Wave transmission lines and networks – Resonators
Reexamination Certificate
1999-07-30
2001-06-05
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Resonators
Reexamination Certificate
active
06242992
ABSTRACT:
1. BACKGROUND OF THE INVENTION
a. Field of the Invention
This invention pertains to resonators for use at microwave frequencies. More particularly, this invention pertains to the use of shorted, slow-wave coplanar transmission lines as microwave resonators or filters and devices for coupling to such resonators and filters.
b. Description of the Prior Art
U.S. Pat. No. 5,777,532 (“532”) discloses an interdigital, slow-wave coplanar transmission line consisting of two conducting strips that are located upon the surface of a substrate and that have interleaved conducting fingers that extend from the one side of each strip towards the opposite strip. The “532” patent also discloses a second, balanced configuration in which a central conducting strip is bounded on both sides by conducting grounds. Conducting fingers extend from the sides of the central conductor towards the grounds and interleave with conducting fingers extending from the grounds towards the central conductor. The capacitance between the interleaved fingers substantially slows the rate at which an electromagnetic wave propagates along the transmission line in comparison to the rate of propagation in the absence of such interleaved fingers. As a consequence a shorter piece of such transmission line provides a substantially greater time delay as compared with a coplanar transmission line that has no such interleaved fingers.
A section of ordinary coaxial transmission line that is short circuited at one end has been used for impedance matching purposes, and in some instances has been used as a frequency filter, or resonator, at MF, HF, VHF and in some instances at LHF frequencies. The input impedance at the input end of the transmission line opposite to the end that is short-circuited, exhibits a high impedance when the transmission line has a length of one-quarter wave, or an odd integer multiple thereof, and exhibits a low impedance when the transmission line has a length of one-half wave or an integer multiple thereof. At microwave frequencies, waveguides have also been used in a similar manner for the same purposes. However, at microwave frequencies, attempts to use short-circuited coplanar transmission lines as resonators and filters have been severely limited because higher order modes, other than the TEM mode(transverse electric and magnetic field mode) begin to propagate at frequencies that are not far removed from the microwave frequencies at which the shorted section of coplanar transmission line resonates.
2. SUMMARY OF THE INVENTION
Instead of using an ordinary coplanar transmission line, the present invention uses a section of an interdigital slow-wave coplanar transmission line, such as that depicted in
FIG. 1
, that is short circuited to ground at one end of the transmission line to function as a resonator. The unexpected advantage of using an interdigital, slow-wave coplanar transmission line as a resonator is that, in contrast to using an ordinary co-planar transmission line, the frequencies at which the higher order modes begin to propagate along the interdigital, slow-wave transmission line are displaced much further from the frequencies at which the shorted, slow-wave transmission line is resonant. Because of the greater displacement, the interdigital, slow-wave coplanar transmission line can be used as a resonator for line lengths of odd integral multiples of a quarter-wave without being degraded by the propagation of higher order modes. Because the transmission line resonators that utilize line lengths that are higher odd-integral multiples of a quarter-wave, tend to exhibit a narrower bandwidth or higher Q resonance, such a multiple quarter wavelength resonator that utilizes an interdigital slow-wave coplanar transmission line provides better performance than a resonator that uses a multiple quarter wavelength of ordinary, coplanar transmission line.
An immediate practical problem associated with such a resonator is providing a means at microwave frequencies for connecting to the input port of the resonator. One could simply connect an external transmission line to the open end of the quarter wavelength interdigital, slow-wave coplanar transmission line. However, in such a circumstance the Q of the resonator would be significantly depressed by the loading of the resonator by the impedance of the transmission line. One solution used in an embodiment of this invention is to “tap down” on the resonator by attaching an electrical conducting wire tap from the external transmission line to some point on the central conductor of the interdigital slow-wave coplanar transmission line that was located nearer to the shorted end of the interdigital transmission line.
FIG. 4A
depicts the use of such a tap.
A second embodiment of invention includes a means for coupling to an interdigital, slow-wave, coplanar transmission line resonator that does not require the addition of wire tap or plated through holes in a substrate. This embodiment, instead, uses a portion of an ordinary coplanar transmission line that is electrically shorted at one end and that is overlaid in close proximity to the resonator, so as to couple the transmission line to the resonator. By using such coupling one avoids any need for wire taps and plated through holes as a means of electrically connecting to the resonator.
REFERENCES:
Gorur, Karpuz and Alkan, “Characteristic of Periodically Loades CPW Structurs” Aug. 1998, IEEE Microwave and Guided Wave Letters, Vot. 8 No. 8 pp. 278-280.
Buck G. Joseph
Glenn Kimberly E.
Pascal Robert
TFR Technologies, Inc.
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