More particularly, it relates to the field of microwave band-reject filters. Still more particularly, it relates to the field of very-narrow band, thin-film, superconductive band-reject filters.

Background Over the past decade, wireless telecommunications systems have become commonplace. One example is in the proliferation of cellular and mobile telephones, where the number of additional devices has pushed current radio technology to its limits within the assigned frequency ranges. More specifically, in this area of wireless telecommunications, each service provider is assigned a particular band (or region) of the electromagnetic spectrum. Within this band the provider's customers'communications have priority over all other signals.

Accordingly, the service provider's equipment must be capable of transmitting and receiving signals within its allocated spectrum while filtering out those signals which are outside its allocated spectrum.

In the U. S. (and some other countries), each area has two cellular systems within the cellular spectrum to promote competition. The two systems may be designated as the"A"carrier and the"B"carrier. Continuing with the U. S. example, the"A"carrier transmits from its base station in the frequency ranges 869- 880 and 890-891.5 MHz and receives in the ranges 824-835 and 845-846.5 MHz.

The"B"carrier transmits in the frequency ranges 880-890 and 891.5-894 MHz and receives in the ranges 835-845 and 846.5-849 MHz. Ideally the"A"carrier receive filter combines a low-loss band pass filter in the range 824-846.5 MHz and a band stop filter in the range 835-845 MHz. The"B"carrier receive filters has a bandpass range of 835-849 MHz with a band-reject filter in the range 845-846.5 MHz. It will be appreciated that in order to maximize the use of the frequency range assigned to the carrier, both the bandpass filter and the band stop filter must have very sharp skirts. That is, the transition from nearly 100% transmission of the signal to essentially no transmission of the signal must be very steep. Additionally, to avoid cross-talk, the band-reject filter must produce a"notch"in the frequency spectrum which is very deep. In the"B"range in particular, the band stop filter must be a very narrow band.

Thin-film superconductive filters hold promise as ideal filters for communications base stations. Much effort has been directed toward developing filter designs in this area which maximize the advantages of superconductive filters within the limitations posed by the technology. Thin-film superconductive filters are generally fabricated by epitaxially depositing a layer of a high temperature superconductor on a crystalline substrate and then using lithographic methods for patterning the film to form inductive and capacitive elements. A number of patents and publications have heretofore addressed design considerations for superconductive filters. For example, Zhang, et al.,"Frequency Transformation Apparatus and Method in Narrow-Band Filter Designs", U. S. Patent Application Ser. No. 08/706,974, which is incorporated herein and made a part hereof by reference, describes a very narrow band lumped-element microstrip filter with capacitively-loaded inductors. Also, Zhang, et al.,"Microstrip Filters for Wireless Communications Using High-Temperature Superconducting Thin Films,"Applied Superconductivity, Vol. 3, No. 7-10,483-496 (1995), which is incorporated herein and made a part hereof by reference, reviews a variety of design approaches to bandpass filters on small substrates.

It is known to construct band-reject filters of a ladder circuit containing series branches of parallel-resonant circuits and shunt resonators. Efforts to design superconductive thin-film band-reject or notch filters are more recent. On example is U. S. Patent No. 5,616,539 issued to Hey-Shipton et al., and titled"High Temperature Superconductor Lumped Element Band-Reject Filters". In this reference, resonators including serpentine inductors are used as the shunt. The shunt structures are coupled to varying series inductors which are connected to the input and output lines (see Hey-Shipton Fig. 9). One disadvantage of the circuit used by Hey-Shipton is that the shunts are very large. As the bandwidth of the notch becomes smaller, the shunts then rapidly become unacceptably large.

Accordingly, there arises a need in the art to provide a band-reject filter having a narrow bandwidth with shunt structures sufficiently compact as to be realizable on a small substrate. The present invention addresses and overcomes the shortcomings of the prior art.

Summarv The present invention provides a very-narrow band, thin-film, superconductive band-reject filter and a method of using and designing such a filter.

While the present invention will be presented in connection with wireless telecommunications systems, and more particularly cellular and mobile telephone communication systems, such environment is only one in which preferred

embodiments of the present invention may be employed. Other representative environments include satellite communications and military communications.

Accordingly, the wireless communications environment should not be construed in a limiting manner.

In a preferred embodiment constructed according to the principles of the present invention, a seven pole notch filter is provided which is especially useful in connection with wireless technology. The filter may be constructed in either an MgO substrate or a LAO substrate. In the case of an LAO substrate, either a traditional lumped element inductor may be provided or a capacitively loaded inductor may be used. In the case of the MgO substrate, capacitively loaded inductors are preferred, however, other types of devices may used.

In preferred embodiments of the present invention, inductive, capacitive, and resistive electrical components are formed utilizing high temperature superconducting materials. The filter is formed from a plurality of impedance inverters connected in series, the inverters each having identical inductors and shunt capacitors in a pi-network. The impedance inverters series is connected at one end to an input pad and at the opposite end to a terminal pad. A plurality of identical resonators are capacitively coupled to the series impedance inverters. Each resonator rejects one particular frequency at its resonance, and the combined multiple rejection of all resonators forms the basis of a band-reject feature in the whole unwanted band.

One feature of the present invention is that the resulting filter provides for extremely sharp skirt rejection. As noted above, in order to maximize the use of the frequency range assigned to the carrier, both the bandpass filter and the band stop filter must have very sharp skirts. That is, the transition from nearly 100% transmission of the signal to essentially no transmission of the signal must be very steep. Additionally, to avoid cross-talk, the band-reject filter must produce a "notch"in the frequency spectrum which is very deep. Filters constructed in accordance with the principles of the present invention exhibit these desirable characteristics.

Another feature of the present invention is that devices constructed in accordance with the principles of the present invention do not require crossovers.

Additionally, because of the uniformity of the inductors, for a given size substrate (e. g., a 2-inch substrate), a larger pole filter may be constructed thereby providing a sharper skirt. Still further, the principles of the present invention provide a circuit transformation methodology.

Therefore, in accordance with one aspect of the invention, there is provided a notch filter, comprising: a first inductor having a first and second end;

first capacitors electrically connected to the first and second ends of the first inductor respectively, wherein the first and second capacitors are connected to ground; resonator structures including a second capacitor and a second inductor connected in parallel, wherein for each resonator structure a third capacitor is connected in series to ground from the resonator structure and a fourth capacitor is connected in parallel to the series combination of the resonator structure and third capacitor to ground; and fifth capacitors are electrically connected in series between the first and second resonator structures to the first and second ends of the first inductor.

According to another aspect of the invention, there is provided a notch filter, comprising: a transmission line; a plurality of networked elements disposed along the transmission line, the networked elements including: an inductor; a capacitor electrically connected to each end of the inductor; a resonator device electrically connected to the inductor; wherein a notch filter is created which provides for bandreject operation.

According to another aspect of the invention, there is provided a notch filter, comprising: a plurality of impedance inverter devices, wherein each of the impedance inverter devices including a shunt capacitor and an inductor, each impedance inverter has a first and second end, and the impedance inverters are connected to one another in series; a plurality of coupling capacitors, wherein a coupling capacitor is connected to each of the first ends of the impedance inverters; and a plurality of resonator elements, wherein a resonator element is connected to each of the coupling capacitors, wherein the resonator element includes a capacitively loaded inductor and wherein each resonator element rejects one particular frequency at its resonance, whereby the combined rejection of the plurality of resonator elements establishes a band-reject area.

According to another aspect of the invention, there is provided a notch filter, comprising: a plurality of impedance inverter devices, wherein each of the impedance inverter devices including a shunt capacitor and an identical inductor, each impedance inverter has a first and second end, and the impedance inverters are connected to one another in series; a plurality of coupling capacitors, wherein a coupling capacitor is connected to each of the first ends of the impedance inverters; and a plurality of resonator elements, wherein a resonator element is connected to each of the coupling capacitors and each resonator element rejects one particular frequency at its resonance, whereby the combined rejection of the plurality of resonator elements establishes a band-reject area.

While the invention will be described with respect to several preferred embodiment filters, and with respect to particular components, it will be understood that the invention is not to be construed as limited in any manner by such

device with which the present invention is utilized. The principles of this invention apply to the field of microwave band-reject filters, and very-narrow band, thin- film, superconductive band-reject filters.

These and other variations of the invention will become apparent to those skilled in the art upon a more detailed description of the invention. Other advantages and feature which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, reference should be had to the drawings which forms a further part hereof and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

Brief Description of the Drawings Referring to the drawing, wherein like numerals represent like parts throughout the several views: Figure 1 is a band-reject ladder circuit containing series branches of parallel-resonant circuits and shunt branches of series-resonant circuits.

Figure 2 is an equivalence circuit.

Figure 3 is the resulting circuit of applying the equivalence circuit of Fig. 2 to the band-reject ladder circuit of Fig. 1.

Figure 4 is the resulting circuit of Fig. 3 after approximating jR by an inductor L and jR by a capacitor C, wherein 27cfoL = R and 2sf0CR = 1 (wherein fo is the center frequency of the band reject filter).

Figure 5 illustrates exact equivalence circuits used to modify the circuit of Fig. 4.

Figure 6 illustrates the new shunt branch of Fig. 4 after splitting the capacitor in the first shut branch and replacing it with the equivalence illustrated in Fig. 5.

Figure 7 is a spurious frequency filter which results for frequencies relatively far away from the reject band.

Figure 8 are further equivalence circuits used to modify the circuit of Fig. 6.

Figure 9 illustrates a lumped element LC ladder network for a 7-pole elliptic function bandstop filter.

Figure 10 illustrates the resulting circuit after modifying the circuit illustrated in Fig. 9, wherein Kij is a microwave ideal device known as an impedance inverter.

Figure 11 illustrates the circuit in Fig. 10 after removing two cascading identical inverters.

Figure 12 illustrates the circuit in Fig. 11 after converting the series resonator to parallel resonators.

Figures 13a-13e illustrate the realization of impedance inverters and admittance inverters in the circuit illustrated in Fig. 12.

Figure 14 illustrates the resulting circuit after the circuit of Fig. 12 is modified by the equivalence structures shown in Figs. 13a-13 e.

Figure 15 illustrates a notch filter circuit constructed in accordance with the principles of the present invention.

Figure 16 illustrates a layout of a notch filter on an MgO substrate using the circuit of Figure 15, a frequency transformed inductor and constructed in accordance with the principles of the present invention.

Figure 17 illustrates a layout of a notch filter on an LAO substrate using the circuit of Figure 15, lumped element inductors and constructed in accordance with the principles of the present invention.

Figure 18 illustrates a layout of a notch filter on an LAO substrate using the circuit of Figure 15, a frequency transformed inductor, and constructed in accordance with the principles of the present invention.

Figure 19 illustrates an exemplary measured response of an HTS notch filter on an MgO substrate.

Detailed Description As noted above, the principles of this invention apply to the design and implementation of a seven pole notch filter which is especially useful in connection with wireless technology. The filter may be constructed in either an MgO substrate or a LAO substrate. A description of the preferred layouts will be deferred pending a discussion of the theory of operation and design methodology.

A. Theory of Operation A very narrow band band-elimination filter, no matter how it is designed (Butterworth, Chebyshev, elliptic or in other manners), includes a ladder circuit containing series branches of parallel-resonant circuits and shunt branches of series-resonant circuits. The circuit in Fig. 1 illustrates this type of circuit, where: Ls1#...#BW(1)Ls2 (e. g, the inductors Lsn are proportional to the reject bandwidth), while Lp, Lp---1/BW (2)

(e. g., the inductors Lpn are proportional to the inverse bandwidth). Accordingly, the following relationship may be observed: Ls/Lp (BW) In equation (3), if the bandwidth BW is 1% or less, then the ratio of the series inductors to those of the shunt inductors will exceed 104 which is unacceptable.

If the frequency band of interest above the reject band is limited, as it usually is, then an approximate method may be used to eliminate this problem. The method is based on the (exact) equivalence shown in Fig. 2. In Fig. 2, R is an arbitrary resistance value. Therefore, jR and jR represent constant (frequency- independent) reactances (positive and negative, respectively). Applying this equivalence to the series branches of the band-reject filter results in the specific case illustrated in Fig. 3, where: La = R2Cs ; and (4) C,=Ls/R'(5) It will be appreciated to those of skill in the art that the value of R is usually selected to be equal to the terminating impedance as a matter of convenience.

However, selection of such a value for R is not necessary (if R is selected in that manner, the selection may save a few components if the filter degree is divisible by 4). Also, computing the series resonant circuit elements may be achieved by simply using the"DUAL"command of the S/FILSYN program manufactured by DGS Associates of Menlo Park, CA.

Naturally, Z may contain two, series connected, parallel resonant circuits as well, in which case R2/Z will contain two series resonant circuits in parallel. Since a frequency-independent reactance does not exist, we approximate jR by an inductor L and jR by a capacitor C such that: 27rfoL=R ; and (6) 27tfoCR=l (7) where fo is the center frequency of the band reject filter. The resulting structure will now take the general form shown in Fig. 4. The pairs of parallel capacitors C may

be combined into a single capacitor. Fig. 4 illustrates the approximate new band- reject structure in which all series Ls are now equal and of magnitude: L &num 1 while all resonant circuit inductances are: L. 1/(BW)(8) Consequently, the inductance ratio is reduced to about 1/ (BW). For later reference, the same should be observed of the capacitance ratios.

Further modification of this structure may be made to improve this ratio to a greater extent. More specifically, the following exact equivalence for this purpose is illustrated in Fig. 5; where: La = (c/ (C+Co))' (9) Ca = (C+Co)/C (10) Cb =C+Co (11) By way of example, the capacitor C in the first shunt branch of FIG. 4 may be split into two, with one of these being used together with the equivalence of FIG. 5 to obtain a new shunt branch as shown in FIG. 6. Here, the value of the inductor in this circuit is selectable between very wide limits (i. e., due to the capacitance ratio being proportional to 1/ (BW) as noted above. For instance, this operation can be used to make all inductors in the circuit, equal to a single common value. In fact, the S/FILSYN program has a command to perform this operation directly.

Since the frequency-independent reactances have been replaced by inductors and capacitors, the effect of this replacement must be determined in order to understand the modified overall filter performance. Clearly, if the reject band is very narrow, the overall performance in the reject band will be very close to the original. Only relatively far away from the reject band can any effects of this approximation be observed. On the other hand, far away from the reject band, the resulting circuit is simply as set forth in Fig. 7. It will be appreciated that this circuit is a simple"image-parameter"lowpass filter with characteristic impedance at zero frequency:

#(L/2C)=R/#212)RI= and a cut-off frequency: fi = 2/ (LC) = 2fo (13) which is about 40% above the reject band. Furthermore, the characteristic impedance at the band-reject frequency fo will be exactly equal to R, hence the filter is"matched"and will contribute nothing to the loss characteristics of the original band reject filter near the reject band. Elsewhere, there will be some deviation from the flat passband performance of the band-reject, but detailed numerical analysis shows that the deviation is at most about. 5 dB from zero frequency up to about 35% beyond fo.

Finally, for certain implementations, a structure is needed where both ends of all inductors must be grounded not directly, but through a capacitor. In such a case, Fig. 8 shows a further equivalence which may be useful. Since the right side of this equivalence is a reducible circuit, one of the values may be selected, say Ca, arbitrarily. After such selection, then the rest of the elements are computed as follows: Cb[(C1/2)2+CCa]-Ca/2(14)# <BR> <BR> <BR> <BR> <BR> <BR> C,=Co(C,+Cb)/(C,+Cb-Co);and(15)<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> L, =LC (C, +Cb)/ (C, Cb) (16) Substituting Cb in the last equation, it can be rewritten as: L a = L [V (C/C, +. 25) +. 5] 2 (17) which shows that La will always be greater than L. Hence, L must be selected to be lower in value than the desired final inductor value.

B. Design Methodolog The circuit in Figure 9 is a lumped-element LC ladder network for a 7-pole elliptic function bandstop filter. It contains both series and parallel resonators. Similar to the cases of microwave bandpass filters, an equivalent

network containing only one type of resonator is usually required for practical realization by microwave structures.

A resonator is uniquely defined by two parameters. For microwave resonators, resonance frequency and slope parameter are usually selected. A reactance slope parameter is used for series resonators since it resonates at the frequency that impedance vanishes. The same is true for the susceptance slope parameter for a parallel resonator.

The circuit of Fig. 9 can be converted to the circuit illustrated in Fig.

10 according to the equations set forth in the following Table 1.

TABLE 1 <BR> <BR> <BR> <BR> <BR> <BR> 2 2<BR> 3=3*343=3*<BR> <BR> <BR> <BR> <BR> <BR> <BR> 2 2<BR> -'33X-]-.].X'33 x2b2x2b2 = =<BR> K12=K23=K34x3b3x3b3 <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> x5 = b5*K452 = K45 = K56 = k67 x5 = b5*K2<BR> x5b5'x5b5' <BR> == x6b6x6b6 <BR> <BR> <BR> <BR> <BR> #<BR> x6=b6*K672x6=b2*K2 It will be appreciated that Kjj is a microwave ideal device designated as an impedance inverter. In making the foregoing conversion, redundant elements are created. For example, impedance inverters which do not contribute to the selectivity of the filter, but make realization of the filter by only one type of resonator possible.

In view of the foregoing, the circuit in Fig. 10 is transformed to the circuit in Fig. 11 by eliminating the two cascading identical inverters. Further, a shunt series resonator directly connected to the signal path cannot be precisely realized by microstrip lumped elements. To solve this issue, admittance inverters are introduced, as in the circuit illustrated in Fig 11, to convert the series resonator to parallel-resonators. Again, those admittance inverters are redundant elements but

are required to convert the equivalent network to a form that can be realized in microwave structures.

The networks illustrated in Figs. 13a through 13 e are used to realize impedance inverters, admittance inverters and parallel resonators found in the circuit of Fig. 12.

Next, the inverters and parallel resonators are replaced by the circuits described above resulting in the circuit illustrated in Fig. 14. Finally, by combining the shunt capacitors that connected in common nodes, the circuit of Fig. 14 can be simplified to the circuit of Fig. 15.

Referring to Fig. 15, the circuit is shown generally at 100. The impedance inverters are shown generally at 101 and are comprised of shunt capacitors 102,103 and inductor 104. The coupling capacitor is illustrated at 105 and the resonator device illustrated at 106. Resonator device 106 is comprised of capacitors 107,108 and 109. Also included in the resonator element 106 is inductor 109. The capacitively loaded inductor is comprised of inductor 109 and capacitor 108.

Impedance inverter 101 shares shunt capacitor 104 with the impedance inverter 101'shown in phantom. In turn, each of the impedance inverters along the transmission line share the shunt capacitor located therebetween.

C. Lavout and Fabrication Figs. 16,17, and 18 illustrate layouts of the circuit of Fig. 15 on superconductive substrates. More specifically, Fig. 16 illustrates a layout of a notch filter on an MgO substrate using frequency transformed inductors; Fig. 17 illustrates a layout of a notch filter on an LAO substrate using lumped element inductors; and Fig. 18 illustrates a layout of a notch filter on an LAO substrate using frequency transformed inductors.

Fig. 19 illustrates the exemplary results from an experimentally measured 7 pole notch filter constructed in accordance with Fig. 16.

It will be appreciated that the capacitive elements are the interdigitized devices illustrated in Figs. 16-18, while the inductive elements are constructed using a half loop and serpentine devices.

The filter devices of the invention are preferably constructed of materials capable of yielding a high circuit Q filter, preferably a circuit Q of at least 10,000 and more preferably a circuit Q of at least 40,000. Superconducting materials are suitable for high Q circuits. Superconductors include certain metals and metal alloys, such a niobium as well as certain perovskite oxides, such as YBa2Cu307-s (YBCO). Methods of deposition of superconductors on substrates and of

fabricating devices are well known in the art, and are similar to the methods used in the semiconductor industry.

In the case of high temperature oxide superconductors of the perovskite-type, deposition may be by any known method, including sputtering, laser ablation, chemical deposition or co-evaporation. The substrate is preferably a single crystal material that is lattice-matched to the superconductor. Intermediate buffer layers between the oxide superconductor and the substrate may be used to improve the quality of the film. Such buffer layers are known in the art, and are described, for example, in U. S. Patent No. 5,132,282 issued to Newman et al., which is hereby incorporated herein by reference. Suitable dielectric substrates for oxide superconductors include sapphire (single crystal A1203) and lanthanum aluminate (LaAl03).

As an example circuit, all inductors are identical within the filter with 100 micron line width. All interdigital capacitor fingers are 50 micro wide.

Equivalent inductance of this capacitively-loaded circuit is about 12 nanoHenries at 1.6 GHz. The whole filter structure may be fabricated on a MgO substrate with a dielectric constant of about 10. The substrate is 0.5 millimeter thick. Other substrates also used in this type of filters could be lanthanum aluminate and sapphire.

The YBCO is typically deposited on the substrate using reactive co- evaporation, but sputtering and laser albation could also be used. A buffer layer may be used between the substrate and the YBCO layer, especially if sapphire is the substrate. Photolithography is used to pattern the filter structure.

While a particular embodiment of the invention has been described with respect to its application for wireless communications, it will be understood by those skilled in the art that the invention is not limited by such application or particular circuits utilized in such devices. Other configurations that embody the principles of this invention and other applications therefor other than as described herein can be configured within the spirit and intent of this invention. Other modifications and alterations are well within the knowledge of those skilled in the art and are to be included within the broad scope of the appended claims.