Bandstop filters with minimum through-line length

Systems and methods are provided for creating higher order microwave bandstop filters with total through-line length of significantly less than one-quarter wavelength at the filter center frequency. The mixed electric and magnetic field coupling bandstop filter topologies provided by embodiments of the present disclosure can be used to reduce the size, weight, and throughline insertion loss of microwave bandstop filters. In an embodiment, if the relative field strengths are intelligently designed for each coupling structure, effective phase offsets can be produced between resonators along the through line. These phase offsets can be used to absorb some or all of the length of the λ/4 inverters between resonators.

FIELD OF THE DISCLOSURE

This disclosure relates to filters, including bandpass filters.

BACKGROUND

Microwave bandstop filters can be used to reflect or absorb unwanted signals in a microwave system. These unwanted signals can originate from co-site or externally generated interference as well as nonlinear components under high-power excitation in the system. For example, a traditional microwave bandstop filter can be composed of resonators coupled to a through line with quarter-wavelength admittance inverters between each resonator. This bandstop filter topology can produce a symmetric notch frequency response and meet a wide variety of practical specifications. However, when the traditional microwave bandstop filter topology is used for high-order filters, the total through-line length becomes long.

A long through-line leads to higher passband insertion loss, increased circuit size and weight, and larger dispersive effects. In addition, the through-line lengths are difficult to tune in production environments yet have appreciable effects on the frequency response of the filter. Thus, conventional bandpass filters have undesirably large passband insertion loss, size, and weight.

Features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, it will be apparent to those skilled in the art that the disclosure, including structures, systems, and methods, may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the disclosure.

For purposes of this discussion, the term “module” shall be understood to include one of software, or firmware, or hardware (such as circuits, microchips, processors, or devices, or any combination thereof), or any combination thereof. In addition, it will be understood that each module can include one, or more than one, component within an actual device, and each component that forms a part of the described module can function either cooperatively or independently of any other component forming a part of the module. Conversely, multiple modules described herein can represent a single component within an actual device. Further, components within a module can be in a single device or distributed among multiple devices in a wired or wireless manner.

Embodiments of the present disclosure provide systems and methods for implementing bandstop filters using minimum through-line lengths between coupled resonators. For example, conventional microwave bandstop filters with λ/4 inverters between each resonator usually assume that the coupling structures between the through-line and the resonators all implement coupling with either electric field, magnetic field, or the same relative mixture of electric and magnetic field. Embodiments of the present disclosure use mixed electric and magnetic field coupling to reduce physical length between coupled lines.

In an embodiment, a bandstop filter in accordance with an embodiment of the present disclosure comprises a number of resonators coupled along a transmission line, with a ratio of electric to magnetic coupling of each resonator set such that that physical length between coupled lines is minimized. For example, in an embodiment, if the relative field strengths are intelligently designed for each coupling structure, effective phase offsets can be produced between resonators along the through line. These phase offsets can be used to absorb some or all of the length of the λ/4 inverters between resonators. Thus, the bandstop filter topologies provided by embodiments of the present disclosure can be used to reduce the size, weight, and throughline insertion loss of microwave bandstop filters.

Bandstop filters can be used in microwave systems to excise unwanted signals.FIG. 1Ais a diagram showing an exemplary in-line microwave bandstop circuit topology comprising a transmission line102to which a number of electromagnetic resonators104are coupled. The electrical length between adjacent resonators is typically close to a quarter wavelength, defined at the center frequency of the filter. The required transmission line lengths in the circuit topology ofFIG. 1Alimit passband insertion loss and place a lower limit on size and weight. As shown inFIG. 1A, any number of additional bandstop filters106and electromagnetic resonators108can be coupled to the circuit.

An exemplary bandstop filter in accordance with an embodiment of the present disclosure comprises a number of resonators coupled along a transmission line, with the ratio of electric to magnetic coupling of each resonator set such that the physical length between coupled resonators is minimized. This approach can be applied to both in-line bandstop topologies as well as other topologies, including reflection-mode.

3. Reflection Mode Topology

A reflection-mode bandstop filter can be constructed by first designing a prototype bandpass filter with a reflection coefficient that is equivalent to the transmission coefficient of the desired bandstop filter.FIG. 1Bis a diagram of a circuit that implements the even- and odd-mode impedances of a bandpass filter designed in accordance with an embodiment of the present disclosure. InFIG. 1B, even- and odd-mode impedances110are connected to two adjacent ports112of a four-port hybrid circuit. The remaining two ports116of the hybrid circuit are used as source and load ports. The combined circuit retains the even-mode impedance of the prototype bandpass filter but inverts the odd-mode impedance. When the odd-mode impedance of any linear network is inverted, the reflection coefficient becomes the transmission coefficient and vice-versa. Therefore, since the initial network was a bandpass filter, a bandstop response is produced.

A significant advantage of reflection-mode topology is that only two resonators are required to be coupled to the through-line regardless of the filter order. For example, in an exemplary fifth-order bandstop filter in accordance with an embodiment of the present disclosure, only two resonators are directly coupled to the through-line. Such a topology allows for minimum through-line length in planar technologies like stripline because a resonator can be placed on both sides of the through-line at the same point. In a fifth-order in-line topology, all five resonators would be coupled to the through line. However, even in three dimensional circuit topologies, coupling five resonators to the through-line at the same point would be difficult or impractical, resulting in a need to lengthen the through-line.

4. Bandstop Filters With Minimum Through-Line Length

A conventional microwave bandstop filter with λ/4 inverters between each resonator assumes that the coupling structures between the through-line and the resonators all implement coupling with either electric field, magnetic field, or the same relative mixture of electric and magnetic field. A bandstop filter in accordance with an embodiment of the present disclosure uses mixed electric and magnetic field coupling to reduce physical length between coupled lines. In an embodiment, if the relative field strengths are intelligently designed for each coupling structure, effective phase offsets can be produced between resonators along the through line. These phase offsets can be used to absorb some or all of the length of the λ/4 inverters between resonators. This concept is illustrated inFIG. 2A.

FIG. 2Ais a diagram showing a resonator202coupled to a node204with a mixture of both electric206and magnetic208coupling (mixed coupling).FIG. 2Afurther shows that this point can be thought of as 360 degrees of electrical phase over zero physical length.FIG. 2Aalso shows an equivalent circuit210for this mixed coupling circuit based on an expansion210of node204to 360-degree phase length using 4 admittance inverters matched to the port impedance. The electric and magnetic couplings inFIG. 2Aare represented by admittance inverters KE and KM+/−, respectively. The point inFIG. 2Acouples to resonator202with negative magnetic, positive electric, and positive magnetic coupling, where negative and positive values are assigned to represent a 180 degree phase shift between two coupling values of similar type. The composite phase offset due to multiple types of coupling between a single node and a resonator can be reduced to equivalent circuit211. Thus, equivalent circuit211is a representation of the node in equivalent circuit210as a phase offset dependent on E and M coupling. In equivalent circuit211, K0=√{square root over ((KE2+KM2))} and

θoffset=±12⁢Arg⁡(2⁢⁢KEKE-jKM-1),
where the sign of θoffsetdepends on the relative orientation of magnetic coupling. In an embodiment, these equations can be implemented in a fourth-order minimum through length bandstop filter design, which is illustrated inFIG. 2B.

FIG. 2Bis a diagram showing a photograph of an exemplary filter using mixed electric and magnetic field coupling to resonators along a through line that implements a fourth-order bandstop filter design in accordance with an embodiment of the present disclosure. InFIG. 2B, each resonator is coupled to the through line over a λ/8 physical length of line and implements a λ/8 electrical shift of the coupling reference plane between it and the next resonator through the use of appropriately designed mixed coupling. InFIG. 2B, the λ/8 physical coupling section for each resonator is followed directly by the λ/8 physical coupling section for the next resonator, so the entire length of the through line is coupled to a resonator. This is possible regardless of the length of the physical coupling section required for the desired coupling values if appropriately-designed mixed coupling is used. The result is a minimum-length design for the implemented fabrication technology and coupling values. With the combination of the λ/8 phase shifts due to the physical lengths of the coupling sections and the λ/8 electrical θoffsetshifts of the coupling reference planes between each pair of resonators, a composite λ/4 inverter exists between each pair of resonators despite there being only λ/8 of physical through line between each resonator.

While this technique enables an improvement over conventional designs that have a total through-line length of N*λ/4, where N is the order of the filter, the total through-line length, N*Lc, where Lc is the length of the coupling section between the through line and each resonator, can still be significant for high-order filters. The combination of reflection-mode circuit techniques and minimum-through-line-length bandstop filter theory can produce bandstop filter designs with total throughline length equal to only the length of a single coupling section, Lc, regardless of filter order. Therefore, the total through-line length becomes only a function of the desired coupling values and fabrication technology tolerances, not filter order, and it can be much shorter than λ/4 for many filter specifications. For high-order filters, dramatic reductions of total length are possible.

Reflection-mode topology can be used to interchange the reflection and transmission responses of a circuit network by placing the network's even and odd mode impedances at the correct ports of the reflection-mode structure.FIG. 3is a diagram showing an example transformation of an elliptic bandpass filter to a highly selective bandstop filter. Elliptic bandpass filters are known for the maximum selectivity that they provide, and embodiments of the present disclosure can use that selectivity in a bandstop mode. InFIG. 3, elliptic bandpass topology302is transformed304to reflection-mode bandstop topology306. The 90-degree hybrid in the reflection-mode bandstop topology306shown inFIG. 3is classically implemented by four quarter wavelength transmission lines of varying characteristic impedance. However, when used in conjunction with an embodiment of the present disclosure, it can be reduced to a single physical point when the correct phase and strength of electromagnetic coupling values are used, as shown inFIG. 2A.

The phase-expanded but zero-length view of a point along a through line shown inFIG. 2Acan be used to understand how the 90 degree hybrid in the reflection-mode bandstop prototype in accordance with an embodiment of the present disclosure collapses into a single point.FIG. 4is a diagram showing a zero-length, phase-expanded point that involves two couplings to one resonator and one coupling to another resonator. The two couplings to the same resonator have the same phase and are of the same type because the couplings that are represented by “1”402and “−1”404inFIG. 4are separated by 360 degrees of phase length. The coupling to the resonator below the hybrid equivalent circuit is of the opposite type because it is separated from the other two couplings by 90 and 270 degrees.

5. Second Order Example

In an embodiment, the even and odd-mode admittances of a prototype lowpass filter can be determined and, the proposed reflection-mode topology can be used to implement a prototype highpass filter with a transmission coefficient equal to the reflection coefficient of the lowpass prototype and vice-versa. The highpass prototype can be transformed to produce a bandstop response using standard circuit techniques. In this example, a second-order, 20 dB equi-ripple Chebychev lowpass filter prototype will be used as the starting point. However, any lowpass prototype filter can be used for the design procedure.FIG. 5is a coupling-routing diagram502for the prototype lowpass filter. The dashed line through the K12coupling504is the symmetry plane used for even-odd mode analysis, and the even- and odd-mode subcircuits can also be seen inFIG. 5. For the even mode, the K12coupling becomes an open-circuited λ/8 length of line with a characteristic impedance of K12, and the input admittance is given by Ye=K02/(p+j/K12), where j is the square root of −1, and p is the frequency variable jω. For the odd mode, the K12coupling becomes a shortcircuited λ/8 length of line with a characteristic impedance of K12, and the odd-mode input admittance is given by Yo=.K02/(p−j/K12). The reflection and transmission coefficients of the network can be found using the equations S11=(1−YeYo)/((1+Ye)(1+Yo)) and S21=(Yo−Ye)/((1+Ye)(1+Yo)).

FIG. 5also shows a 2-pole version506of the proposed reflection mode topology in with a dashed line508that indicates the plane of symmetry for even-odd mode analysis. It is important to note that the lower path through the resonator is symmetric about the dashed line, while the upper path is antisymmetric about the dashed line due to the opposite signs of the unity-magnitude inverters. An antisymmetric path will have opposite terminations in even-odd mode analysis relative to the symmetric case. For example, analysis of the even mode will use short circuit terminations in the asymmetric path. For the even mode, the antisymmetric path is shorted to ground, and the even-mode input admittance is given by Ye=K22/(0.5(p+jB2)), where B2is a frequency-invariant suseceptance that manifests as a shift of the center frequency of resonator2. Note that the K2couplings to the source and load are in-phase due to the 360 degree phase shift between the source and load ports.

For the odd mode, the lower path through resonator2is shorted to ground. In an embodiment, the even mode admittances have the same form; however, the forms of the odd mode admittances are inverses of each other. Therefore, in an embodiment, the reflection-mode topology can produce a highpass response with a transmission coefficient equal to the lowpass prototype's reflection coefficient.

Comparing the input admittances forFIGS. 2B and 5, it can be seen that the even mode admittances have the same form. However, the forms of the odd mode admittances are inverses of each other. Therefore, the reflection-mode topology can produce a highpass response with a transmission coefficient equal to the lowpass reflection coefficient if K1, K2, B1, and B2are designed properly. Solving for the desired quantities yields B1=1/K12, K1=√{square root over (2)}K0, B2=−1/K12, and K2=K0/√{square root over (2)}.

If this 2-pole highpass filter was translated into a physical bandstop filter design, the total through-line length could be limited to only that which is needed to obtain the desired magnitudes of K1and K2if K1and K2use the proper combination of electric and magnetic coupling such that their offset values produce an intrinsic phase shift that makes the total shift equal to an odd multiple of λ/4. Depending on the design bandwidth, manufacturing technology, and characteristic impedance values, the amount of λ/4 shift required to be obtained from a physical length of transmission line can be very small.

6. Fifth Order Example

While the example shown in. the previous section could reduce the total through-line length of a second-order filter to the length of one coupling section, the proposed bandstop filter concept is especially beneficial in high-order bandstop filter designs. The through-line length does not increase beyond the length needed to couple to the first two resonators of the filter as the filter order grows. Therefore, high-order bandstop filters can be made with total through-line length equal to the length of one coupling section.FIG. 6Ais a diagram showing the coupling-routing diagram602of a fifth-order bandpass filter.

Using the same even and odd-mode analysis procedure shown in the second-order example, the even- and odd-mode admittances of the fifth order bandpass filter can be found and set equal to the even and odd-mode admittances of the fifth-order reflection-mode bandstop topology604. The result is a 30-dB equi-ripple bandstop response with four reflection zeros. This response was used as a target specification to design and fabricate a suspended-stripline prototype circuit for verification. InFIG. 6, resonators1and3are coupled to the through line. To realize the bandstop topology inFIG. 6with the shortest possible through line, a physical coupling topology that implements the phase differences between the required electric and magnetic coupling coefficients over minimum through-line length can be designed for the chosen resonator technology.FIG. 6Bis a diagram showing an exemplary expansion based on the source and load nodes of the coupling routing diagram ofFIG. 6Athat more clearly shows the phase relationships between the coupling values.

FIG. 7is a diagram showing models702and a photograph704of a fabricated circuit board in accordance with an embodiment of the present disclosure. In an embodiment, the center frequency of the filter is 3 GHz, and it uses a 5th-order 30-dB equi-ripple elliptic response for high selectivity. The circuit board fits between two sides of a metal housing to produce a suspended stripline circuit.FIG. 8is a diagram showing models802of the housing. This embodiment of the present disclosure allows this filter to have a through line length that is less than one fifteenth of a wavelength while producing a 5th -order bandstop response. A conventional 5th-order bandstop filter would require a through line length of one wavelength. This significant difference allows a filter designed in accordance with an embodiment of the present disclosure to have a substantially reduced physical size relative to conventional designs. It also enables the design of bandstop filters with extremely low passband insertion loss. For example, a filter designed in accordance with an embodiment of the present disclosure has less than 0.1 dB passband insertion loss across S band away from its 30 dB equi-ripple notch.

Any representative signal processing functions described herein can be implemented using computer processors, computer logic, application specific integrated circuits (ASIC), digital signal processors, etc., as will be understood by those skilled in the art based on the discussion given herein. Accordingly, any processor that performs the signal processing functions described herein is within the scope and spirit of the present disclosure.

The above systems and methods may be implemented as a computer program executing on a machine, as a computer program product, or as a tangible and/or non-transitory computer-readable medium having stored instructions. For example, the functions described herein could be embodied by computer program instructions that are executed by a computer processor or any one of the hardware devices listed above. The computer program instructions cause the processor to perform the signal processing functions described herein. The computer program instructions (e.g., software) can be stored in a tangible non-transitory computer usable medium, computer program medium, or any storage medium that can be accessed by a computer or processor. Such media include a memory device such as a RAM or ROM, or other type of computer storage medium such as a computer disk or CD ROM. Accordingly, any tangible non-transitory computer storage medium having computer program code that cause a processor to perform the signal processing functions described herein are within the scope and spirit of the present disclosure.

While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the disclosure. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments.