FIELD OF THE INVENTION The present invention relates to slot line band pass filters.

BACKGROUND OF THE INVENTION The prior art provides several types of filters for use with radio frequency signals including high pass, low pass, band pass, notch and other types of filters fabricated in lumped or distributed form. Filters of these types have been formed in a variety of transmission media.

With respect to band pass filters, filters of this type have been formed in microstrip transmission media using distributed elements. Microstrip transmission media generally consists of one or more thin conducting strips of finite width parallel to a single extended conducting ground plan. In its common form, the strips are fixed to an insulting substrate attached to the ground plane. Filters fabricated in microstrip transmission media are disadvantageous in that the formation is process intensive involving metalization on

two sides of a substrate and occasionally the formation of interconnecting vias therebetween to achieve proper grounding.

Prior art band pass filters also include some filters formed in coplanar (CPW) waveguide transmission media. Coplanar waveguide transmission media consists of a single thin conducting strip of finite width situated between two semi-infinite ground planes and separated from them by finite gaps. The conducting strips and ground planes are affixed to the same planar surface of an insulating substrate of arbitrary thickness. An example of a CPW band pass filter is disclosed in Coplanar Waveguide Band Pass Filter-A Ribbon-of-Brick- Wall Design, by Lin et al., IEEE, 1995.

Slot line transmission media is another type of known transmission media. One beneficial aspect of this transmission media is that it affords uniplanar fabrication. A need does exist, however, to provide band pass filters in slot line transmission media, particularly filters with improved performance, desirable rejection profiles and compact designs.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a slot line band pass filter.

It is another object of the present invention to provide a slot line band pass filter that affords flexibility in the design of performance characteristics.

It is another object of the present invention to provide such a band pass filter that is compact in size.

These and related objects of the present invention are achieved by use of the slot line band pass filter disclosed herein.

The attainment of the foregoing and related advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of a slot line band pass filter in accordance with the present invention.

Fig. 2 is a diagram of another embodiment of a slot line band pass filter in accordance with the present invention.

Fig. 3 is a diagram of another embodiment of a slot line band pass filter in accordance with the present invention.

Fig. 4 is a frequency diagram illustrating pass bandwidth.

Fig. 5 is a diagram of another embodiment of a slot line band pass filter in accordance with the present invention.

Fig. 6 is a diagram of another embodiment of a slot line band pass filter in accordance with the present invention.

Fig. 7 is a diagram of another embodiment of a slot line band pass filter in accordance with the present invention.

Fig. 8 is a diagram of another embodiment of a slot line band pass filter in accordance with the present invention.

DETAILED DESCRIPTION Slot line transmission media generally consists of two semi-infinite coplanar conductors affixed to the same side of an insulating substrate of arbitrary thickness and separated by a finite gap. With respect to other transmission media, slot line embodiments are relatively non-consumptive of substrate area and provide flexibility of component layout. Slot line embodiments also provide the benefits of uniplanar fabrication, including all circuit elements being formed on one side of the substrate and avoiding the formation of interconnecting vias. The filters described herein are preferably formed on a substrate that may include fused silica, ceramic, plastic, Teflon, glass, air, or the like. The positive and negative conductors are preferably configured as strip lines (as shown), though the use of ground planes is also contemplated and is within the present invention.

Referring to Fig. 1, a diagram of a slot line band pass filter in accordance with the present invention is shown. The filter 10 is coupled to an input signal line 5 and an output signal line 8, each consisting of a positive line (indicated as +V/2) and a negative line

(indicated as-V/2). Filter 10 has a positive half coupled to the positive lines and a negative half coupled to the negative lines. The positive and negative halves are preferably symmetric about a dash-dot center line 20.

The potential at center line 20 is the difference between the signals on the positive and the negative conductors and is effectively ground, i. e., a virtual ground.

Filter 10 comprises a first (or input) positive signal conductor 32 and a second (or output) positive signal conductor 33, separated by a gap 35. Filter 10 also comprises a first (or input) negative signal conductor 42 and a second (or output) negative signal conductor 43, separated by gap 45. It should be recognized that the positive and negative signal conductors can be interchanged such that a signal is propagated from a positive to a negative conductor or vice versa as is known in the art. Hence the terms positive and negative are used here for convenience in describing the filter and it is to be understood that they may be interchanged.

A positive resonator 37 is provided in a preferably substantially parallel relationship with conductors 32,33 and a negative resonator 47 is provided in a preferably substantially parallel relationship with conductors 42,43. The conductors 32,33 and resonator 37 form a positive signal line 30 while conductors 42,43 and resonator 47 form a negative signal line 40. The symmetric arrangement of signal lines 30,40 supports two

fundamental modes of signal propagation, an odd mode and an even mode.

In operation, signals propagating at the design or center frequency are electromagnetically coupled from conductor 32 to conductor 33 through resonator 37 on the positive side and from conductor 42 to conductor 43 through resonator 47 on the negative side.

The selection of component geometry to achieve a desired center frequency and bandwidth are generally as follows. The center frequency is achieved by configuring the first and second positive conductors 32,33 and the first and second negative conductors 42,43 to have a length of one-quarter wavelength of the center frequency, fc, and by configuring the resonator to have a length of approximately one-half wavelength at that frequency. It should be recognized that undesirable pass bands occur at multiples of the half wavelength frequency, which can be attenuated by compensating the filter to equalize even and odd mode phase velocities.

With respect to bandwidth, this is determined by the ratio of the even mode to odd mode capacitance which in turn is determined by the positioning of conductors 32,33,42,43 and resonators 37,47 with respect to center line 20 and to each other. The odd mode capacitance exists between the preferably straight line defined by conductors 32,33 (hereinafter sometimes referred to as the"positive conductor line 31") and resonator 37 and between the preferably straight line defined by conductors 42,43 (hereinafter sometimes referred to a

"negative conductor line 41") and resonator 47. The magnitude of this capacitance is dependent upon the respective distance, dodd-modei of resonators 37,47 from conductor lines 31,41. As the distance between the conductor lines 31,41 and the respective resonator 37,47 increases, the odd mode capacitance decreases.

The even mode capacitance exists between resonators 37,47 and virtual ground center line 20, and between positive and negative conductor lines 31,41 and virtual ground center line 20. As distance, deven-modeS of the conductor lines 31,41 and resonators 37,47 from the center line 20 decreases, the even mode capacitance or capacitive coupling increases.

The relationship of even mode and odd mode capacitances to bandwidth is that the bandwidth of a filter increases as odd mode capacitance increases and as even mode capacitance decreases. Thus, as the distance between positive and negative conductor lines 31,41 and their respective resonators 37,47 increases, the filter bandwidth is reduced. Similarly, as the distance between lines 31,41 and their respective resonators 37,47 decreases, the filter bandwidth is increased.

Referring to Fig. 2, a diagram of another embodiment of a slot line band pass filter in accordance with the present invention is shown. In the embodiment of Fig. 2 resonators 37,47 are placed outside of signal line 31,41.

Such an arrangement reduces even mode capacitance because the distances between resonators 37,47 and virtual ground center line 20 increases. It should also be recognized

that resonators 37,47 may be positioned in-part inside of signal line 31,41 and in-part outside of signal line 31,41 as indicated by dashed lines 37', 47' (the bottom half of resonators 37,47 would not be present in such an embodiment). The ability to position resonators on either side of their respective conductor lines gives a designer significant flexibility in the selection of pass bandwidth and filter layout.

Another benefit of the slot line band pass filter of the present invention is higher impedance, particularly compared to microstrip embodiments. This higher impe- dance allows for significantly wider bandwidth for a given spacing between conductor line and resonator, and thus obviates the need for additional up impedance transforming to achieve wide bandwidths within manu- facturable conductor line to resonator spacing.

While the resonators of Figs. 1 and 2 and others herein are shown in a generally symmetric arrangement about center line 20, it should be recognized that the resonators can be arranged asymmetrically.

Referring to Fig. 3, a diagram of an alternative embodiment of a slot line filter 110 in accordance with the present invention is shown. Filter 110 includes a positive signal line 130 comprised of first (or input) and second (or output) conductors 132,134 and three resonators 133,138,139. Conductors 132,134 are formed in a linear arrangement with resonator 133 to form a positive conductor line 128 and are separated from resonator 133 by gaps 131,135, respectively. Resonators

138,139 flank the linear arrangement of first conductor 132, resonator 133 and second conductor 134. Filter 110 also includes a negative signal line 140 comprised of first and second conductors 142,144 and three resonators 143,148,149. Conductors 142,144 are formed in a linear arrangement with resonator 143 to form a negative conductor line 129 and are separated from resonator 143 by gaps 141,145, respectively. Resonators 148,149 flank the linear arrangement of first conductor 142, resonator 143 and second conductor 144.

In operation, signals propagating at the design frequency are coupled from first conductor 132 to resonator 138 and then to resonator 133 from where they are coupled to resonator 139 and then to second conductor 134. Signal propagation occurs in an analogous manner in negative signal line 140. Each resonator 133,138,139, 143,148,149 is preferably approximately one-half wavelength of the design frequency. It should be recognized, however, that the coupling and frequency are adjusted according to filter type, e. g., Chebychev, Butterworth, elliptic, etc., amongst other known para- meters.

With respect to implementing a desired design frequency, this is achieved by the relative arrangement of conductors 132,134,142,143 and resonators 133,138,139, 143,148,149 as discussed above with respect to Fig. 1A.

It should also be recognized that the resonators 133,138,139,143,148,149 may be arranged other than as shown in Fig. 3, for example, one or more resorlators

(including all resonators) may be provided outside of the conductors (for example, as shown in Fig. 5 below) or arranged asymmetrically. The provision of multiple resonator segments provides the designer with enhanced latitude in achieving desired filter characteristics, including bandwidth and rejection profile.

Referring to Fig. 4, a diagram of pass band frequency is shown. The diagram and the equations below demonstrate that filter rejection outside the pass band, i. e., the steepness of the filter response, increases proportionately with the order of the filter. The order of the filter is defined as the number of half wave resonators per positive or negative signal line and thus filter 10 is a first order filter, while filter 110 is a third order filter. The relationship between filter rejection, bandwidth and filter order is approximately as follows: R = (20) (n) logic (A/B), where A = half of filter pass bandwidth, B = frequency offset from center where rejection is calculated, and n = filter order.

This formula is used to approximate the required filter order to achieve a given level of rejection, R, at a given offset frequency, B.

Referring to Fig. 5, a diagram of another embodiment of a slot line band pass filter 210 in accordance with the present invention is shown. Filter 210 includes a positive and a negative input conductor 232, 242 and a

positive and a negative output conductor 233,243. A plurality of overlapping resonators 235-237,245-247 and 251-252 couple signals of a design frequency from input to output. The input and output conductors are preferably one-quarter wavelength of the design frequency and the resonators are preferably one-half wavelength of the design frequency.

Electromagnetic energy is coupled through filter 210 via two paths. A first path is sequentially through resonators 235,251,236,252 and 237, while a second path is sequentially through resonators 245,251,246, 252 and 247. Filter 210 illustrates resonators that are inside and outside of the input and output conductors.

Supplemental resonators 261 and 262 are connected (or otherwise coupled) to resonators 236 and 246, respectively. The supplemental resonators are preferably of a length that forces voltage to be zero (and current to be a maximum) and are preferably coupled at the mid- point of resonators 236 and 246, though they may be otherwise configured to obtain a desired filter characteristic. In the embodiment of Fig. 5, the supplemental resonators are preferably one-half wavelength of the design frequency. Additional supplemental resonators could be provided to achieve a desired pass (or rejection) profile.

Referring to Fig. 6, a diagram of a slot line band pass filter 510 in accordance with present invention is shown. Though illustrated in slot strips, the filter of

The operation of filter 510 is generally as follows.

Resonator 540 is approximately one-quarter wavelength of fc such that a loop formed by segment 526, resonator 540 and segment 556 has a length of approximately one-half wavelength of fc (180 degrees phase shift). This boundary condition establishes that there will be a maximum voltage present across the positive and negative interconnection leads 543 for frequencies at fc. At frequencies below fc, as frequency approaches zero (DC), there increasingly appears to be a short circuit across lead pair 543 which causes increased signal rejection.

Above fc, as frequency approaches 2fc, the length of the above-described loop approaches a wavelength of fc or 360 degrees, which is a short circuit and thus a rejection maxima. As frequency continues to increase above 2fc, and approaches 3fc, insertion loss switches from a rejection maxima to a rejection minima. This cyclical pattern continues as frequency increases.

It should be recognized, however, that circuit parasitics such as bond wire length begin to play an increasing role in influencing insertion loss. Resonator 570 is optionally included to increase rejection of frequencies above fc. Its length will depend on the parasitics for which it is designed to compensate. Its length and positioning relative to resonator 540 are preferably determined empirically or by using modeling software of a type known in the art. Spacing between resonator 540 and resonator 570, L1, is optimized so that rejection minima due to resonator 540 occur at the same

frequencies as rejection maxima from resonator 570, thus producing a resultant insertion loss versus frequency profile having the desired band pass and rejection characteristics. Resonator 570 can be removed, with the effect of increasing the band pass bandwidth and reducing rejection to frequencies above the pass band.

Conversely, in addition to including resonator 570, a third resonator (not shown) shorter in length than resonator 570 and spaced apart from resonator 570 at some optimal spacing, can be included for further suppression of higher frequency signals. This can extend to multiple resonators of various lengths and spacings. Also, replicas of filter 510, with or without resonator 570 included, can be cascaded together resulting in a broader pass band and increased out-of-band rejection levels.

Referring to Fig. 7, a diagram of another embodiment of a slot line band pass filter 310 in accordance with the present invention is shown. Filter 310 includes an input 312 and output 314 which are interchangeable. In- put 312 contains positive and negative slot line connectors 315,316 for connecting to the positive and negative slot lines (not shown). Connector 315 is coupled through an inductive trace 318 and an interdigitated capacitor 322 to a first loop 325, while connector 316 is coupled through inductive trace 319 and capacitor 322 to first loop resonator 325. A second loop resonator 335 is positioned adjacent the first loop resonator and a third loop resonator 345 is positioned adjacent the second resonator loop. The Lhird loop

resonator includes an interdigitated capacitor 352 and is coupled through inductive traces 358,359 to positive and negative connectors 355,356, respectively.

The input inductive traces 318,319 and capacitor 322 form a series inductor, shunt capacitor matching network as do output inductors 358,359 and capacitor 352.

In operation, a signal current input at the positive connector 315 moves in a clockwise direction around loop resonator 325 and exits at the negative connector 316.

Loop resonator 335 is positioned a desired distance from loop resonator 325 such that a signal in loop resonator 325 is electromagnetically coupled to loop resonator 335, thus there is mutual inductance between these loop resonators. Third loop resonator 345 is similarly coupled to second loop resonator 335. The gaps 330,340 between loop resonators 325,335 and loop resonators 335,345, respectively, and slots 321,331 and 341 determine the amount of coupling between the loop resonators. The width of gaps 330,340 determines the bandwidth of the filter. Wider spacing results in narrower bandwidth, while closer spacing results in broader bandwidth.

The center frequency of filter 310 is determined by the length (wavelength) of each loop resonator. The width of the signal traces and of the slots which define the loops also effect center frequency and bandwidth.

Though filter 310 illustrates the use of three loop resonators (to form a third order filter), it should be recognized that fewer or more loops may be utilized. An

increase in the number of loops results in a sharper out of band rejection profile.

Referring to Fig. 8, an alternative embodiment of the band pass filter of Fig. 7 in accordance with the present invention is shown. In the filter 310'of Fig.

6, loops 325'and 345'are not continuous but include gaps 326 and 346, respectively. The provision of gaps 326 and 346 induces electrical coupling between the loops. The continuous formation of loop 335'also induces magnetic coupling. It should be recognized that gaps such as 326,346 could be provided in similar locations in loop 335', thus rendering loop 335'non- continuous and inducing primarily electric as opposed to magnetic coupling. Loop 335'could also be wholly removed to form a two loop electrically coupled filter.

The embodiments illustrated therein are intended to demonstrate a diversity of conductor size, shape and spacing. It should be recognized that other filter arrangements are contemplated and are within the scope of the present invention. They include filters that differ in the quantity of resonators used and filters with components that have different geometric configurations such as curved, zigzag, amorphous, etc. The embodiments herein are intended to be illustrative are not limiting.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations ot the invention hollowing, in general, Lhe

principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.