A resonator comprising an equivalent circuit including an LC series circuit having a first capacitance element and inductance elements connected in series to both sides of the first capacitance element, and a second capacitance element connected parallel to the LC series circuit. A plurality of resonators having this construction may be magnetically connected in series to provide a filter, by utilizing the two inductance elements constituting parts of the resonator and without necessitating separate coupling members such as capacitors or coils.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a microwave resonator suitable for a 
filter or an oscillator used in a microwave frequency range including and 
above the UHF band. 
(2) Description of the Prior Art 
Examples of known resonators employed in the above frequency range include 
a dielectric coaxial resonator and a ceramic resonator. The dielectric 
resonator comprises an equivalent circuit including one capacitance 
element and one inductance element connected in parallel. On the other 
hand, the ceramic resonator comprises an equivalent circuit including a 
series circuit having one inductance element and one capacitance element, 
and a further capacitance element connected parallel to the series 
circuit. 
Since either resonator includes only one inductance element in the 
equivalent circuit, it is impossible to magnetically connect a plurality 
of resonators by utilizing the inductance element. Therefore, a filter 
formed of resonators connected in a multistep mode requires separate 
coupling members such as capacitors, and has the disadvantage of being 
bulky in construction. 
SUMMARY OF THE INVENTION 
A primary object of the present invention, therefore, is to provide 
resonators easy to be magnetically connected to each other without 
necessitating separate coupling members. 
Another object of the invention is to provide a small and economical 
resonator which may be formed by pattern or screen printing on a single 
dielectric substrate. 
In order to achieve the above and other objects, a preferred embodiment of 
the present invention comprises two pairs of capacitor electrode layers, 
one layer of each pair formed on each of the front and back faces of a 
dielectric substrate and opposed to one another across the dielectric 
substrate, the two capacitor electrode layers formed on each of the front 
face and back face of the dielectric substrate being interconnected by one 
coil layer. One pair of capacitor electrode layers opposed to each other 
across the dielectric substrate constitutes a capacitance element, and 
each of the coil layers constitutes an inductance element. Therefore, the 
above resonator comprises a circuit construction in which a capacitance 
element is connected in parallel in an equivalent manner to an LC series 
circuit including a capacitance element and inductance elements, the LC 
series circuit being arranged across the capacitance element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, a resonator according to the present invention 
comprises an equivalent circuit including an LC series circuit having a 
first capacitance element C1 and inductance elements L1 and L2 connected 
in series to both sides of the first capacitance element C1, and a second 
capacitance element C2 connected in parallel to the LC series circuit. As 
shown in FIGS. 2A, B, C, the resonator comprising this equivalent circuit 
is formed on a dielectric plate or substrate. In FIGS. 2A, B, C, number 1 
denotes the dielectric substrate which may be of a ceramic or other 
dielectric material. The substrate 1 has a front face 1a and a back face 
1b including, respectively, capacitor electrode layers 2a, 2b, 3a and 3b 
and coil layers 4a and 5a which are formed, for example, of screen printed 
silver paste. The electrode layers 2a and 3a are opposed to each other 
across the substrate 1 along one side thereof, one on the front face 1a 
and the other on the back face 1b. Likewise, the electrode layers 2b and 
3b are opposed to each other across the substrate 1 along the opposite 
side thereof. Therefore, the electrode layers 2a, 2b, 3a and 3b constitute 
the capacitance elements C1 and C2 having capacities determined by the 
permittivity and thickness of the substrate 1 and by the areas over which 
the capacitor electrodes layers are opposed to each other. On the other 
hand, the coil layers 4a and 5a are formed so as to connect the two 
capacitor electrode layers 2a and 2b on the front face of substrate 1, and 
the two capacitor electrode layers 3a and 3b on the back face of 
substrate 1, at opposite ends of the substrate, respectively. The coil 
layers 4a and 5a constitute the inductance elements L1 and L2 in high 
frequency. 
In the resonator according to this embodiment particularly, the two coil 
layers 4a and 5a have a selected length and are formed close to ends of 
the substrate 1 so that the resonator may be magnetically connected to 
other resonators. Further, it is desirable that the two coil layers 4a and 
5a in one resonator be spaced from each other as much as possible in order 
to prevent the coil layers 4a and 5a from being magnetically connected to 
each other and from having an electrostatic capacity. In the illustrated 
example the coil layers 4a and 5a are spaced from each other to a maximum 
extent. 
To resonate this resonator in fr=504 MHz, L1 and L2 are derived, for 
example, from the following equation provided C1=18 pF: 
##EQU1## 
If L1=L2 here, then L1=L2=2.77 nH. C2 may be selected according to an 
impedance applied, regardless of the resonant frequency. For this 
embodiment 53 pF was selected since C2=53 pF resulted in a maximum value 
of Q factor when the impedance was 50.OMEGA.. Where the dielectric 
substrate 1 has a permittivity .epsilon. set to 80 and a 0.4 mm thickness, 
the capacitance elements C1 and C2 having the above electrostatic capacity 
are realized by selecting the dimensions to be l1=7 mm, l2=1.5 mm, l3=7 mm 
and l4=3.5 mm. The inductance of inductance elements L1 and L2 is realized 
by selecting the dimension l5=l6=6 mm. The coil layers 4a and 5a have 
width W1, W2 which are not related to the inductance value. However, the 
greater the width, the smaller is the resistance and the higher becomes 
the value of Q factor which is desirable. In this example, W1=W2=1.5 mm. 
FIG. 3 shows frequency characteristics of the resonator incorporating the 
specific dimensions noted above. As seen from the characteristics, a very 
high value of Q factor 148 has been achieved. The reasons for making such 
a very high value of Q factor possible appear to be that the above 
resonator comprises the peculiar equivalent circuit construction that find 
no parallel in the prior art, and that the resistance has been reduced by 
increasing the width of the coil layers 4a and 5a. 
It will be understood that in the above resonator the dielectric substrate 
1 has the capacitor electrode layers 2a, 2b, 3a and 3b and the coil layers 
4a and 5a formed in the same pattern on its front face and back face. By 
uniformalizing the pattern for the front and back faces, printing masks of 
the same pattern may be used for manufacture which leads to a very high 
productivity. 
While in the above embodiment the dielectric substrate 1 has the 0.4 mm 
thickness, by varying its thickness the capacity of the capacitance 
elements C1 and C2 may be varied to realize different resonant 
frequencies. Therefore, the thickness of substrate 1 may be selected to be 
appropriate with relation to a frequency to be used. 
FIG. 4 is a front view of an example of a bandpass filter where three of 
the resonators as in the foregoing embodiment are magnetically 
interconnected. In this example, three resonators A, B and C are arranged 
in a multistep mode and are interconnected by a magnetic coupling occuring 
between coil layers 4a and 5a of the adjacent resonators. As illustrated 
by this example, the resonators A, B and C may just be arranged side by 
side to establish the magnetic connection therebetween, which is made 
possible by forming the coil layers close to the ends of the dielectric 
substrate 1. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore unless otherwise such changes and modifications depart from 
the scope of the present invention, they should be construed as being 
included therein.