WO2005036692A1

WO2005036692A1 - Frequency and bandwidth tunable band-pass filter

Info

Publication number: WO2005036692A1
Application number: PCT/US2004/033005
Authority: WO
Inventors: Robby L. Alvarez; C. Warren Haas; James D. Mccambridge; Charles Wilker
Original assignee: E.I. Dupont De Nemours And Company
Priority date: 2003-10-06
Filing date: 2004-10-06
Publication date: 2005-04-21

Abstract

Cascaded filters, at least one of which is tunable, provide a frequency and bandwidth tunable band-pass filter. Preferably, the filter is comprised of two tunable high temperature superconductor filters.

Description

TITLE FREQUENCY AND BANDWIDTH TUNABLE BAND-PASS FILTER

This application claims the benefit of U.S. Provisional Application No. 60/509,332, filed on October 6, 2003, which is incorporated in its entirety as a part hereof for all purposes.

Field of the Invention This invention relates to a frequency and bandwidth tunable band-pass filter comprised of cascaded filters, at least one of which is tunable, and particularly to such a filter comprised of two tunable high temperature superconductor filters.

Background of the Invention Band-pass filters have a multitude of uses in electrical circuits. In particular, high temperature superconductor (HTS) filters have applications in telecommunication, instrumentation and military equipment . The HTS band-pass filters have the advantages of extremely low in-band insertion loss, high off-band rejection and steep skirts. The HTS band- reject filters have the advantages of extremely high in-band rejection, low off-band insertion loss and steep skirts. The advantages of both types of filters are due to the extremely low loss in the HTS materials.

In some instances there is a need to have a bandpass filter that is both frequency and bandwidth tunable. This provides a filter that can be used in a variety of applications. Another advantage is that the yields of a manufacturing process can be significantly improved if the filter can be subsequently tuned. Shen, U.S. 6,522,247, discloses a method for tuning the center frequency of an HTS band-pass or band-reject filter by adjusting the distance between a moveable plate and the HTS filter wherein the moveable plate is covered with an HTS film on at least the portion of the plate opposite the HTS filter element. Such tuning may change the inter-resonator coupling and thereby cause the filter's bandwidth and the shape of the frequency response to change. One or more inter- resonator coupling circuits can be used to compensate for these potential side effects.

An object of the present invention is to provide a filter that, is readily both frequency and bandwidth tunable .

Summary of the Invention This invention provides a frequency and bandwidth tunable band-pass filter comprising two or more cascaded filters, wherein at least one of the cascaded filters is frequency tunable. Preferably two of the cascaded filters are independently frequency tunable. Preferably, any cascaded filter that is frequency tunable is a frequency tunable high temperature superconductor filter.

In one embodiment, this invention provides a frequency and bandwidth tunable band-pass filter comprising a band-pass filter cascaded with a band- reject filter, wherein at least one of the cascaded filters is frequency tunable. Preferably both cascaded filters are independently frequency tunable. Preferably, the tunable band-pass filter comprises a frequency tunable high temperature superconductor bandpass filter cascaded with a frequency tunable high temperature superconductor band-reject filter. In another embodiment, this invention provides a frequency and bandwidth tunable band-pass filter comprising two cascaded band-pass filters, wherein at least one of the cascaded filters is frequency tunable. Preferably both cascaded filters are independently frequency tunable. Preferably, the tunable band-pass filter comprises two cascaded frequency tunable high temperature superconductor band-pass filters.

In yet another embodiment, any frequency tunable high temperature superconductor filter in the above embodiments is comprised of high temperature superconductor spiral resonators.

Another embodiment of this invention is a frequency tunable high temperature superconductor filter, comprised of high temperature superconductor spiral resonators, that includes: (a) an HTS filter circuit comprising a substrate having a front side and a back side, an HTS filter element comprised of at least two HTS spiral resonators on the front side of the substrate, at least one inter- resonator coupling mechanism, an input coupling circuit comprising a transmission line with a first end thereof connected to an input connector of the filter circuit and a second end thereof coupled to one of the at least two spiral resonators, an output coupling circuit comprising a transmission line with a first end thereof connected to an output connector of the filter circuit and a second end thereof coupled to a second of the at least two spiral resonators; (b) a plate having a front surface and a back surface, the front surface spaced a distance apart from and opposite to the HTS filter circuit on the front side of the substrate, wherein the front surface of the plate is covered with a HTS film on at least the portion of the front surface opposite the at least two resonators; and (c) means to adjust the distance between the HTS film on the front surface of the plate and the at least two HTS spiral resonators on the front side of the substrate .

In this embodiment, there are preferably uniform HTS films disposed on the back side of the substrate and the back surface of the plate to serve as ground planes, and there are conductive films disposed on these uniform HTS films to serve as contacts to the case containing the filter. Preferably, these conductive films are gold films.

Brief Description of the Drawings Figure 1 shows the S_2i of a frequency and bandwidth tunable band-pass filter comprised of a band- pass filter cascaded with a band-reject filter, wherein at least one of the cascaded filters is independently frequency tunable.

Figure 2 shows the S₂ι of a frequency and bandwidth tunable band-pass filter comprised of two cascaded band-pass filters, wherein at least one of the cascaded filters is independently frequency tunable.

Figure 3 shows the S₂₁ of a frequency and bandwidth tunable band-pass filter comprised of a frequency tunable wide-band low-pass filter and a fixed wide-band high-pass filter.

Figure 4 shows the S₂ι of a frequency and bandwidth tunable band-pass filter comprised of a wideband low-pass filter, a band-reject filter and a fixed wide-band high-pass filter, wherein at least one of the wide-band low-pass filter and the band-reject filter is independently frequency tunable.

Figure 5 shows a frequency tunable HTS band-pass filter.

Figure 6 shows a frequency tunable HTS band-reject filter.

Detailed Description of the Preferred Embodiments The present invention provides a frequency and bandwidth tunable band-pass filter comprised of cascaded filters, at least one of which is tunable. Various combinations of filters can be cascaded to result in the desired frequency and bandwidth tunable band-pass filter. Cascaded filters are filters that are connected in series, i.e. the output of one filter is the input to the next filter, where the series may contain any useful number of filters connected in that manner .

S_2ι is used herein to characterize and describe the operation of the frequency and bandwidth tunable band- pass filter. S₂ι is the magnitude of the scattering coefficient from the input port of the filter to the output port and therefore is a measure of the transmission through the filter. It is an important parameter for practical applications of a filter. S₂ι should be nearly 1, i.e., about 0 dB in the band-pass region. The magnitude of S_2X outside the band-pass region should be as low as possible.

In one embodiment, a band-pass filter can be cascaded with a band-reject or notch filter, wherein at least one of the cascaded filters is frequency tunable. TheS₂ι's for the band-pass filter, S¹ ₂ι, and the band- reject filter, S² ₂i, are shown in Figure 1. Either the band-pass or the band-reject filter or both are independently frequency tunable, and the arrows shown with each S₂ι indicate the possible movement of that S₂ι as that filter is tuned. Also shown in Figure 1 is the resultant band-pass of the cascaded filters, S^c ₂ι, i.e. the band-pass of the frequency and bandwidth tunable band-pass filter. As one or both of the cascaded filters are tuned, the S^c ₂ι changes accordingly. The results shown in Figure 1 are for the situation in which the reject band of the band-reject filter occurs at lower frequency than the pass band of the band-pass filter. This combination of cascaded filters also results in a frequency and bandwidth tunable band-pass filter when the reject band of the band-reject filter occurs at higher frequency than the pass band of the band-pass filter.

In another embodiment, two band-pass filters can be cascaded, wherein at least one of the cascaded filters is frequency tunable. TheS₂ι's for the first band-pass filter, S¹ ₂ι, and the second band-pass filter, S² ₂i, are shown in Figure 2. Either one of the bandpass filters or both are independently frequency tunable, and the arrows shown with each S_2i indicate the possible movement of that S₂ι as that filter is tuned. Also shown in Figure 2 is the resultant band-pass of the cascaded filters, S^c ₂ι, i.e. the band-pass of the frequency and bandwidth tunable band-pass filter. As one or both of the cascaded filters are tuned, the S^c ₂ι changes accordingly.

In still another embodiment of the invention, a wide-band low-pass filter can be cascaded with a wideband high-pass filter, wherein at least one of the filters is frequency tunable. For example, the wideband low-pass filter can be frequency tunable, and the cascaded wide-band high-pass filter can be a fixed wide-band high-pass filter. The fixed wide-band high- pass filter can be a RF choke, i.e., a fixed inductor. TheS₂ι's for a frequency tunable wide-band low-pass filter, S¹ ₂i, and a fixed wide-band high pass filter, S² ₂i, are shown in Figure 3. The arrows shown with S¹ ₂ι indicate the possible movement of S^x ₂ι as that filter is tuned. Also shown in Figure 3 is the resultant bandpass of the cascaded filters, S^c ₂ι, i.e. the band-pass of the frequency and bandwidth tunable band-pass filter. As the low-pass filter is tuned, the S^c ₂ι changes accordingly. In a related embodiment, the tunable wide-band low-pass filter can be cascaded with a tunable band-pass filter.

In yet another embodiment, a wide-band low-pass filter can be cascaded with a fixed wide-band high-pass filter and a band-reject filter, wherein at least one of the low-pass filter or the band-reject filter is frequency tunable. The fixed wide-band high-pass filter can be a RF choke, i.e., a fixed inductor. TheS₂ι's for a wide-band low-pass filter, S¹ ₂ι, a fixed wide-band high pass filter, S² ₂ι, and the band-reject filter, S³ ₂i, are shown in Figure 4. Either the low- pass filter or the band-reject filter or both are independently frequency tunable, and the arrows shown with each S₂ι indicate the possible movement of that S₂ι as that filter is tuned. Also shown in Figure 4 is the resultant band-pass of the cascaded filters, S^c ₂ι, i.e. the band-pass of the frequency and bandwidth tunable band-pass filter. As one or both of the tunable cascaded filters are tuned, the S^c ₂ι changes accordingly.

Various other embodiments comprising three or more cascaded filters of the types discussed above can also provide a frequency and bandwidth tunable band-pass filter. In all the embodiments of the invention it is preferred that two of the cascaded filters are independently frequency tunable. When only one of the cascaded filters is frequency tunable, the resultant band-pass can only be broadened or narrowed along with a change in the center frequency of the band-pass . One edge of the band-pass is fixed by the cascaded filter that is not tunable. When two of the cascaded filters are independently frequency tunable, there is considerably more flexibility in adjusting the resultant band-pass. The band-pass can be broadened or narrowed leaving the center frequency unchanged or the center frequency can be changed leaving the width of the band-pass unchanged. Alternatively, the center frequency and the width of the band-pass can both be changed .

Preferably, the tunable filters used in the above embodiments of the instant invention are HTS filters and most preferably these tunable HTS filters are comprised of spiral resonators. A preferred way of achieving a tunable HTS filter has been provided by Shen, U.S. 6,522,247. This is accomplished by providing a moveable plate for tuning the center frequency of an HTS band-pass or band-reject filter by adjusting the distance between the moveable plate and the HTS filter. The moveable plate is covered with an HTS film on at least the portion of the plate opposite the HTS filter element. No foreign materials or bias circuit are introduced into the HTS filter circuit. A preferred embodiment is to provide the HTS filter with a tuning structure, comprising the aforementioned plate spaced a distance apart from the HTS filter circuit, and connected to an actuator which can change the position of the plate relative to the HTS filter circuit. This embodiment enables tuning of the center frequency of the HTS mini-filters without performance deterioration. The tunable HTS filter is enclosed in a vacuum dewar assembly having a cyrogenic source, such as a cryocooler unit, connected to it.

It is desirable to reduce the size of the components needing cooling and this can be accomplished by using HTS mini-filter configurations and, in particular, spiral resonators.

The HTS spiral resonators can have a wide variety of shapes including a rectangular-shaped single spiral resonator with rounded corners, a circular-shaped single spiral resonator, a rectangular-shaped double spiral resonator, a circular-shaped double spiral resonator, a mirror symmetrical rectangular-shaped double spiral resonator with rounded corners, a 180° rotational rectangular-shaped double spiral resonator with rounded corners, a double mirror symmetrical rectangular-shaped spiral resonator with rounded corners, a 180° rotational symmetrical rectangular- shaped spiral resonator with rounded corners, and a 90° rotational symmetrical square-shaped quadruple spiral resonator with rounded corners .

Opposite the front side, i.e., the side containing the spiral resonators, of the HTS filter circuit is the plate, which interacts with the magnetic field of the spiral resonators in the HTS filter circuit, changing the resonant frequency thereof as the relative distance between the plate and the HTS filter circuit changes. The movement of plate relative to the HTS filter circuit thus "tunes" the center frequency of the HTS filter.

The inter-resonator coupling of the HTS filter circuit may simply be a gap between adjacent resonators in which the electromagnetic fields of the two resonators overlap. During the tuning process, however, this type of inter-resonator coupling may change, which in turn can cause the filter's bandwidth and the shape of the frequency response to change. These side effects may deteriorate the filter' s performance. The HTS filter element preferably includes one or more compensating inter-resonator coupling circuits to compensate for these potential side effects.

A preferred coupling circuit comprises an HTS transmission line at least in part disposed between an adjacent pair of HTS resonators such that the transmission line couples such adjacent pair. The coupling can occur, for example, by directly attaching the HTS transmission line to a resonator, inserting the HTS transmission line into a slot between two split branch lines at the end of a resonator, placing the HTS transmission line close by and parallel to the edge of a resonator, or any combination of the above. The moveable plate utilized in the tunable HTS filters comprises a substrate having a front surface and a back surface, the front surface facing the HTS filter circuit and the back surface facing the second inner surface of the enclosure. At least a portion of the front surface of the plate is covered with an HTS film, which portion is at a minimum the area on the front surface corresponding to the position of the spiral resonators on the front side of the HTS filter circuit. For ease of construction, the HTS film may, however, cover the entire front surface or any other portions thereof, for example, an area slightly larger than that corresponding to the spiral resonators on the front side of the HTS filter circuit, or the entire front surface except for the two end locations facing the input and output circuit areas of the HTS filter circuit. The back surface is preferably covered with a blank HTS film over which a blank conductive film has been deposited, particularly if a piezoeletric actuator is attached to this back surface.

Preferably, the superconducting materials of the HTS filters have a transition temperature, T_c, greater than about 77K. In addition, the substrates for the

HTS filter circuit and plate should have a dielectric material lattice matched to the HTS film deposited thereon, with a loss tangent less than about 0.0001.

In all of the embodiments described above, it is preferred that the high temperature superconductor is selected from the group consisting of YBa2Cu3θ ,

Tl Ba2CaCu2θ₈, TlBa2Ca₂Cu3θ₉, (TlPb) Sr₂CaCu₂0₇ and (TlPb)Sr₂Ca₂Cu₃09.

The means to adjust the distance between the HTS film on the front surface of the plate and the spiral resonators on the front side of the HTS filter circuit may, for example, be a non-conductive actuator attached to the plate and to the HTS filter circuit. The actuator can take any number of forms. A simple form is a screw mechanism attached to the back surface of the plate through the enclosure, which mechamism can be rotated manually and/or by mechanical (e.g., with a lever) and/or electromechanical devices (e.g., a motor) .

In another embodiment the actuator is constructed from a piezoelectric material, which allows the relative distance between the plate and HTS filter circuit to be controlled and adjusted by applying voltage to the actuator (or actuators) . The actuator of the HTS filter is one or more (depending on configuration discussed below) piezoelectric blocks made of a piezoelectric material operating at temperature below 80K and having a sensitivity better than 5xl0^"5 /volts/cm. Preferred piezoeletric materials meeting these conditions include, for example, PZT [lead zirconate titanate (PbZr)Ti0₃] and barium titanate (BaTi0₃) . The actuator can be attached to the plate in a number of different configurations. For example, one end of a piezoelectric block (with a metallic surface) can be attached to the back surface of the plate, with the other end attached to the surface of the metallic enclosure containing the HTS filter circuit and plate. As another example, one end of four substantially identical piezoelectric blocks (each with a metallic surface) can be attached to each corner of the front surface of the plate, with the other end of each non- conductively attached to the surface of the metallic enclosure or each corresponding corner of the HTS filter circuit .

To control the piezoelectric actuators, a metallic wire can be electrically connected to the metallic surface on a piezoelectric block (for example, either directly or via the conductive layer on the back surface of the plate) with the opposite end of the metallic wire being connected to at least one tuning connector. The tuning connector can in turn be connected to a control device to apply a pre-determined control voltage.

The various independently frequency tunable filters used in this invention can best be understood in reference to the Figures 5 and 6. Fig. 5 shows a frequency tunable HTS band-pass filter. In Fig. 5a shows an HTS filter circuit 1, and a HTS top plate 2. Fig. 5b shows a substrate la of the filter circuit 1. An HTS circuit pattern lb is deposited on front side of substrate la. A blank HTS film lc is deposited on the back side of substrate la serving as the ground plane of the filter. A metallic film Id made of gold or silver is deposited on the surface of blank film lc. The HTS circuit pattern lb comprises four HTS spiral resonators 9a, 9b, 9c and 9d, input transmission line 10a, output transmission line 10b, inter-resonator coupling transmission lines 11, 11a and lib to form a 4 -pole band-pass filter, as shown in Fig. 5c. The HTS filter circuit 1 is attached to the bottom of metallic case 5. Input connector 3a, output connector 3b and tuning connector 7 are inserted into the side wall of metallic case 5. As shown in Fig. 5c, the input connector 3a and output connector 3b are connected to the input and output transmission lines 10a and 10b, respectively. As shown in Fig. 5b, the HTS top plate 2 comprises a substrate 2a with HTS films 2b and 2c deposited on the front side and back side of substrate 2a, respectively. A metallic film 2d made of gold or silver is deposited on top of film 2c. As shown in Fig. 5a, an actuator 4 made of piezoelectric material with one side attached to the back side metallic film 2d of the HTS top plate and the opposite side attached to the metallic lid 6 on top of the case 5 is used to move the HTS top plate for tuning the center frequency of the HTS filter. A wire 8 with one end connected to a tuning connector 7 and the other end connected to the actuator 4 via the metallic film 2d is used to apply tuning voltage to the actuator 4.

Fig. 6 shows a frequency tunable HTS band-reject filter. Fig. 6a shows an HTS filter circuit 21, and an HTS top plate 22. Fig. 6b shows the substrate 21a of the HTS filter circuit 21. An HTS circuit pattern 21b is deposited on front side of substrate 21a. A blank HTS film 21c is deposited on the back side of substrate 21a serving as the ground plane of the filter. A metallic film 21d made of gold or silver is deposited on the surface of film 21c. In Fig. 6c, the HTS circuit pattern 21b comprises four HTS spiral resonators 29a, 29b, 29c and 29d, an HTS main transmission line 30, and inter-resonator coupling transmission lines 31, 31a and 31b to form a 4-pole HTS band-reject filter. The main transmission line 30 is in the zigzag form at the locations between the resonators. The purpose of such zigzag is for adjusting the phase to obtain maximum in-band rejection. The filter circuit 21 is attached to the bottom of metallic case 25. The input connector 23a, output connector 23b, and tuning connector 27 are inserted into the side wall of case 25. The input connector 23a and output connector 23b are connected to two ends of main transmission lines 30 to provide off- band signal pass through. As shown in Fig. 6b, the HTS top plate comprises a substrate 22a with HTS films 22b and 22c deposited on the front side and back side of substrate 22a, respectively. A metallic film 22d made of gold or silver is deposited on top of film 22c. As shown in Fig. 6a, an actuator 24 made of piezoelectric material with one side attach to the back side metallic film 22d of the HTS top plate and the opposite side attached to the metallic lid 26 is used to move the HTS top plate for tuning the center frequency of the HTS filter. A wire 28 with one end connected to a tuning connector 27, and the other end connected to the actuator 24 via the metallic film 22d, is used to apply tuning voltage to the actuator 24.

When the frequency and bandwidth tunable band-pass filter is comprised of two independently frequency tunable cascaded HTS filters, these two HTS filters may be on two separate substrates or on the same substrate. In both instances, two separate and independently adjustable plates must be provided, one for each HTS filter, in order that the two filters are independently tunable. Preferably, both filters are enclosed within the same metallic case.

Claims

CLAIMSWhat is claimed is:

1. A frequency and bandwidth tunable band-pass filter comprising two or more cascaded filters, wherein at least one of the cascaded filters is frequency tunable.

2. The frequency and bandwidth tunable band-pass filter of claim 1, wherein two of the cascaded filters are independently frequency tunable.

3. The frequency and bandwidth tunable band-pass filter of claim 1 or 2 , wherein a cascaded filter that is frequency tunable is a frequency tunable high temperature superconductor filter.

4. The frequency and bandwidth tunable band-pass filter of claim 3, wherein a frequency tunable high temperature superconductor filter is comprised of high temperature superconductor spiral resonators.

5. The frequency and bandwidth tunable band-pass filter of claim 1, wherein a band-pass filter is cascaded with a band-reject filter.

6. The frequency and bandwidth tunable band-pass filter of claim 5, wherein both cascaded filters are independently frequency tunable.

7. The frequency and bandwidth tunable band-pass filter of claim 5 or 6 , wherein a cascaded filter that is frequency tunable is a frequency tunable high temperature superconductor filter.

8. The frequency and bandwidth tunable band-pass filter of claim 7, wherein a frequency tunable high temperature superconductor filter is comprised of high temperature superconductor spiral resonators .

9. The frequency and bandwidth tunable band-pass filter of claim 1, wherein two band-pass filters are cascaded.

10. The frequency and bandwidth tunable band-pass filter of claim 9, wherein both cascaded filters are independently frequency tunable.

11. The frequency and bandwidth tunable band-pass filter of claim 9 or 10, wherein a cascaded filter that is frequency tunable is a frequency tunable high temperature superconductor filter.

12. The frequency and bandwidth tunable band-pass filter of claim 11, wherein a frequency tunable high temperature superconductor filter is comprised of high temperature superconductor spiral resonators.

13. The frequency and bandwidth tunable band-pass filter of claim 1, wherein a wide-band low-pass filter is cascaded with a wide-band high-pass filter.

14. The frequency and bandwidth tunable band-pass filter of claim 13, wherein the wide-band low-pass filter is frequency tunable and the wide-band high-pass filter is a fixed inductor.

15. The frequency and bandwidth tunable band-pass filter of claim 1, wherein a wide-band low-pass filter is cascaded with a band-pass filter.

16. The frequency and bandwidth tunable band-pass filter of claim 15, wherein both cascaded filters are independently frequency tunable.

17. The frequency and bandwidth tunable band-pass filter of claim 1, wherein a wide-band low-pass filter is cascaded with a fixed wide-band high-pass filter and a band-reject filter.

18. The frequency and bandwidth tunable band-pass filter of claim 17, wherein the fixed wide-band high- pass filter is an inductor and the wide-band low-pass filter and the band-reject filter are both independently frequency tunable.

19. The frequency and bandwidth tunable band-pass filter of claim 17 or 18, wherein a cascaded filter that is frequency tunable is a frequency tunable high temperature superconductor filter.

20. The frequency and bandwidth tunable band-pass filter of claim 19, wherein a frequency tunable high temperature superconductor filter is comprised of high temperature superconductor spiral resonators.

21. The frequency and bandwidth tunable band-pass filter of claim 4, wherein a frequency tunable high temperature superconductor filter comprised of high temperature superconductor spiral resonator comprises: (a) an HTS filter circuit comprising a substrate having a front side and a back side, an HTS filter element comprised of at least HTS two spiral resonators on the front side of the substrate, at least one inter- resonator coupling mechanism, an input coupling circuit comprising a transmission line with a first end thereof connected to an input connector of the filter circuit and a second end thereof coupled to one of the at least two the spiral resonators, an output coupling circuit comprising a transmission line with a first end thereof connected to an output connector of the filter circuit and a second end thereof coupled to a second of the at least two the spiral resonators; (b) a plate having a front surface and a back surface, the front surface spaced a distance apart from and opposite to the HTS filter circuit on the front side of the substrate, wherein the front surface of the plate is covered with a HTS film on at least the portion of the front surface opposite the at least two the spiral resonators; and (c) means to adjust the distance between the HTS film on the front surface of the plate and the at least two HTS the spiral resonators on the front side of the substrate.

22. The frequency and bandwidth tunable band-pass filter of claim 21, wherein the frequency tunable high temperature superconductor filter comprised of high temperature superconductor spiral resonator further comprises uniform HTS films disposed on the back side of the substrate and the back surface of the plate to serve as ground planes and conductive films disposed on the uniform HTS films to serve as contacts to a case containing the frequency and bandwidth tunable bandpass filter.