Switching Device for Switching Signals of Very High and Ultrahigh Frequencies

The present invention relates to a signal switching device used in electrical equipment handling signals of very high and ultrahigh frequencies.

Signal switching devices of the type described are widely used in television receivers, video tape recorders, communication equipment, electronic instruments, and so on all of which handle the signals of very high and ultrahigh frequencies. In a signal switching device, there exist stray capacitance and inductance between the component parts thereof so that signal leakage due to electrostatic, and electromagnetic coupling occurs, resulting in poor signal isolation. In general, the higher the frequency of the signal, the poorer or lower the signal isolation becomes, and the leakage signal causes the harmful raidiation. Furthermore the reflection is caused by the poor impedance characteristic (voltage standing wave ratio) of the signal switching device so that the quality of transmission of the signals of very high and ultrahigh frequencies is much degraded. In order to overcome these defects, it has been proposed to use a switch with a plurality of circuits, to increase the dimensions of the switching device, and to employ the improved wiring designs. However, in prior art signal switching devices, signal isolation is reduced to the order of 6 to 10 dB per octave. In other words, the conventional signal switching devices cannot attain the signal isolation of the order of higher than 50 to 60 dB that is required in very high and ultrahigh frequency bands.

The first object of the present invention is therefore to provide a very high and ultrahigh frequency signal switching device which prevents the coupling between the contacts, improves the signal isolation 30 to 40 dB higher than the conventional switching devices, and considerably suppresses the unwanted radiation.

The second object of the present invention is to provide a very high and ultrahigh frequency signal switching device which may be provided by the simple modification of a conventional signal switching device.

The third object of the presenet invention is to provide a very high and ultrahigh frequency signal switching device which may be combined with coaxial-line connectors so as to reduce insertion loss.

The fourth object of the preset invention is to provide a very high and ultrahigh frequency signal switching device in which shielding plates and a cover is suitable shape and with suitable dimensions are used so that the impedance characteristic in the very high and ultrahigh frequency bands may be improved and that the better voltage standing wave ratio may be attained.

The fifth object of the present invention is to provide a very high and ultrahigh frequency signal switching device which is inexpensive to manufacture and is highly reliable and dependable in operation.

Briefly stated, the present invention provides a very high and ultrahigh frequency signal switching device comprising an insulating base, a plurality of stationary contacts attached to the insulating base, a movable contact for establishing the electrical connection between the desired pair of the stationary contacts, and shielding means interposed between the adjacent stationary contacts. The shielding means is directly electrically connected to a metal housing in order to ensure the electrical isolation between the adjacent stationary contacts.

FIG. 1 is a perspective view illustrating only the major parts of a prior art signal switching device;

FIG. 2 is a perspective view illustrating the major parts of a first embodiment of the present invention;

FIG. 3 is a perspective view illustrating the outer appearance of the first embodiment;

FIG. 4 is a perspective view thereof turned upside down;

FIG. 5 is a perspective view illustrating only the essential parts of a second embodiment of the present invention;

FIG. 8 is a perspective view illustrating only the essential parts of a third embodiment of the present invention having two arrays of the stationary contacts;

FIG. 7 is a perspective view thereof turned upside down;

FIG. 8 is a perspective view illustrating only the major parts of a fourth embodiment of the present invention;

FIG. 9 is a perspective view of a side plate thereof;

FIG. 10 is a perspective view illustrating only the essential parts of a fifth embodiment of the present invention;

FIG. 11 is a perspective view of a metal cover thereof;

FIG. 12. is a perspective view of a sixth embodiment of the present invention using a box-shaped metal housing;

FIGS. 13 and 14 are perspective views of shielding plates used in the sixth embodiment;

FIG. 15 is a perspective view of the assembled sixth embodiment;

FIG. 16 is a graph of frequency-separation or isolation characteristic curves of a prior art signal switching device and of the fourth and sixth embodiments of the present invention; and

FIG. 17 is a perspective view according to FIG. 12 with the shielding plates of FIG. 13 mounted therein.

Prior to the description of the preferred embodiments of the present invention, the prior art signal switching device will be described briefly with reference ot FIG. 1 in order to point out some specifically the defects thereof. The signal switching device shown in FIG. 1 is a single pole double throw slide switch comprising an insulating base 1, three fixed or stationary contacts 2, 3, and 4, and a sliding contact 5. The sliding contact 5 is shown as bridging between the contacts 2 and 3 while breaking the path between the contacts 3 and 4. That is, the high frequency wave signal applied to the stationary contact 2 passes through the slider 5 to the stationary contact 3. There exists a stray capacitance between the opened contacts 3 and 4 due to the air gap and the base 1 with a dielectric constant. There also exists a stray inductance between the contacts 3 and 4. In addition, there also exist various stray capacitance and inductance between the base 1, stationary contacts 2, 3 and 4 and the slider 5. These various stray capacitance and inductance produce electrostatic and electromagnetic couplings between the stationary contacts 3 and 4 which cause signal leakage. The higher the frequency of the signal, the higher the degree of the signal leakage becomes. When the stationary contact 4 is connected to, for instance, an antenna circuit, the high frequency signal leaked to the stationary contact 4 is radiated from the antenna. In case of the UHF signal, its level drops to 20 to 30 dB because of the signal leakage from the stationary contact 3 to the contact 4.

Next referring to FIGS. 2 through 4, the first embodiment of the present invention will be described, As shown in FIG. 2, the signal switching device in acordance with the present invention comprises an insulating base 6, five stationary contacts 7, 8, 9, 10, 11 and a sliding contact 12. The sliding contact 12 is so designed as to bridge between the stationary contact 9 and the stationary contact 7 or ll. The contacts 8 and 10 which are interposed between the stationary contacts 7 and 9 and between the stationary contacts 9 and 11 serve as shielding means which are gounded. That is, the grounding terminals 8 and 10 serve not only to shield the signals leakage between the stationary contacts 7, 9 and 11 but also to reduce the stray capacitance and inductance between them when the grounding terminals 8 and 10 are electrically connected to a metal frame, housing or chassis as will be described in detail hereinafter.

Referring to FIGS. 3 and 4, the grounding terminals 8 and 10 are bent at a right angle and fitted into bifurcated ends of lugs 16 and 17 extended from a metal casing 14 as best shown in FIG. 4 so that electrical connection may be established between the grounding terminals 8 and 10 and the metal casing 14. Since the stationary contacts 7, 9 and 11 are isolated from each other in the manner described above, the high frequency signal leakage may be improved by higher than 10 dB compared with the prior art signal switching means. The sliding contact 12 is atttached to a slider 13 with a projection which extends through an opening 15 formed through the top of the metal casing 14. As with a conventional slide switch, the sliding contact 12 may be positioned by moving the slider 13 back and forth.

The second embodiment of the present invention shown in FIG. 5 is substantially similar in construction to the first embodiment except the arrangement for attaining the more improved isolation between the stationary contacts 7, 9, and 11. That is, the grounding terminals 8 and 10 electrically contact shielding plates 18 and 19, respectively, made of a metal, and are fitted into the grooves or slits of projections 16a, 16b, 17a, and 17b formed integral with the metal casing 14. The shielding plates 18 and 19 are higher than the grounding terminals 8 and 10. The grounding terminals 8 and 10 may be connected to the shielding plates 18 and 19, and soldered if so desired. The high frequency signal leakage may be reduced by 20 to 30 dB compared with the prior art signal switching device.

The third embodiment shown in FIGS. 6 and 7 comprises two signal switching devices of the type of the second embodiment which are ganged together. The base 6 is further provided with stationary contacts 7', 9', and 11', grounding terminals 8' and 10', a sliding contact 12' which is attached to a slider 13', all of which correspond to the stationary contact 7, 9, and 11, the grounding terminals 8 and 10, and the sliding contact 12 attached to the slider 13, respectively, of the second embodiment. The corresponding stationary contacts 7, 7', 8, 8', 9 and 9', are isolated from each other by shielding plates 20 and 21 which are fitted into center grooves 22 formed through the opposed side walls of the metal casing 14. The longitudinal shielding plates 20 and 21 are also made into electrical contact with the transverse shielding plates 18 and 19 as best shown in FIG. 7.

In the fourth embodiment of the present invention shown in FIG. 8, the second embodiment shown in FIG. 5 is combined with coaxial-line connectors in order to improve the impedance characteristic. Three coaxial-line connectors 24, 25, and 26 are attached to a metal cover 23 which in turn is attached to the metal casing 14 in such a way that the cover 23 electrically contacts the casing 14 and the shielding plates 18 and 19 in electrical contact with the grounding terminals 8 and 10 and that the core conductors 24', 25', and 26' of the coaxial-line connectors 24, 25, and 26 electrically contact the stationary terminals 7, 9, and 11, respectively. The characteristic impedance between the stationary contacts 7, 9, and 11 and the core conductors 24', 25', and 26' is dependent upon the volume of the spaces defined by the cover 23 and the shielding plates 18 and 19. Therefore, the shielding plates 18 and 19 must be positioned so as to match the characteristic impedance. In the fourth embodiment, the shielding plates 18 and 19 are provided with projections 18' and 19', and a side plate 27 made of a metal has its slots 27a and 27b fitted over these projections 18' and 19', respectively, so that the side plate 27 may be attached to the cover 23 and may be made into electrical contact therewith. The cover 23 and the side plates 27 serve to prevent leakage to the exterior from the terminals 7, 9, and 11 and the base 6.

The fifth embodiment shown in FIG. 10 is a modification of the fourth embodiment shown in FIG. 8. In the fifth embodiment, the coaxial-line connectors 24, 25, and 26 are attached to the extended side wall portions 14a, 14b, and 14c, respectively, of the casing 14 so as to be at right angles with respect to the stationay contacts 7, 8, and 9, respectively. The shielding plates 18 and 19 are fitted into the grooves formed through the extended wall portions 14a, 14b, and 14c so that the electrical connection between them may be attained. A cover 28 made of a metal (See FIG. 11) is attached to the assembly shown in FIG. 10. The cover 28 is provided with three openings 28a, 28b, and 28c through which the coaxial-line connectors 24, 25, and 26 extendeoutwardly, respectively, and two slots 28d and 28e which are fitted over the upright projections 18' and 19', respectively, of the shielding plates 18 and 19. Thus the electrical connection between the cover 28 and the casing 14 may be attained, and the signal leakage may be prevented.

In the sixth embodiment, within a metal box 29 is disposed a high frequency signal switching device SW which is substantially similar in construction to the second embodiment (See FIG. 5) except that it is not provided with the shielding plates 18 and 19. The coaxial-line connectors 24, 25, and 26 are attached to the side walls of the metal box 29 in such a way that their outer conductors electrically contact the metal box 29 and that their core conductors are connected to the stationary contacts 7, 9, and 11, respectively.

In order to isolate the stationary contacts 7, 9, and 11 from each other, shielding plates 30 and 31 shown in FIG. 13 are used. The shielding plate 30 is fitted into the metal box 29 in such a way that its opening 30a is fitted over the signal switching device SW and it engages the grounding terminal 8 and the projections 16a and 16b of the metal casing 14. In like manner, the shielding plate 31 is fitted into the metal box 29 in such a way that its opening 31a is fitted over the switching device SW and that it engages with the grounding terminal 10 and the projections 17a and 17b of the metal casing 14.

In order to provide better isolation between the stationary contacts 7, 9, and 11, it is required that the shielding plate has two planes or more.

For this purpose, shielding plates 32 and 33 of the type shown in FIG. 14 may be used instead of the shielding plates 30 and 31 shown in FIG. 13. These shielding plates 32 and 33 have openings 32a and 33a, respectively, and are fitted into the metal box 29 in a manner substantially similar to that described above with reference to the shielding plates 30 and 31.

The shielding plates 30 and 31 or 32 and 33 are fitted into the metal box 29 so as to attain the electrical connection therewith and to define the electrical spaces therein matching the characteristic impedance between the stationary contacts 7, 9, and 11 and the coaxial-line connectors 24, 25, and 26, respectively. Thereafter, a metal bottom 34 is attached to the metal box 29 so that the switching device with the coaxial-line connectors is completed as shown in FIG. 15. The slider 13 is extended from the top of the metal box 29.

FIG. 16 is a graph illustrating the relationship between the frequency and separation or isolation. The curve (a) is the characteristic curve of othe prior art signal switching device. It is seen that the effective operating range is less than 100 MHz in the high frequency band. The curve (b) is the characteristic curve of a prior art signal switching device which was modified to some extent according to the present invention. It is seen that this switching device cannot be used in the very high frequency band. The curve (c) is the characteristic curve of the fourth embodiment (See FIG. 8). It is seen that this signal switching device may attain the sufficient separation or isolation at each band in the ultrahigh frequency range. The curve (d) is the characteristic curve of the sixth embodiment (See FIGS. 12 through 15) of the present invention.

The satisfactory effects may be attained by any of the embodiments of the present invention described hereinbefore, but if it is required to attain the higher isolation than that attained by one of the preferred embodiments, the suitable combination of the preferred embodiments may be used.