BRIEF DESCRIPTION OF THE DRAWINGS
Various objects and advantages and a more complete understanding of the present invention are apparent and more readily appreciated by reference to the following Detailed Description and to the appended claims when taken in conjunction with the accompanying Drawings, wherein:
FIG. 1 is a circuit diagram of a high-power PIN diode switch according to the prior art;
FIG. 2A is a top view of a quarter-wavelength, resonant transmission-line element according to the prior art;
FIG. 2B is a cross-sectional side view of the quarter-wavelength, resonant transmission-line element shown in FIG. 2A according to the prior art;
FIG. 3 is a circuit diagram of a PIN diode switch in accordance with an illustrative embodiment of the invention;
FIG. 4A is a top view of a transmission-line element in accordance with an illustrative embodiment of the invention;
FIG. 4B is a cross-sectional side view of the transmission-line element shown in FIG. 4A in accordance with an illustrative embodiment of the invention;
FIG. 5A is a top view of a transmission-line element in accordance with another illustrative embodiment of the invention;
FIG. 5B is a cross-sectional side view of the transmission-line element shown in FIG. 5A in accordance with this illustrative embodiment of the invention;
FIG. 6A is a top view of a transmission-line element in accordance with yet another illustrative embodiment of the invention;
FIG. 6B is cross-sectional side view of the transmission-line element shown in FIG. 6A in accordance with this illustrative embodiment of the invention; and
FIG. 7 is a top view of high-power PIN diode switch in accordance with an illustrative embodiment of the invention.
DETAILED DESCRIPTION
In one illustrative embodiment of the invention, a PIN diode single-pole, single-throw (SPST) switch is provided that has low cost, high stability and reliability, and small size. The PIN diode switch comprises a series PIN diode and direct-current (DC) biasing circuit in which DC-conducting and radio-frequency (RF)-isolating elements are microstrip-line-type, folded, quarter-wavelength, resonant transmission lines including a plurality of substantially parallel sections that are magnetically coupled and electrically connected in series. The substantially parallel sections are arranged in a manner that mutually reinforces their local magnetic fields. This results in an increase in the characteristic impedance and a decrease in the RF losses of the microstrip line.
The closer the adjacent substantially parallel sections are placed to each other, the stronger the interaction between their magnetic fields, the smaller the RF losses, and the smaller the size of the resonant transmission line. Lower loss and smaller size allow the PIN diode switch to operate more reliably and to be assembled in smaller and less expensive housing.
Referring now to the drawings, where like or similar elements are designated with identical reference numerals throughout the several views, and referring in particular to FIG. 3, it is a circuit diagram of a PIN diode switch 300 in accordance with an illustrative embodiment of the invention. The circuit of FIG. 3 includes PIN diodes 305 and 310; blocking capacitors 315, 320, 325, and 330; and transmission-line elements 335 and 340. In this particular embodiment, transmission-line elements 335 and 340 are microstrip-line-type, folded, quarter-wavelength, resonant transmission lines. Input RF power is fed to input terminal 345, and output RF power is taken from output terminal 350. Control port 355 provides a terminal for biasing PIN diode 305. Control port 360 provides a terminal for biasing PIN diode 310. For simplicity, the circuit making up the bias controller is not shown in FIG. 3 but is within the capability of one of ordinary skill of the art to design. The transmission and isolation modes of operation of a series-shunt SPST switch in general are described above.
To aid thermal management in the illustrative embodiment shown in FIG. 3, all elements are disposed on a highly thermally conductive, electrically isolating substrate such as aluminum oxide ceramic. All interconnections and quarter-wavelength, resonant transmission-line elements 335 and 340 are fabricated using microstrip technology. Blocking capacitors 315, 320, 325, and 330 and PIN diodes 305 and 310 are disposed on the same substrate using surface-mount soldering technology.
To minimize the size of PIN diode switch 300, transmission-line elements 335 and 340 are implemented using a folded design, but the folded design differs from the prior-art meandering shape shown in FIG. 2A. To compensate for the harmful effects of the close proximity of adjacent sections of the line, the topology of the sections is configured in such a way that the local magnetic field of each individual section increases (constructively interferes with) the magnetic field of adjacent sections.
FIG. 4A is a top view of a transmission-line element 400 in accordance with an illustrative embodiment of the invention. In this embodiment, transmission-line element 400 is a quarter-wavelength, resonant transmission line. A cross-section A-B of transmission-line element 400 is shown in FIG. 4B.
Referring to both FIGS. 4A and 4B, substantially parallel sections (“sections”) 405 of transmission-line element 400 are formed by printing an electrically conductive trace (e.g., a microstrip line) 410 on the surface of thermoconductive insulating substrates 415 and 420 such as aluminum oxide. The electrically conductive trace 410 can be composed of any suitable conductor such as copper, aluminum or alloys. The two thermoconductive insulating substrates (415 and 420) are assembled together, as shown in FIG. 4B. The substrates 415 and 420 are separated by an electrically conductive ground plane 425. In one embodiment, a printed metallization layer on the opposite side of at least one substrate provides ground plane 425.
Sections 405 of the trace 410 associated with substates 415 and 420 are connected electrically in series (e.g., through the use of jumpers). Trace 410, however, is electrically isolated from ground plane 425. In this embodiment, trace 410 is effectively “wrapped around” the attached substrates 415 and 420. As indicated in FIG. 4B, RF currents in adjacent sections 405 of trace 410 flow in the same direction on a given substrate 415 or 420. In FIG. 4B, the circled crosses above the sections 405 associated with substrate 415 indicate current flow into the page, away from the reader. The circled dots below the sections 405 associated with substrate 420 indicate current flow out of the page, toward the reader. In this case, the magnetic coupling M results in a mutual increase of the magnetic fields of adjacent sections 405. Because the substrates 415 and 420 are separated magnetically and electrically from ground plane 425, there is no electromagnetic interaction between the two sides of the assembly.
The mutually reinforced magnetic field of the plurality of substantially parallel sections 405 exceeds that of a straight line. At the same time, the distribution of the electric field of each section 405 of the line remains almost the same as for the straight line because the major part of the energy of the electric field is confined in the body of the substrate between the trace and ground plane 425. But the ratio of magnetic field energy to electric field energy defines the characteristic impedance of the transmission line. Consequently, the characteristic impedance of a folded transmission line constructed in accordance with the principles of the invention becomes higher than that of a straight line. This means an increase in input impedance of the transmission-line element and a proportional decrease in energy loss.
The illustrative embodiment shown in FIGS. 4A and 4B is well suited for applications in which PIN diode switch 400 controls a moderate level of RF power and air cooling is sufficient to remove the thermal power dissipated by the elements of the switch.
FIG. 5A is a top view of a transmission-line element 500 in accordance with another illustrative embodiment of the invention. A cross-section A-B of transmission-line element 500 is shown in FIG. 5B. The embodiment shown in FIGS. 5A and 5B, which is similar to that shown in FIGS. 4A and 4B, includes a heat sink 505 between substrates 415 and 420. Using a water-cooled heat sink allows more power to be dissipated. Therefore, the PIN diode switch 500 can control higher RF power.
FIG. 6A is a top view of a transmission-line element 600 in accordance with yet another illustrative embodiment of the invention. A cross-section A-B of transmission-line element 600 is shown in FIG. 6B. In this embodiment, two or more spatially separated groups (605 and 610) of substantially parallel sections 615 of the transmission line are disposed on a single planar surface. Within each group of substantially parallel sections, the direction of RF current flow is the same, causing the local magnetic field of the sections in that group to be mutually reinforced. In the particular example of FIGS. 6A and 6B, two spatially separated groups are employed, and the distance between those two groups of sections 605 and 610 (“D1” in FIG. 6A) is made sufficiently larger than the width of trace 625 (“D2” in FIG. 6A) to render negligible the magnetic coupling between the groups 605 and 610. Thus, the magnetic coupling M2 has a negligible effect on the total magnetic field configuration compared to the much stronger magnetic coupling M1. In some embodiments, the substrate is attached to a heat sink 630. In this embodiment, trace 625 is shown as a rectangular spiral. However, a rectangular spiral is shown only for illustration purposes. Other shapes are also realizable.
FIG. 7 is a top view of a high-power PIN diode switch 700 in accordance with an illustrative embodiment of the invention. The elements shown in FIG. 7 corresponding to those shown in FIG. 3 are designated by the same reference numerals. In this particular embodiment, the quarter-wavelength, resonant transmission-line elements 335 and 340 are constructed in a single-plane fashion, as discussed in connection with FIGS. 6A and 6B. The grounded terminals of capacitors 320 and 325 and transmission-line element 340 are connected to a ground plane 635 (not shown in FIG. 7) disposed on the other side of substrate 620 (see FIGS. 6A and 6B) by way of vias (through holes). The components of PIN diode switch 700 are disposed on a thermoconductive dielectric substrate 705.
By way of illustration, one particular implementation of a PIN diode switch in accordance with the principles of the invention has overall dimensions of 50 mm×100 mm×15 mm. This PIN diode switch has an operating frequency range from 55 MHz to 65 MHz. Two such PIN diode switches installed at the output of an RF generator provide switching of 5 kW of RF power between two independent loads. The insertion loss measured under these conditions remains below 0.05 dB. The isolation between two outputs measured at the 5-kW level is greater than 45 dB.
A PIN diode switch according to the invention is simple in structure and, as such, is inexpensive, yet it is capable of providing excellent performance.
In conclusion, the present invention provides, among other things, a high-power PIN diode switch suitable for applications such as plasma processing systems. Those skilled in the art can readily recognize that numerous variations and substitutions may be made in the invention, its use, and its configuration to achieve substantially the same results as achieved by the embodiments described herein. Accordingly, there is no intention to limit the invention to the disclosed illustrative forms. Many variations, modifications and alternative constructions fall within the scope and spirit of the disclosed invention as expressed in the claims.
Patent Application
20080030285
