Small reed switches are used for applications where R.F. signals must be switched on or off in very short time periods. They typically consist of ferromagnetic contacts that are hermeticallys ealed in a glass vial or enclosure and are selectively activated by the magnetic field resulting from curent flow in a coil wound around the enclosure. A prior art application of a reed switch is illustrated in FIG. 1, which s a sectional view cut through the switch assembly as mounted on a conventional circuit board. The reed switch 10 can be made into a coaxial sltructure by surrounding the glass vial or enclosure 11 of the switch with an electrically conductive and nonferromagnetic tube 14 that acts as the outer conductor of a coaxial transmission line including switch 10. Because of their small size, and the tight tolerance space between the two contacts provided within such a reed switch 10, these switches can be made to open and close very quickly. In the example shown in FIG. 1, the reed switch 10 is surrounded by a nonmagnetic bobbin 12 haing an actuating coil 13 wrapped about it. Switch 10 is arranged parallel to the plane of the printed circuit board 15 and is located within an aperture cut out through the printed circuit board to receive the switch and surrounding bobbin. The leads 20 and 21 of reed switch 10 are directly joined to conductive transmission lines 16 on one side of the printed circuit board. The surrounding conductive tube 14 can be electrically connected to the ground plane 22 on the remaining side of the printed circuit board by short conductors 17.
The main limitation encountered with respect to prior art use of reed relays in the manner shown in FIG. 1 is the impedance mismatches which occur at the ends of the reed switch 10 when mounted in the signal path on the printed circuit board. Because of manufacturing tolerances, there are always nominal gaps at the ends of the vial or enclosure 11 between its coaxial structure and the transmission lines 16 on the printed circuit board 15 (gaps 18 in FIG. 1). These gaps create an impedance mismatch which is random in nature and difficult to correct. In addition, since the diameter of the glass enclosure 11 is generally reduced at the ends of the reed switch 10 becaues it is melted during manufacture of the switch assembly, and since the conductive surrounding tube 14 cannot easily conform to this irregular and somewhat unpredictable enclosure shape, there is a further impedance mismatch introduced because of these gaps (gaps 19 in FIG. 1). The problem becomes more pronounced at higher frequencies, making the approach shown in FIG. 1 not practical for signals substantially above 2 GHz.
The present invention was designed to substantially eliminate both described sources of impedance mismatch inherent in past applications of reed relays to printed circuit board applications, thereby permitting the utilization of the reed relay technology for fast switching applications at much higher R.F. frequencies. The invention also permits the relay builta bou the reed switch technology to be manufactured as a complete, stand-alone component, not dependent upon the charcteristics of a printed circuit board or specific mounting details for its performance specifications.
USPTO Backgrounds
USPTO Backgrounds