Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1998-11-13
2001-01-23
Lee, Benjamin C. (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S572100, C343S818000, C343S711000, C343S7000MS, C343S823000, C455S215000, C342S044000
Reexamination Certificate
active
06177872
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to impedance matching networks, and, more particularly, to impedance matching networks for circuits connected to loads with high reflection coefficients.
BACKGROUND OF THE INVENTION
In order to reduce reflections, improve signal quality, and supply the greatest power from a source to a load, the impedance “looking into the load” from the source should match the output impedance of the source. Additionally, since a mismatched line has different properties at different frequencies, a mismatched circuit is generally unsuitable for broadband or multi-frequency use. Consequently, relatively broadband circuit applications are particularly needful of impedance matching circuits.
Impedance matching is discussed in The Art of Electronics, Paul Horowittz and Winfield Hill, Cambridge University Press, Second Edition, New York, 1989 pp. 879-882 and in R. E. Collin, “Foundations For Microwave Engineering”, McGraw-Hill, 1992.
Matching the impedance of a load having a high reflection coefficient is particularly difficult, sometimes requiring complex circuits that are particularly difficult to implement using conventional technologies. For example, in order to produce a matching circuit having the appropriate impedance, a conductor line on a printed circuit board may need to be thinner than may be accommodated by conventional photoligthographic techniques. The term, “a load having a high reflection coefficient” refers to a load that has a high reflection coefficient with respect to a conventional fifty-ohm system impedance. Additionally, the terms load and source may be interchanged, depending upon one's perspective so that, for example, an antenna and an RF tag circuit may be respectively viewed as source and load, or load and source, depending upon perspective. Related applications and issued patents
Related U.S. Patents assigned to the assignee of the present invention include: U.S. Pat. Nos. 5,528,222; 5,550,547; 5,552,778; 5,554,974; 5,538,803; 5,563,583; 5,565,847; 5,606,323; 5,521,601; 5,635,693; 5,673,037; 5,682,143; 5,680,106; 5,729,201; 5,729,697; 5,736,929; 5,739,754; and 5,767,789. Patent applications assigned to the assignee of the present invention include: application U.S. Pat. No. 5,673,037; Ser. No. 08/621,784, filed on Mar. 25, 1996 entitled, “Thin Radio Frequency Transponder with Leadframe Antenna Structure” by Brady et al. (pending); application Ser. No. 08/626,820, Filed: Apr. 3, 1996, entitled, “Method of Transporting RF Power to Energize Radio Frequency Transponders”, by Heinrich et al.; application Ser. No. 08/694,606 filed Aug. 9, 1996 entitled, “RFID System with Write Broadcast Capability” by Heinrich et al.; application Ser. No. 08/681,741, filed Jul. 29, 1996 entitled, “RFID Transponder with Electronic Circuit Enabling and Disabling Capability”, by Heinrich et al.; application Ser. No. 08/592,250 (See also PCT International Application No. PCT/EP95/03903 filed Sep. 20, 1995, and U.S. application Ser. No. 08/330,288 filed Oct. 27, 1994, now abandoned, on which the PCT application is based); U.S. Pat. No. 5,729,201; application Ser. No. 08/909,719; application Ser. No. 08/621,784; application Ser. No. 08/660,249; application Ser. No. 08/660,261; application Ser. No. 08/790,640; application Ser. No. 08/790,639; and application Ser. No. 08/681,742. The above identified U.S. Patents and U.S. Patent applications are hereby incorporated by reference. Additionally, Patent Applications entitled, “Radio Frequency Identification Transponder Having a Spiral Antenna”, “Radio Frequency Identification Transponder Having a Helical Antenna”, “RFID Transponder Having Improved RF Characteristics”, and “Radio Frequency Identification Transponder Employing a Patch Antenna”, filed on the same day as this application and assigned to the same assignees as this application is assigned are also hereby incorporated by reference.
The applicants claim priority under 35 U.S.C. 119 (e) for provisional applications having attorney docket numbers YO897 660P1, YO897-661P1, and YO997-038P1, respectively filed on March 16, 17, and 13.
SUMMARY
An impedance matching circuit in accordance with the principles of the present invention employs series-connected transmission lines, such as microstrip lines, to match a high reflection coefficient source impedance, such as that associated with RF tag circuitry, with a load impedance such as that associated with an antenna input. The matching circuit is formed on a substrate material and accommodates the relatively limited capabilities of lithographic circuit formation techniques. The terms load and source may be interchanged, depending upon one's perspective so that, for example, an antenna and an RF tag circuit may be respectively viewed as source and load, or load and source, depending upon perspective.
In an illustrative embodiment the new impedance matching circuit may be constructed of three series-connected transmission line sections. A first section, the section that is to be connected to the source, transforms the high source impedance into a relatively low-valued impedance that is substantially resistive. Additionally, the reflection coefficient looking into the source from one end of the first section is substantially equal to the reflection coefficient looking into the source from the other end of the first section. The reflection coefficient looking into the source from a first end of the first section, at the source/first section interface, is given, for example, by a ratio of differences and sums of the impedances of the first section and of the source. Similarly, the reflection coefficient looking into the source from second end of the first section is given by the ratio of the differences and sums of the source impedance as transformed to the second end of the first section and the characteristic impedance of the first section.
A second section of the illustrative three-section embodiment is a quarter wave transformer that transforms the low impedance developed by the first section into an intermediate impedance value. The third section transforms this intermediate value impedance into an impedance that substantially matches that of the load.
The second section may be transformed to any of a number of values, but, perhaps most conveniently, if the second section transforms the input impedance looking into the source into a 50 ohm impedance and the load is a 50 ohm impedance, the third section may be a transmission line of any length having an intrinsic impedance of 50 ohms. Consequently, in this case, the matching circuit affords some latitude in the selection of the third section's length because the third section may be made of such a length as minimizes interference between the second section and the load. Similarly, with higher-impedance loads, within the 50 to 800 ohm range corresponding to a wide variety of antenna impedances, for example, the length of the third section may be chosen to suit a wide range of impedances. As a result, a single impedance matching circuit design may be employed to match a source to a variety of loads, thus reducing the manufacturing costs of devices that employ the new matching circuit.
Although the new impedance matching circuit may be employed with a variety of sources and loads, it is particularly well suited to application with RFID tags.
In a further aspect of the invention, a free standing quarter wave transformer is employed to couple one terminal of the RFID circuitry to a ground plane on the opposite side of the substrate from the RFID circuitry. By employing the quarter wave transformer, a via-free connection is made between the circuitry on one side of the substrate and a ground plane on the other side. This via-free connection improves mechanical reliability and reduces manufacturing costs.
REFERENCES:
patent: 3914762 (1975-10-01), Klensch
patent: 4068232 (1978-01-01), Meyers et al.
patent: 4075632 (1978-02-01), Baldwin et al.
patent: 4326203 (1982-04-01), Kaloi
patent: 4360810 (1982-11-01), Landt
patent: 4782345 (1988-11-01), Landt
pa
Brady Michael John
Duan Dah-Weih
Kodukula Venkata S. Rao
Intermec IP Corp.
Lee Benjamin C.
O 'Melveny & Myers LLP
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