Filter having resonant frequency adjusted with dielectric layer

[0002] This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-046878 filed on Feb. 24, 1999, the content of which is incorporated herein by reference.

[0003] 1. Field of the Invention

[0004] The present invention relates to a filter having resonators, the resonant frequency of which is adjusted with a dielectric layer formed on the resonators, and to a method of adjusting the resonant frequency of such resonators.

[0005] 2. Description of Related Art

[0006] A filter characteristic such as resonant frequency has to be adjusted after its manufacturing process is completed, because the characteristic usually deviates from its target due to various deviation factors such as dielectric constant of a substrate, thickness of layers, accuracy of a mask, manufacturing process conditions and the like. Such characteristic adjustment, or tuning is especially necessary for narrow band and low ripple filters.

[0007] Conventionally, a resonant frequency of a filter has been tuned by adjusting an effective dielectric constant of a resonator with a screw that carries a dielectric member at its tip, or by trimming a resonator pattern with a laser beam. In both methods, it is necessary to carry out the adjustment or tuning while the filter characteristic is being measured. Such adjustment under measurement is not easy, especially when a filter is constituted by a large number of resonators.

[0008] The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide a filter, the resonant frequency of which is easily adjusted, and more particularly, to provide a filter having plural resonators, in which the resonant frequency of each resonator is easily adjusted independently from one another. Another object of the present invention is to provide an improved method of adjusting the resonant frequency of the resonators, which can be carried out without watching a measuring instrument.

[0009] A filter is composed of a dielectric substrate, plural resonators formed on one surface of the substrate and a ground plane formed on the other surface of the substrate. A dielectric layer is formed on each resonator to adjust or tune the resonant frequency of the resonator. The resonators and the ground plane may be made of a superconductive material, and each resonator is ring-shaped with one portion opened with a narrow gap. The resonant frequency of each resonator is equalized to a target resonant frequency by accumulating a proper amount of a dielectric material as a layer formed on the resonator.

[0010] The resonators are designed so that their resonant frequencies are a little higher than a target resonant frequency, because the resonant frequency is adjusted by the dielectric layer only in the direction to decrease the resonant frequency. After the plural resonators are formed on the substrate, the resonant frequency of each resonator is measured and compared with the target frequency, thereby determining frequency deviation of each resonator. The larger the deviation is, the higher amount of the dielectric material is accumulated on the resonator. Thus, the resonant frequencies of all the resonators are adjusted to the target frequency.

[0011] The dielectric layer may be formed in a photolithography process. Preferably, the dielectric layer having the same thickness is formed on each resonator, and the amount of the dielectric material is controlled by changing the areal size of the dielectric layer covering each resonator. The resonant frequency of each resonator is adjusted independently from one another by changing the covering area of the dielectric layer. It is preferable to pattern all the dielectric layers, each covering each resonator with a respective areal size, at the same time using a single mask in the photolithography process. Preferably, one resonator showing the highest frequency deviation is fully covered with the dielectric layer while other resonators are partially covered. Alternatively, the amount of the dielectric layer may be controlled by changing its thickness while keeping the covering area constant.

[0012] Since the resonant frequency deviation from the target frequency of each resonator is measured, and the amount of the dielectric material required to eliminate such deviation is determined before the adjustment process, it is not necessary to measure the resonant frequency during the adjustment process. Thus, the adjustment or tuning process is simplified, especially when a large number of resonators are used in a filter.

[0013] Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.

[0014] FIG. 1 is a plan view showing a filter having plural resonators, the resonant frequency of each of which is adjusted with a dielectric layer;

[0015] FIG. 2 is a plan view showing a filter having plural resonators, the resonant frequency of which is not adjusted;

[0016] FIG. 3 is a table showing resonant frequencies and frequency deviations of each resonator;

[0017] FIG. 4A is a plan view showing a resonator on which a dielectric layer is formed near a pattern gap;

[0018] FIG. 4B is a plan view showing a resonator on which a dielectric layer is formed at the center of a pattern;

[0019] FIGS. 5A-5D are plan views showing various patterns of the resonator;

[0020] FIG. 6 is a plan view showing a method of measuring a resonant frequency of a selected resonator, other resonators being short-circuited by conductive members; and

[0021] FIG. 7 is a graph showing frequencies at which a peak insertion loss appears in a resonator under measurement and other resonators, respectively.

[0022] A preferred embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a filter after resonant frequency adjustment is completed, while FIG. 2 shows the filter before the adjustment. First, referring to FIG. 2, a filter is composed of a dielectric substrate 1, plural resonators 11-18 formed on the dielectric substrate 1 and a ground plane (not shown) formed on the rear surface of the substrate 1. The filter is a distributed-constant-type filter and has a microstrip line structure. The plural resonators 11-18 are positioned along a circle having a certain radius from the center of the round substrate 1. A loop length of each resonator is designed to be one half of a wave length (λ), and one portion of the loop is open toward the center of the substrate 1. The open portion of each loop forms respective gaps 11a-18a. A terminal 20 for an input signal is tapped from the resonator 11, and a terminal 21 for an output signal is tapped from the resonator 18. The resonators 11-18 and the ground plane are made of a superconductive material, and the substrate 1 is made of a dielectric material. In this particular embodiment, each resonator 11-18 is designed so that its resonant frequency becomes 2 GHz. In other words, the target resonant frequency is 2 GHz.

[0023] To adjust any deviation from the target resonant frequency, the resonant frequency of each resonator is first measured in a method described later, and then dielectric layers 31, 33-38 are formed on the resonators to be adjusted as shown in FIG. 1. In this particular example shown in FIG. 1, the resonator 12 need not be adjusted or tuned, while all other resonators have to be adjusted. The dielectric layers 31, 33-38 are made of a dielectric material such as CeO, MgO, SiO₂or the like.

[0024] The resonant frequency of each resonator 11-18 is measured as shown in FIG. 6. FIG. 6 shows an exemplary situation where one resonator 14 is selected as a resonator to be measured. In order to avoid interference due to electromagnetic coupling between the selected resonator 14 and non-selected resonators, all the gaps of the non-selected resonators are short-circuited with conductive members 41-43, 45-48. The conductive material may be silver paste, a conductive tape or the like, which is easily removable. Thus, the resonant frequency of the resonator 14 can be precisely measured without interference with other resonators. An input probe 51 and an output probe 52 are placed as shown in FIG. 6, and the resonant frequency of the resonator 14 is measured. When the non-selected resonators are short-circuited, their resonant frequencies shift to a high side, becoming about double, that is, from about 2 GHz to about 4 GHz. This resonant frequency shift is shown in FIG. 7. The resonant frequency (the frequency at which insertion loss shows a peak) of the resonator 14 that is selected to be measured is about 2 GHz, while the resonant frequencies of non-selected resonators that are short-circuited are around 4 GHz. In this manner, the resonant frequency of the selected resonator 14 is precisely measured. Non-selected resonators 11-13, 15-18 are measured in the same manner as the resonator 14 by selecting one by one and short-circuiting other resonators.

[0025] The results of resonant frequency measurement are shown in FIG. 3. Resonator numbers 11-18 are shown in the first row, the respective resonant frequencies in the second row in terms of GHz, the frequency deviations from the target frequency (2 GHz) in the third row, and the area to be covered with the dielectric layers to eliminate the deviations in the fourth row. As seen in FIG. 3, all of the resonant frequencies are equal to or a little higher than the target frequency 2 GHz, because the resonators are intentionally so designed. The reason for this is that the resonant frequency can be adjusted only to the lower side, not to the higher side, by accumulating the dielectric member on the resonator. The frequency deviations of the resonators 11-18 are 0.0075, 0, 0.015, 0.01, 0.03, 0.0075, 0.015 and 0.005 GHz, respectively. The No. 12 resonator has the target resonant frequency, requiring no adjustment. The No. 15 resonator shows the highest deviation from the target frequency, requiring a highest degree of adjustment. The deviations of other resonators are all inbetween those of No. 12 and No. 15.

[0026] In other experiments, it has been proved that the resonant frequency decreases by 0.06 GHz, if a whole surface of the resonator is covered by the dielectric layer having a thickness of 1 μm. Also, the resonant frequency decrease is substantially proportional to the amount of the dielectric material covering the resonator surface. Accordingly, if the whole surface of No. 15 resonator is covered with 0.5 μm-thick dielectric layer, its resonant frequency decreases by 0.03 GHz to the target frequency 2 GHz. In the same manner, the area to be covered with the 0.5 μm-thick dielectric layer is determined for other resonators, as shown in the fourth row of the table in FIG. 3. FIG. 1 shows each resonator covered with the dielectric layer having the respective areas shown in FIG. 3 to eliminate the frequency deviation. Thus, the resonant frequencies of all the resonators 11-18 are adjusted to the target frequency 2 GHz, all the deviations being eliminated.

[0027] The dielectric material 31, 33-38 may be accumulated or formed on the resonators 11, 13-18 by using photolithography technology that is widely used in semiconductor manufacturing processes. For example, the dielectric material may be accumulated on the resonator to partially cover its surface by a liftoff process. The mask to be used in the photolithography precess may be the one to individually cover each resonator, or the one to cover all the resonators at the same time. FIGS. 4A and 4B show positions of the dielectric layer partially covering the resonator surface. The dielectric layer 31-38 may be positioned at the open end portion of the resonator 11-18 as shown in FIG. 4A, and it may be positioned at the center portion as shown in FIG. 4B. The degree of resonant frequency decrease is higher when the dielectric layer is positioned at the open end portion than when it is positioned at the center portion. Accordingly, it is preferable to place the dielectric layer at the open end portion if a large amount of frequency shift is required, while it is preferable to place it at the center portion if fine tuning is required.

[0028] Since the resonant frequency of each resonator is adjusted to the target frequency by accumulating individually different amount of the dielectric material, no damage is given to the resonator material such as a superconductive material in the adjusting process. Also, even if a large number of resonators are used in a filter, the adjustment process can be easily carried out. In addition, the frequency adjustment or tuning can be performed with high precision through the photolithography process.

[0029] The shape of the resonator is not limited to the ring shape shown in FIGS. 1 and 2, but it may be variously changed. FIGS. 5A-5D show some of the variations of the resonator shape. Though the dielectric layer is formed on the resonator surface with a uniform thickness, and the covering area is altered depending on the frequency deviation to be adjusted in the foregoing embodiment, the dielectric layer thickness may be altered while keeping the covering area constant. Though the filter using a superconductive material is shown in the foregoing embodiment, the present invention may be applied also to a filter using a usual conductive material.

[0030] While the present invention has been shown and described with reference to the foregoing preferred embodiment, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.