embodiments of the invention, it is intended that all matters in the foregoing description and shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. Thus, the scope of the invention should be determined by the appended claims and their legal equivalence.