Limited Bandwidth Microwave Filter

The present invention relates generally to microwave filters and more particularly to filters with limited bandwidth which are compatible with microwave monolithic integrated circuit (MMIC) design requirements.

Conventional passive microwave filters consist of appropriately coupled or connected transmission line segments, which, in a monolithic integrated circuit environment, consume a disproportionate amount of chip area and are hence not well suited for on-chip integration and, also, do not take advantage of the semiconducting properties of the substrate material. Critical chip area may be conserved by resorting to lumped-element filter configurations (involving inductors, capacitors, resistors, etc.), but only at the cost of incurring appreciably higher signal transmission losses and poorly defined passband edges.

In the prior art, an obvious solution to these problems has been to combine space-saving lumped-element circuitry with on-chip amplifying devices so as to compensate for the lumped-element-derived derived losses. One approach has been to employ the familiar active filter techniques used at audio frequencies, involving high-gain amplifiers and lumped-element embedding circuitry. This approach, however, is not practical at microwave frequencies due to the lack of sufficient gain in the available active devices and the presence of large signal time delays associated with these devices. Alternative approaches have been to encompass the use of transistor circuits with positive feedback to establish negative resistance characteristics for the purpose of loss compensation. In these circuits, the drawback lies with their inherent susceptibility to oscillation, especially when narrow bandwidths are involved.

An approach to circumvent the above stated problem with oscillation is set forth in U.S. Pat. No. 4,661,789, where the filter structures are based on transversal and recursive principles. Filtering action is achieved through frequency-dependent constructive and destructive interference engineered among signal components derived from the signal input and subjected to differing amounts of time delay and amplitude weighting. Mathematically, the transfer function of such a filter is described as a polynomial or rational function of terms composed of products of frequency-independent weighting factors and complex exponential functions with frequency-proportional arguments. This approach is particularly attractive for microwave applications, as it neither requires high-gain amplifiers, nor is it hampered by long time delays intrinsic to currently utilized active devices. However, in narrow-band situations where larger amounts of aggregate time delay are required than can readily be realized in a practical circuit with simple transmission lines or lumped-element approximations thereof another solution must be sought.

Prior art transversal and recursive filters have typically contained a designated main signal path with transfer characteristics of the same type as those sought for the overall filter, but lacking adequate frequency selectivity. For instance, if the overall filter is to exhibit bandpass behavior with sharp cutoff properties, the main signal path would typically show bandpass behavior, but without well defined filter lower and upper cutoff frequency characteristics.

In prior art situations, filter sections have been positioned within transversal and recursive structures so as to affect deliberately and principally the amplitude properties of the main signal path. Depending on the selected arrangement, other auxiliary transversal or recursive signal paths are also affected by the filter sections, but to subordinated degree.

The prior art has no more than one signal transfer branch that is both common to only a single signal path and also deliberately contains frequency-selective componentry. In the prior art there are no transversal band-reject microwave filters.

The prior art, employing mostly signal paths with nonselective or only weakly selective frequency behavior, necessitates a large number of transversal branches to achieve acceptable overall filter frequency selectivity in narrowband situations, thus resulting in large structures.

It is an object of the present invention to provide for the incorporation of means to mimic long signal time delays over limited frequency ranges without necessitating commensurately long transmission line segments, thereby providing the key to realizing narrowband transversal and recursive filters in a practical and compact form.

A further object of the present invention is to apply the disclosed filtering techniques to any type of filtering requirement, including low-pass, band-pass, high-pass, band-reject, and all-pass situations.

These and other objects are achieved by a circuit means to distribute a transversal filter input signal among input ports of two-port network branches, as well as a circuit means to combine signals at output ports of such branches to form a transversal filter output signal. The array of two-port branches contains either two or more feedforward branches with frequency-selective signal transfer characteristics or, in the alternative, the feedforward branches contain one or more principal branches yielding a low-order or zeroth-order approximation to a specified overall transversal filter magnitude response. This is in conjunction with one or more auxiliary feedforward branches that exhibit band-edge signal transfer cut-off characteristics more abrupt than those of the low-order approximation. Transversal filter responses and individual branch transfer responses can be low-pass, bandpass, high-pass, band-reject or all-pass--with bandpass branch responses particularly useful--while involving lumped-element, distributed element, or lumped-distributed-element implementations.

Furthermore, the present invention encompasses a microwave recursive filter design that is analogous to the transversal filter, but with frequency-selective feedback branches employed either in addition to or in place of frequency-selective feedforward branches, and with circuit means to guide feedforward and feedback signals in the appropriate directions.

FIG. 1 is a schematic block diagram of the invention.

FIG. 2 is a schematic block diagram of an exemplary three-branch transversal filter.

FIG. 3 is a schematic circuit diagram of an exemplary three-branch transversal bandpass filter.

FIG. 4 depicts the microstrip layout of the Mark II 1% bandwidth three-branch transversal bandpass filter with isolation resistors.

FIGS. 4(a) and 4(b) are expanded views of the input and output coupling networks of FIG. 4.

FIG. 5 is a schematic block diagram of an exemplary three-branch recursive filter.

FIG. 6 is a schematic circuit diagram of an exemplary two-branch band-reject transversal filter.

FIG. 7(a) is a schematic for a varactor-tuned band-reject filter.

FIG. 7(b) depicts a magnetically-tuned yttrium-iron-garnet (YIG)-based multipole planar bandpass filter.

The present invention encompasses a microwave transversal or recursive filter arrangement that supports multiple paths between filter input and filter output through an array of individual twoport network branches with optional mutual signal coupling among selected branches.

The limited bandwidth microwave filter 10 is conceptionally composed of three main subnetworks, as shown in FIG. 1--an input coupling network 12, a frequency-selective branch network 14, and an output coupling network 16. Each subnetwork may include appropriate amplifying devices or other nonreciprocal devices to boost pertinent signal levels, to provide signal isolation between selected subnetwork ports, or to prevent undesired interactions among pertinent signal components.

An input coupling network 12 is comprised of one primary or input port 15 and a multiplicity of secondary ports 13 designed to distribute a signal incident on the primary port 15 (the composite filter input port) among designated secondary ports 13 and to control distribution of signals incident on any one given secondary port 13 to other secondary ports 13 and to the input port 15.

The output coupling network 16 is comprised of a multiplicity of primary ports 17 and one output or secondary port 18, or the composite filter output, designed to combine signals incident on the primary ports 17 into a composite signal output appearing at the secondary port 18 and to control distribution of signals incident on a primary port 17 or the secondary port 18 to other primary ports 17.

The input and output coupling networks 12 and 16, respectively, will typically be comprised of transmission line sections for adjustment of signal phasing, power splitters and combiners, with or without isolation resistors, and directional couplers of various types for establishing desired interactions among designated signal components, nonreciprocal elements, and various lumped and distributed circuit elements for impedance matching and signal phasing purposes.

The frequency-selective branch network 14 may encompass transmission line sections and nonreciprocal elements, but most importantly it contains individual filtering structures of differing frequency selectivity. A main purpose of the filtering structures is to establish individual signal branches exhibiting differing degrees of rapidly changing transmission phases as functions of frequency. These structures can mimic, over limited frequency bands, the phase characteristics of very long transmission lines, such as would be required in the prior art narrowband transversal and recursive filters, yet consume only a fraction of the circuit area otherwise needed. A simple resonator with input and output coupling is an example of a structure that provides rapid phase change versus frequency. This invention would typically use filtering structures made up of multiple resonator sections appropriately coupled together.

In a typical branch of the frequency-selective branch network 14, an array of individual two-port channels, each having a primary (input) port 13a and a secondary (output) port 17a, connect the respective secondary (output) ports 13 of the input coupling network 12 to the respective primary (input) ports 17 of the output coupling network 16. Two or more of these channels, or branches, exhibit directional signal transfer characteristics whose frequency-selective, non-linear-phase amplitude and phase responses as functions of frequency differ from branch to branch. Directionality is either established within the branch network through use of nonreciprocal circuit elements, established through the directional signal distribution properties of input and output coupling networks 12 and 16, respectively, or established by the branch network 14 in collaboration with either one or both of the coupling networks 12 and 16, respectively.

In the present invention, the technical approach disclosed has limited bandwidth over which the rapidly changing desired phase characteristics of individual frequency-selective branches can be maintained and narrowband magnitude characteristics that invariably go along with the phase responses. These frequency-selective characteristics are an essential ingredient of the invention in that they can concentrate desired effects at specific critical frequency points and thus locally help shape the magnitude and phase properties of an overall filter response. The rapid phase changes versus frequency associated with each of the frequency-selective filter components contained in the multiport branch network, as well as the frequency selectivity of their amplitude characteristics, all combine to yield an efficient and very compact overall transversal, recursive, or mixed-transversal-recursive filter design.

Mathematically, the pertinent transfer functions are polynomials or rational functions of magnitude-weighted exponential terms where the weighting factors vary distinctly with frequency and the exponential arguments generally are not frequency-proportional, as they are in prior art transversal and recursive filters.

The frequency-selective branch network 14 constitutes a critical part of the overall limited bandwidth microwave filter 10, and the preferred embodiments. In a typical implementation, the multiport network 14 consists of an array of individual branches comprising cascade connections of active elements and passive filter segments. Deliberate mutual signal coupling among such branches may be incorporated as part of the invention, if desirable. Individual passive filter segments contained in the frequency-selective branch network 14 may be of the low-pass, highpass, band-pass, band-reject and all-pass type, and may involve distributed circuit elements (transmission line elements), lumped circuit elements, and combinations thereof. The active elements may consist of off-the-shelf microwave monolithic integrated circuit (MMIC) amplifiers, allowing for a building-block approach to microwave active filter design, as used in the audio range, but based on very different principles.

A first preferred embodiment, as shown in FIG. 2, is a three-branch transversal filter 20 design with isolation resistors 29a, 29b and 29c where all branches of the input coupling network 12, the frequency-selective branch network 14 .and the output coupling network 16 (described and discussed in relation to FIG. 1) provide signal transmission in a forward direction. The common direction for all signal components is maintained by a common orientation of the isolating active elements. The input and output coupling networks 12 and 16, respectively, have in-phase power splitting and combining, as well as preamplification as part of input coupling network 12 to minimize the overall noise figure. Further, in this embodiment, it is noted that all the element parameters, including the amplifier gains, differ from branch to branch.

In the first preferred embodiment of the three-branch transversal filter 20, as shown in FIG. 2, the output of a preamplifier 21 (comprised of a MMIC amplifier, such as Model TGA 8021, manufactured by Texas Instruments, Inc. of Dallas TX) together with additional input and output matching circuitry, contained in input coupling network 12, branches into three output transmission line segments 22a, 22b and 22c which respectively match the input impedances of amplifiers 23a, 23b and 23c (Manufacturer's Part No. EG-6345, manufactured by Texas Instruments, Inc. of Dallas. TX) and provide signal phase adjustments. In the amplifiers 23a, 23b and 23c, respective signal components are amplified to precompensate for subsequently incurred signal transmission losses.

Output ports 33a, 33b and 33c of the input coupling network 12 are connected to the filter segments 24a, 24b and 24c in the frequency-selective branch network 14 to present signal components propagating through respective branches with differing phase shifts and amplitude variations versus frequency. The frequency-selective branch network 14 consists of two or more filter networks 24a, 24b and 24c and amplifiers 25a, 25b and 25c to affect overall transversal filtering through constructive and destructive interactions among branch signal components that have been subjected to pertinent phasing and amplitude weighting. Multiple filter sections (i.e., 24bb and 25bb) in the main branch 35 and in the auxiliary branches 36a and 36b (i.e., 24aa, 25aa and 24cc, 25cc, respectively) may be used to achieve overall filter response characteristics of suitable selectivity, with higher-order filter characteristics obtained for higher numbers of filter sections in each branch.

Upon completion of the desired filtration, the output of each of the branches is coupled to the transmission line segments 31a, 31b and 31c at the input ports 27a, 27b and 27c of the output coupling network 16. The impedance is matched in transmission line segments 31a, 31b and 31c to the line impedance of the output 18. Isolation resistors 29a, 29b and 29c are connected between input circuits of the coupling network 16 analogous to the way such resistors are used in common two-way Wilkinson-type signal splitters and combiners.

A second preferred embodiment of the invention is shown in FIG. 3 by the mask layout of a microstrip one-percent-bandwidth three-branch transversal bandpass filter 40, without power splitter 98 and power combiner 138 isolation resistors and is referred to as the Mark I (Mk I) version of the filter. The input and output coupling networks 12 and 16, respectively, consist of a simple three-way power splitter or junction 54 and a three-way power combinet, or junction 94, respectively, segments of cascaded impedance-transforming microstrip transmission lines 55, 56 and a 50-ohm phase equalization line 59 in the auxiliary branches 103 and 105; and microstrip transmission lines 55, 57 and 58 in the main branch. Microstrip transmission lines 55, 56, 59 and 55, 57, 58 in each cascade of lines are followed by an amplifier 64 in each branch together with input cascade short matching lines sections, or amplifier matching networks, 61 and 62 and output cascade short matching line sections, or amplifier matching networks, 66 and 67, respectively.

The frequency-selective branch network 14 contains three independent multisegment capactive end-coupled resonator filters 76, 77 and 82 of varying construction and electrical characteristics. The distributed-element configuration was chosen for its ease of implementation. It should be noted that lumped-element filter segments would be typically substituted for the distributed-element filter segments if space is a concern, such as in an on-chip realization.

The three-branch transversal filter 40 is composed of a 50-ohm input microstrip transmission line ("line" for short) 52, leading to a three-way splitter, or junction 54, that connects to three microstrip transmission lines, or impedance matching circuits. Each microstrip transmission line is effectively a quarter wavelength long at band center and consists of a combination of short and long cascaded lines 55, 56 and 55, 57, respectively, with characteristic impedances all in excess of 50 ohms. These short and long cascaded lines 55, 56, and 55, 57 are followed by 50-ohm phase equalization lines 58 and 59, respectively. It will be noted that the length of the equalization line segments 55, 57 and 58 in the main branch 107 differs from the equalization segments 55, 56 and 59 in each of the two auxiliary branches 103 and 105. This difference in length establishes the desired interference among branch signals. The phase equalization lines 58 and 59, respectively, are connected to amplitude equalizing gain modules, each composed of a MMIC variable gain amplifier or device 64 (Manufacturers Part No. EG 6345, manufactured by Texas Instruments, Inc. of Dallas, TX) located in between input and output amplifier matching circuits each of which is similarly composed of two cascaded short lines 61, 62 and 66, 67, respectively, of appropriate lengths and characteristic impedances to impedance match the MMIC devices 64 to 50 ohms at the amplifier inputs 63 and outputs 65. The output amplifier matching network 66, 67 is connected to a second set of phase equalizing 50-ohm lines 68 and 69, respectively, followed by capacitively end-coupled resonator filters 76, 77 and 82 in each of the three branches 103, 105 and 107. The capacitively end-coupled resonant filters 76, 77 and 82 are thereafter connected to a third set of phase equalizing and impedance transforming lines 88, 88a, 88b; 89, 89a, 89b; and 95, 95a, 95b in the output coupling network 16; which connect to the output three-way combinet 94.

The line segment impsdances and lengths are so selected as to transform the three filter output impedances into a composite 50-ohm overall output impedance at band center at the three-way combiner 94. For the end-coupled resonator filters 76, 77 and 82, the two in the auxiliary branches 76 and 77 possess two quasi-half-wave sections 74 and 75, respectively, and one approximately quarter-wave long spacer section 78, all separated by gaps 72 of equal dimensions. The main branch filter 82 has two quasi-half-wave sections 99 with equal end gaps 79 and one wider center gap 73.

In a third preferred embodiment, a mask layout of a 1% bandwidth three-branch transversal bandpass filter with power splitter 98 and power combiner 138 isolation resistors, 80, referred to as the Mark II (Mk II), is shown in FIG. 4. The design and construction of this embodiment is similar to that of the previously discussed Mk I embodiment 40.

Referring to FIGS. 4a and 4b, the difference between the Mark I 40 and the Mark II 80 is the incorporation of an input assembly 98 and an output assembly 138 at the input 15 and output 18, respectively. The first arrangement in the input assembly 98 is comprised of isolation resistors 94a, 94b, and line section 96a; and is connected to the line segment 102a of the auxiliary branch 142 and the line segment 101 of the main branch 144. The second arrangement in the input assembly 98 is composed of isolation resistors 94c, 94d and line section 96b; and is connected to the line segment 101 of the main branch 144 and the line segment 102b of the auxiliary branch 143. The third arrangement in the input assembly 98 is composed of the isolation resistors 94e, 94f and two segments of line 93a and 93b are connected at the center point by a jumper wire 93c; and terminated at the line segments 102a and 102b of the auxiliary branches 142 and 143, respectively. At the output 18, a similar circuit exists. The first arrangement of the output assembly 138 is comprised of the isolation resistors 108a, 108b and line segments 112a; connected to the line segment 135a of the auxiliary branch 142 and line 121 of the main branch 144. The second arrangement of the output assembly 138 consists of isolation resistors 108c, 108d and line segment 112b; and is connected between the line segment 121 of the main branch 144 and line segment 135b of the auxiliary branch 143. The third arrangement in the output assembly 138 is composed of isolation resistors 108e, 108f and line segments 113a, 113b connected at the center point by a jumper wire 113c; and connected to the line segment 135a of the auxiliary circuit 142 and line segment 135b of the auxiliary circuit 143. The input 98 and output 138 assemblies of the design act as a three-way Wilkinson-type power splitter/combiner with signal isolation achieved among the ports of the main 144 and auxiliary 142, 143 branches facing the branch filters 115, 114, 117.

The three branch filters 114, 117 and 115 are of similar construction to the filters 76, 77 shown in the auxiliary branches 103, 105 of the Mk I embodiment 40 of FIG. 3, with varying gap widths 124, resonator lengths 106 and line lengths, 126, but with the difference that a stepped impedance (dog-bone) resonator 106 is substituted for each quasi-half-wave section. Each "dog-bone" resonator 106 is comprised of a high-impedance line 106b inserted between two 50-ohm lines 106a and 106c and shifts 2nd harmonic parasitic satellite bands to higher frequencies. It is to be noted, that all dogbone sections 106 are of different lengths with different resonant frequencies and passbands, the construction of which is well-known in the art.

Further, in the Mk II 80 version one or more sets of amplifiers, or gain modules, 132, of identical construction to those in the Mk I version 40, are provided between the branch filters 114, 115, 117 with input and output matching circuits 104 and 118 and phase equalizing 50-ohm lines 118, 135a, 135b. The additions of the additional amplifier 104 add isolation and gain to the circuit. Also, to prevent signal interaction between the auxiliary branches 142, 143 and the main branch 144, metallic branch walls 116 are placed equidistant between the branches 142, 143 144.

In a fourth preferred embodiment, FIG. 5, a three-branch recursive filter 30 is shown. The three-branch recursive filter 30 is distinguished from the three-branch transversal filter 20, shown in FIG. 2, in that the signal components are transmitted in both forward and reverse directions, instead of only in the forward direction as in the transversal filter 20. The signal directions are maintained with appropriately oriented active devices having nonreciprocal transmission properties. The filtration process in FIG. 5 is assisted by the use of directional couplers 42 and 44 and active elements in the input 12 and output 16 coupling networks, and the frequency-selective branch network 14.

Referring to FIG. 5, the input signal 15 is passed to the frequency-selective branch network 14 through a preamplifier 32 (Manufacturers Part No. TGA 8021, manufactured by Texas Instruments, Inc. of Dallas, TX), directional coupler 42, and transmission line segment 34b. The transmission line segments 34a, 34b and 34c operate to provide impedance matching to the nominally 50-ohm port impedance of the frequency-selective branch network 14 and to provide signal phasing. In the main branch 48, the signal is further amplified by an amplifier 38 to compensate for filter transmission losses before being connected to the frequency-selective branch network 14.

In the main branch 48, the signal passes through one or more filter segments 36 and amplifiers 39 to transmission line segment 43b where the impedance is matched to that of the output directional coupler 44. In the output coupler 44, a portion of the signal is detected and fed back, through auxiliary branches 53 to the input directional coupler 42. In the input directional coupler 42 the fed-back signal is combined with the original input signal 15 carried by the main branch 48 to achieve frequency-selective constructive and destructive signal interactions. The signals appearing at the isolation ports of the input and output directional couplers 42 and 44, respectively, are grounded at 47 through termination resistors 46.

The feedback signals taken from the output coupler 44 are matched in impedance by transmission line segments 43a, 43c before being amplified by one or more amplifiers 39a, 39b prior to being filtered by one or more filter segments 37a, 37b. After the feedback signal passes through the filter segments 37a, 37b, the impedance is matched to the impedance of the input directional coupler 42 by transmission line segments 34a and 34c. Passing through input directional coupler 42, portions of the feedback signals are recombined with the input signal 15 to complete the recursive loop. The transmission line segments 34a, 34c in the auxiliary branches 53 differ in length from transmission line segment 34b in the main branch 48 to properly match the impedance of the input signal 15 to the main and auxiliary branches 48 and 53, respectively. The design of these impedance matching circuits is well known to individuals practicing in the art.

A fifth preferred embodiment of the filter is the 0.5%-3dB bandwidth two-branch band-reject transversal filter 150 shown in FIG. 6. Signal rejection is achieved through destructive signal interaction between the signal in the main branch 166 and the signal transmitted through the resonant auxiliary branch 164, with the sharpness of the notch determined by the appropriate amplitude weighting of both signals for cancellation and the phase slope differential between the main branch 166, and resonant auxiliary branch 164 in the vicinity of resonance of employed bandpass filters.

In this embodiment, the input signal 15 is transmitted through a 50-ohm impedance line segment 152 connected to the main and auxiliary branches 166, 164, respectively, of the transversal filter. In the auxiliary branch 164, the signal 15 from the line segment 152 is connected to the auxiliary branch 164 through connection point 155 and filtered by two filter segments 162, 177 separated by a MMIC variable gain amplifier 172 (Manufacturers Part No. EG-6345). The first filter segment 162 is separated from the input 50-ohm impedance line 152 by a gap 158. This first filter segment is a single-resonator end-coupled filter 162 consisting of a microstrip half-wave resonator 154. The filter segment 162 is so designed so as to produce a narrow bandpass single-resonant response. A 50-ohm line segment 156 is separated from the half-wave resonator 154 by a gap 159. An amplifier input matching circuit 168 is connected to the 50-ohm segment 156 and matches the circuit impedance at resonance to the input impedance of the MMIC variable gain amplifier 172. The amplifier 172 transmits the signal through an output matching circuit 174 to a second 50 -ohm line segment 176 to match the impedance to the second filter segment 177. This filter segment 177 also consists of a microstrip half-wave resonator 178. In the second filter segment 177 the half-wave resonator 178 is isolated with gaps 182 and 183 from line segment 176 and the connection point 206 where the outputs of main and auxiliary branches 164, 166, respectively, reconnect with the final line segment 204.

In the main branch 166, the signal from the input line segment 152 is connected at connection point 155 to an input matching circuit 186 through a second line segment 184 to match the impedance to the MMIC variable gain amplifier 188 by the matching segment 186. Amplification compensates for losses in the main branch 166. The output of the amplifier 188 is matched with an amplifier output matching circuit 192 to a third line segment 194. Connected to the third line segment 194, at a connection point 195, is an amplitude equalization network 197 composed of a 100-ohm ballast resistor 196 and a half-wave 50-ohm open-circuited stub 198. The purpose of the equalization network is to flatten out the amplitude ripple inherent in the MMIC variable gain amplifier 188. A third line segment 202 in the main branch 166 between connection point 195 and connection point 206 matches the impedance of the output of the main branch 166 to the output line 204 where the output of the auxiliary branch 164 and the output of the main branch 166 are combined through line 204 to provide the filtered output 18.

In modifications of the above-described preferred embodiments, a tuned filter replaces filter blocks 24a, 24b and 24c in FIG. 2 and filter blocks 36, 37a and 37b in FIG. 5 with tuned resonator filters. Such tuned resonator filters may be either bandpass or bandreject filters and may be either of the voltage-tuned or of the magnetically-tuned ferrite-based type.

An exemplification of the voltage-tuned filter is a filter having voltage-tuned capacitive elements (such as varactor diodes) coupled to fixed resonator structures, such as the vatactor-tuned band-reject filter 210, as shown in FIGS. 7(a). The input 212 and output 216 of the filter are connected to a microstrip line 214, which in turn connects to a pair of quarter-wave microstrip resonators 222 through capacitive coupling gaps 218, with the resonators 222 connected to ground at their other ends through varactors 224.

Another type of tuned filter uses magnetically-tuned resonators (such as yttrium-iron-garnet (YIG) resonators) that are frequency-tuned through application of a variable magnetic field. These are commonly called magnetically-tuned YIG-based filters. One example of such a filter is the multi-pole planar YIG-based bandpass filter 220 shown in FIG. 7(b), which filter consists of three microstrip coupling lines 226, 228 and 232, respectively, separated by two YIG patch resonators 234, over a ground plane 236. The first microstrip coupling line (the input microstrip coupling line) 226 has the circuit input 238 at one end and the other end 242 is grounded. The second microstrip coupling line (interresonator coupling line) 228 is grounded at both ends 244 and 246, respectively. The third microstrip coupling line (output microstrip coupling line) 232 is grounded at one end 248 and has the circuit output 250 at the other end. The design of such filters is well known among individuals skilled in the art and need no further detailed discussion. SEE, G.L. Matthaei et al., Microwave Filters, Impedance-Matching Networks, and Coupling Structure, McGraw-Hill, pg. 1043, and Hunter et al., Electronically Tunable Microwave Bandstop Filters, Trans. Microwave Theory and Tech., IEEE, Vol MTT-30, No. 9, pp. 1361-1367, September 1982.

Each branch of the modifications to the above-described embodiments 20, 30, 40 and 150, or selected ones, may also include optional phase shifters (not shown) to provide variable phase shift among transversal and recursive branch signals as the filter center- or cut-off-frequency or bandwidth is tuned. Such tuning is needed to maintain proper phasing relationships among branch signals to achieve required constructive and destructive signal interferences across the filter frequency tuning band and achieve a specified filtering characteristic across the tuning band. Phase shifters (not shown) may be part of the frequency-selective branches 14 or be incorporated into the input and output coupling networks 12, 16, respectively, FIG. 1. The design and construction of phase shifters (not shown) are well known to those skilled in the art, and comprise a wide variety of implementations, including in the MMIC form.

It will be noted that in the embodiments of the invention, the auxiliary branch filters are narrow bandpass filters, each made up of a single resonator or multiple ones with sharp resonant behavior and rapid phase changes in the vicinity of band center. This permits the effects of auxiliary signal paths to be concentrated in a narrow frequency interval, such as around the designated cut-off frequency of a transversal or recursive filter characteristic to be synthesized. As an example, a rapid, readily achievable phase variation with a total phase differential of 180 degrees or so can be utilized to provide a sharp boost to main-signal-path transfer characteristics at a given critical frequency immediately followed by a sharp dip or null, and vice versa. Consequently, sharp composite filter skirts can be achieved with only a relative small number of signal branches. The savings in number of branches and in required cumulative time delay offered by the invention are particularly apparent in narrowband situations.

Although the individual branches of the various embodiments may appear to be similar in construction, however, the various elements of the filters have different line lengths because their filter passband center frequencies differ. Also, gaps between various elements of main branches always differ from those found in the auxiliary branches. The feeder line lengths in the input and output coupling networks that distribute the signal to the branch networks and collect the signals from them always differ because of the phase differences in the individual branches.

In all embodiments, the line lengths and characteristic impedances may be determined with computer-aided design tools so that all sections work together to yield the desired transversal filtering effects in the selected frequency band. The specific parameter values are not unique and, therefore, are not related to the invention as they will vary from case-to-case.

The first truly practical microwave active filter design concept is promised by this invention. It has direct application to receiver systems of all kinds, including RADAR, Electronic Warfare systems, and communications systems--terrestrial, airborne, and space-based. With the ability to readily achieve bandwidths of one percent and less in a compact and unconditionally stable fashion, the invention promises to have a particularly strong impact on the design of communications systems. The invention spans virtually all implementation technologies, including hybrid-circuit and MMIC designs, and including distributed-element, lumped-element, and mixed-element designs. Any filtering scheme falls within the scope of the current invention that employs nonreciprocal feedforward and/or feedback auxiliary signal branches with frequency-selective magnitude and phase characteristics. However, not all branches need to be frequency-selective and nonreciprocal.

High-order filter transfer functions can be achieved through appropriate parallel-, series-, and cascade-connections of filter building blocks designed in accordance with the basic invention.

While maintaining all of the advantages of the U.S. Pat. No. 4,661,789 transversal and recursive filter (i.e., not susceptible to oscillation, not requiring high-gain amplification, and tolerant to long signal time delays intrinsic to available active elements), the invention extends the approach to narrowband (1% to 10% fractional bandwidth) and ultra-narrowband (1.0% and less fractional bandwidth) filters with MMIC compatibility. When utilized in hybrid configurations, the present invention allows a buildingblock approach to filter design through utilization of off-the-shelf MMIC amplifiers--an option thus available at microwave frequencies for the first time.

The present invention distinguishes itself from the prior art by including filter sections with pronounced frequency selectivity (not just sections used for low-Q, nonresonant impedance matching purposes) in auxiliary feedforward and/or feedback branches of the overall filter network that are each common to only one transversal or recursive auxiliary signal path. These branches may, but do not have to, include a nonreciprocal gain device, such as an amplifier, to provide directionality and compensate for passive component losses, in addition to establishing proper signal weighting distribution.

The present invention should not be confused with so-called directional filters. This type of filter is based upon transversal type operation with two frequency-selective branches. Unlike the present invention, directional filters are reciprocal circuits, and thus do not actually fall into the transversal filter category. Also, the present invention should not be confused with transversal and recursive filters found in the prior art that achieve required phase differences among branch signal components by employing passive filter sections in lieu of traditional uniform delay line segments. Such filter sections have been used not only to satisfy signal phasing requirements as substitutes for transmission line sections, but also, to shape overall magnitude responses. However, filter sections used in prior art situations have remained exclusively associated with portions of a transversal or recursive filter that distribute signal components to respective feedforward or feedback branches, rather than directly incorporated into such branches, as in the current invention.

Numerous modifications and adaptations of the present invention will be apparent to those skilled in the art. Thus it is intended that the following claims cover all modifications and adaptations which fall within the true spirit and scope of the present invention. Although the invention has been described in relation to various exemplary preferred embodiments thereof, it will be understood by those skilled in the art that still other variations and modifications can be affected in other embodiments without detracting from the scope and spirit of the invention.