Wave transmission lines and networks – Miscellaneous – Multipactor applications
Reexamination Certificate
2002-05-29
2004-03-02
Tokar, Michael (Department: 2817)
Wave transmission lines and networks
Miscellaneous
Multipactor applications
C333S202000, C333S219000, C505S210000
Reexamination Certificate
active
06700459
ABSTRACT:
FIELD OF THE INVENTION
The present inventions generally relate to microwave filters, and more particularly, to microwave filters designed for narrow-band applications.
BACKGROUND OF THE INVENTION
Filters have long been used in the processing of electrical signals. For example, in communications applications, such as microwave applications, it is desirable to filter out the smallest possible passband and thereby enable dividing a fixed frequency spectrum into the largest possible number of bands.
Such filters are of particular importance in the telecommunications field (microwave band). As more users desire to use the microwave band, the use of narrow-band filters will increase the actual number of users able to fit in a fixed spectrum. Of most particular importance is the frequency range from approximately 800-2,200 MHz. In the United States, the 800-900 MHz range is used for analog cellular communications. Personal communication services are used for the 1,800 to 2,200 MHz range.
Historically, filters have been fabricated using normal, that is, non-superconducting materials. These materials have inherent lossiness, and as a result, the circuits formed from them having varying degrees of loss. For resonant circuits, the loss is particularly critical. The quality factor (Q) of a device is a measure of its power dissipation or lossiness. Resonant circuits fabricated from normal metals in a microstrip or stripline configuration have Q's at best on the order of four hundred. See, e.g., F. J. Winters, et al., “High Dielectric Constant Strip Line Band Pass Filters,” IEEE Transactions On Microwave. Theory and Techniques, Vol. 39, No. 12, December 1991, pp. 2182-87.
With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from high temperature superconductor (HTSC) materials. The microwave properties of HTSC's have improved substantially since their discovery. Epitaxial superconductive thin films are now routinely formed and commercially available. See, e.g., R. Hammond et al., “Epitaxial Tl
2
Ba
2
Ca
1
Cu
2
O
8
Thin Films With Low 9.6 GHz Surface Resistance at High Power and Above 77° K,” Applied Physics Letters, Vol. 57, pp. 825-27 (1990). Various filter structures and resonators have been formed from HTSC's. Other discrete circuits for filters in the microwave region have been described. See, e.g., S. H. Talisa, et al., “Low- and High-Temperature Superconducting Micro-wave filters,” IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 9, September 1991, pp. 1448-1554, and “High Temperature Superconductor Staggered Resonator Array Bandpass Filter,” U.S. Pat. No. 5,616,538.
Currently, there are numerous applications where microstrip narrow-band filters that are as small as possible are desired. One such application involves the use of dual-mode filters (DMF's), which generate two orthogonal modes that occur at the resonant frequency. DMF's include patch dual-mode microstrip patterned structures, like circles and squares. These structures, however, take up a relatively large area on the substrate. More compact dual-mode microstrip ring structures, which occupy a smaller area on the substrate than do patch structures, have been designed.
For example,
FIG. 1
shows a two-pole dual-mode filter structure
40
, which includes an electrically conductive meander loop resonator
42
and a dielectric substrate
44
on which the resonator
42
is disposed. The resonator
42
includes a resonator line
46
that is formed into a loop that has a square envelope. The resonator line
46
is routed, such that it forms four arms
48
, each with a single meander
50
. The filter structure
40
further includes orthogonal ports
52
and
54
, which are used to couple to the resonator
42
. The filter structure
40
also includes a small patch
56
, which is attached to an inner corner of one of the meanders
50
for perturbing the electric field pattern. As a result, a pair of degenerative modes will be coupled when either of the ports
52
and
54
is excited. The degree of coupling will depend on the size of the patch
56
. Without the patch
56
, no perturbation will result, and thus only the single mode will be excited. In this case, when the port
52
is used, only one of the degenerate modes will be excited, and when the other port
54
is used, the field pattern is rotated 90° for the associated degenerate mode. As illustrated, the resonator
42
generally exhibits four-quadrant symmetry to maintain orthogonality between the two degenerative modes. See J. S. Hong, “Microstrip Bandpass Filter Using Degenerate Modes of a Novel Meander Loop Resonator,” IEEE Microwave and Guided Wave Letters, vol. 5, no. 11, pp. 371-372, November 1995.
As another example,
FIG. 2
shows a two-pole dual-mode filter structure
60
, which includes an electrically conductive meander loop resonator
62
and a dielectric substrate
64
on which the resonator
62
is disposed. The resonator
62
includes a resonator line
66
that is formed into a loop with a square envelope. The resonator line
66
is routed, such that it forms four arms
68
, each with three meanders
70
. The filter structure
60
further includes orthogonal fork-shaped coupling structures
72
and
74
, which are distributed between the arms
68
and meanders
70
. The filter structure
60
also includes a patch
76
, which is attached to the inner corner of one of the meanders
70
to effect the dual-mode coupling as previously described in the filter structure
40
of FIG.
1
. See, e.g., Z. M. Hejazi, “Compact Dual-Mode Filters for HTS Satellite Communication System,” IEEE Microwave and Guided Wave Letters, vol. 8, no. 8, pp. 1113-1117, June 2001.
As still another example,
FIG. 3
shows two-pole dual-mode filter structure
80
, which includes an electrically conductive meander loop resonator
82
and a dielectric substrate
84
on which the resonator
82
is disposed. The resonator
82
is similar to the resonator
62
shown in
FIG. 2
, with exception that it includes a resonator line
86
that is routed, such that it forms four arms
88
, each with five meanders
90
. The filter-structure
80
further includes orthogonal fork-shaped coupling structures
92
and
94
, which are distributed between the arms
88
and meanders
90
. The filter structure
80
also includes a patch
96
, which is attached to the inner corner of one of the meanders
90
to effect the dual-mode coupling as previously described in the filter structure
40
of FIG.
1
. See, e.g., Z. M. Hejazi, “Compact Dual-Mode Filters for HTS Satellite Communication System,” IEEE Microwave and Guided Wave Letters, vol. 8, no. 8, pp. 1113-1117, June 2001.
At lower frequencies, however, even these ring structures can become quite large, since resonance occurs when the ring is approximately a full electrical wavelength long. In addition, these ring structures do not necessarily address the problems associated with parasitic coupling, which becomes more prevalent as circuits are squeezed into smaller spaces. When coupling multiple resonators to make more complex narrow-band filters, the area required to accommodate the filter can grow undesirably large in order to minimize unwanted parasitic coupling between resonators and to test the package. This is particularly an issue for narrow bandwidth filters, where the desired coupling between resonators is very small, making the spacing between resonators greater. Thus, the overall size of the filter becomes even larger. For very high Q structures, like thin film HTS, significant Q degradation can occur due to the normal metal housing.
Another issue that arises in the design of narrow-band filter structures is the ability to accurately model these structures in the presence of unknown parameters, such as parasitic coupling and the introduction of mode exciting perturbations within the electrical field. In addition, computer models often use ideal capacitors to model the external capacitive coupling of dual-mode microstrip resonators. Because
Hernandez Matthew
Hey-Shipton Gregory L.
Raihn Kurt F.
Mai Lam T.
O'Melveny & Myers LLP
Superconductor Technologies Inc.
Tokar Michael
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