Strip-line resonator
BACKGROUND OF THE INVENTION 
     The present invention relates to a resonator in the VHF-UHF band for use in filters and oscillators. 
     Strip-line resonators have been developed for application in the VHF-UHF band to overcome problems associated with coaxial resonators. Resonators of the strip-line type comprise a pair of dielectric substrates each of which is metallized on outer surface for connection to ground. A strip line which is metallized on the inner surface of one of the substrates is sandwiched between the substrates which are bonded together by mechanical means or adhesive. One difficulty associated with the conventional strip-line resonator is that the mechanical bonding is not satisfactory for operation under vibration and the use of adhesive results in a reduction of unloaded Q value or results in a resonator which is subject to humidity. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a strip-line resonator which withstands vibration, is immune to humidity and exhibits a high Q value. 
     According to the invention, a strip-line resonator comprises a first dielectric substrate having a conductive film on one surface thereof for connection to ground and a strip line on the other surface thereof and a second dielectric substrate having one surface thereof making contact with the strip line and a conductive film on the other surface thereof for connection to ground. The invention is characterized by the use of a fused glass having a low melting point for bonding the first and second substrates together. Lead borosilicate glass is excellent material for this purpose. 
     Another object of the present invention is to provide a low-cost strip-line resonator which facilitates frequency adjustment. This object is achieved by leaving an open-circuit end portion of the strip line exposed and providing the exposed end with plural slits or forming a capacitive gap with a grounded conductor. By trimming the slitted end portion or varying the capacitive gap, the frequency of the resonator can be readily accomplished. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1a is a side view of one embodiment of the strip-line resonator; 
     FIG. 1b is a cross-sectional view taken along the line 1b--1b of FIG. 1a; 
     FIG. 2 is an illustration of the detail of an exposed open-circuit end portion of the strip line of FIG. 1b; 
     FIG. 3a is a side view of a second embodiment of the invention; 
     FIG. 3b is a plan view of the second embodiment; 
     FIG. 4a is a side view of a modification of the second embodiment; 
     FIG. 4b is a plan view of the modification of FIG. 4a; 
     FIG. 5a is a plan view of a third embodiment of the invention; 
     FIG. 5b is a cross-sectional view taken along the line 5b--5b of FIG. 5a; 
     FIG. 6 is a side view of a modification of FIG. 5a; 
     FIG. 7a is a plan view of a fourth embodiment of the invention; 
     FIG. 7b is a cross-sectional view taken along the line 7b--7b of FIG. 7a; 
     FIG. 7c is a cross-sectional view taken along the line 7c--7c of FIG. 7a; and 
     FIG. 8 is a plan view illustrating a modification of the embodiment of FIG. 7a. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1a and 1b, a half wavelength resonator of a U-shaped strip line comprises a lower substrate 10 of a dielectric material and an upper substrate 11 of a similar dielectric material. Lower substrate 10 has its lower surface entirely metallized to form a film 12 which is to be grounded. The upper surface of lower substrate 10 is metallized to form a U-configured strip line 13. Upper substrate 11 has a smaller length than the length of lower substrate 10 and has its upper surface entirely metallized to form a film 14 which is to be grounded. The lower and upper substrates are bonded together by means of fused glass of the type having a low melting point and a low dielectric loss such as lead borosilicate glass as shown at 15 and 16. The left edges of these substrates are aligned with each other so that open-circuit end portions 13a of the U-configured strip line 13 are exposed as illustrated. Due to the use of fused glass, upper and lower substrates 10 and 11 are strongly bonded together and the resonator is immune to temperature and humidity variations associated with conventional adhesive materials. 
     The frequency of the resonator is trimmed by forming a plurality of parallel slits 13b as shown in FIG. 2 at the open-circuit end portions 13a using a laser beam. The slits may be formed simultaneously as the strip line is metallized. Since the open-circuit end portions are capacitive, precision frequency adjustment is made by trimming the slitted portions transversely to a desired extent as shown at A and B while monitoring an instrument. 
     A modified embodiment is shown in FIGS. 3a and 3b  which differs from the previous embodiment in that instead of the slitted open-circuit end portions, the grounded conductive layer 12 is turned around the right edge of substrate 10 and a comb-like, springy conductive member 17 is bonded to the up-turned end of layer 12 and bent downwards to provide a capacitance gap 18 between it and the open-circuit ends. By appropriately adjusting the degree of the bend, the capacitance of the open-circuit ends and hence the frequency of the resonator is precisely determined. 
     Alternatively, a dielectric strip 20 having a metallized film 19 is fitted under pressure in the capacitance gap 18 as shown in FIGS. 4a and 4b. Instead of adjusting the gap 18, frequency adjustment is made by moving the metallied strip 20 transversely of the open-circuit ends 13a. After adjustment, the comb-like member 17 is preferably soldered to the metallized film 19. This arrangement has an advantage in that by suitably choosing the material for the dielectric strip 20 it is possible to provide a wide adjustable range of frequencies or compensate for temperature variation. 
     In FIGS. 5a and 5b, a pair of rectangular parallel strips 33a, 33b are metallized on the upper surface of a dielectic substrate 31 below which is also metallized as at 32 which contacts the bottom of a metal casing 36. Strips 33a, 33b are spaced apart a distance greater than the thickness of the substrate 31 and extend from the left edge of substrate 31 to a point short of the right edge of substrate 31 which contacts with the right end wall 36a of casing 36 to form open-circuit ends. A generally U-shaped, plastically deformable conductive member 34 is bonded to the strips 33 so that it overhangs the left end of substrate 31 which is spaced from the left end wall 36b of casing 36. 
     The frequency of the resonator is determined by the shape and size of U-shaped end member 34 and the angle of its plane to the plane of film 33. More specifically, rough frequency adjustment is determined by the length and width of the end member and fine adjustment can be made by bending it appropriately with respect to the horizontal. The rough frequency adjustment is made by preparing a plurality of end members having different sizes to choose from. Since the metallized film is known to have an electrical resistance higher than copper or silver-plated members and since the current density is at maximum in the U-shaped end portion of the strip line, it is an advantage of the present embodiment that such a high current portion can be formed of low-resistance conductors, and hence the unloaded Q value of the resonator can be increased. 
     In the present embodiment the impedance ratio of the straight portion 33a to U-shaped portion 34 differs from the impedance ratio of straight portion 33b to U-shaped portion 34. Due to this difference, the spurious resonance deviates from integral multiples of the fundamental frequency. A further feature of the invention is that if the resonator is employed in a filter or the like, no response occurs at the harmonics of the fundamental frequency and spurious components can thus be effectively suppressed. 
     As shown in FIG. 6, the straight portions 33a, 33b are alternatively sandwiched between lower substrate 31 and upper substrate 35 which are bonded together by fused lead borosilicate glass 37. The upper surface of substrate 35 is coated with a metallized film 36 for grounding as in the embodiment of FIG. 1. 
     FIGS. 7a, 7b and 7c are illustrations of a further embodiment of the invention. In this embodiment, lower dielectric substrate 41 has its lower surface metallized and soldered to the bottom of a metal casing 43 and has its upper surface formed with a U-shaped, metallized strip line 44. An upper dielectric substrate 42 having a smaller length than lower substrate 41 is placed on the lower substrate and has a metallized coat 45. As best shown in FIG. 7b, the casing has an upper member having a pair of guides 47 along the length of the resonator. Upper substrate 42 is disposed in the guides so that it is movable longitudinally with respect to the lower substrate to vary its area of contact with the strip line 44, while making pressure contact with it by leaf springs 46 which form part of the upper member of the casing. Frequency adjustment is made by moving the upper substrate along the guides. The frequency is at minimum when the upper substrate is located to the right end of the casing and maximum when located to the left end. After frequency adjustment is made, leaf springs 46 are bonded to metal coat 45 by fused lead borosilicate or solder. 
     While mention has been made of half wavelength resonators having open-circuit ends, the present invention could equally be as well applied to quarter wavelength resonators. One example of such resonators is shown in FIG. 8 in which the strip line is of a single rectangular shape having an open-circuit end 51 and a short-circuit end 52 which is connected to an up-turned end of a metal coat which is metallized on the lower surface of a substrate 50.