EP0165158A1 - Dielectric filter with a variable centre frequency - Google Patents

Abstract

Dielectric filter with variable central frequency comprising at least one dielectric resonator (RES1, RES2) consisting of a body (4, 14) of dielectric ceramic coated with electrodes (1, 2, 3, 11, 12, 13) as well as electromagnetic coupling means (II, S1) for signal input and output on the filter. According to the invention, the filter is characterized in that it comprises a variable capacitance (D1, D2) connected between the electrodes (1, 2, 11, 12) of the resonator, in a capacitive zone of said resonator (RES1, RES2) , allowing the central frequency of said filter to be varied continuously.

Description

The present invention relates to a dielectric filter with variable central frequency comprising at least one dielectric resonator constituted by a dielectric ceramic body coated with electrodes as well as means for electromagnetic coupling of signal input and output on the filter.

Ultra-high frequency (UHF) filters are widely used in the art, for example, in telecommunications to choose a specific frequency band. In frequencies up to about 700 MHz, filters with piezoelectric resonators can be used. However, for the entire UHF band, extending up to 3 GHz, these are no longer suitable and use is made of dielectric filters which have performances similar to cavity filters but which are much less expensive and bulky. In addition, in certain applications, the bandwidth of the piezoelectric filters proves to be too narrow.

Dielectric filters of this type have been described in French application 82.18236 filed on October 23, 1982 in the name of THOMSON-CSF. Such filters of the parallelepiped type use compositions of the (Zr, Sn) TI0 _{4 type} .

Using such parallelepiped resonators, bandpass filters can be produced by placing a plurality of resonators next to each other. Generally, it is preferred to use quarter-wave type resonators because of their shorter length. The electrode metallizations of these resonators are respectively mutually parallel and perpendicular to the insulating substrate on which said substrates are fixed. The coupling between these resonators is made by mutual inductance. The connection to the input and the output of the filter can be done either by welding directly on the electrode of the first and last resonator respectively, or by means of a loop placed above the first and last resonator as an exciter and collector. In a manner known per se, to obtain a filter having a band determined bandwidth, resonators having a specific and determined frequency are used and by coupling said resonators according to the coupling coefficients required by the calculation of the filter, and this by carefully adjusting the distance between the resonators. When it is desired to make a multichannel filter using these resonators, a plurality of resonators generally placed close to each other is used. The natural frequency of each resonator is then tuned to the different frequencies of the channels and it is sufficient to switch the input and the output of the filter to the desired resonator to select the corresponding channel. However, such a solution requires the use of as many resonators as channels, and per filter pole. This results in a significant bulk of the filter, which becomes prohibitive when the number of channels desired is greater than two.

The invention makes it possible to solve the problem posed and to use only a single resonator, or a single set of resonators coupled together according to the quality of the desired filter, while allowing the selection of a large number of channels.

The dielectric filter with variable central frequency according to the invention is characterized in that it comprises a variable capacitance connected between the electrodes of the dielectric resonator, in a capacitive zone of said resonator, making it possible to continuously vary the central frequency of said filter. It is noted that such filters allow a displacement of the central frequency, with a substantially constant passband, of several times the value of this passband, with a very low level loss.

According to a preferred embodiment, the filter according to the invention is characterized in that it comprises a plurality of dielectric resonators arranged close to one another and cut off from each other electromagnetically by air.

Preferably, each resonator is of the quarter-wave type, and consists of a parallelepiped bar having four large faces and two small faces metallized on its large opposite faces. one from the other and on a small face located in the extension of said large faces. The resonators are preferably secured to a support having a low loss factor, by one of their large non-metallized faces. In the case of a plurality of resonators, the small metallized faces will preferably be located on the same side of the large faces.

In the case of a resonator of the aforementioned quarter-wave type, which behaves like an LC circuit whose inductive part is located near the metallization of the small face and whose capacitive part is located between the metallizations of the the electrode of the two large faces, near the small non-metallized face, the input and / or output of the signal will take place on a link placed on an electrode of large face at a point close to the small metallized face while the variable capacity will be connected between the ends of the electrodes placed on the large faces, near the small non-metallized face.

According to a particularly advantageous embodiment of the invention, facilitating the small size of the filter, the variable capacity will be a diode with variable capacity, controlled by a DC voltage generally disposed outside the filter.

• - la figure 1, une vue schématique d'un résonateur parallélépipédique quart-d'onde utilisé dans le cadre de l'invention ;
• - la figure 2, une vue d'une implantation schématiqué d'un filtre passe-bande à deux pôles utilisant deux résonateurs illustrés sur la figure 1 ;
• - la figure 3, un schéma de réalisation d'un filtre passe-bande à deux pôles à fréquence centrale variable selon l'invention ;
• - la figure 4, une représentation des courbes de réponses d'un filtre tel que représenté sur la figure 3, avec différentes tensions d'alimentation sur les diodes varicaps.

The invention will be better understood with the aid of the following embodiments, given without limitation, in accordance with the figures which represent:

• - Figure 1, a schematic view of a quarter-wave parallelepiped resonator used in the context of the invention;
• - Figure 2, a view of a schematic layout of a two-pole bandpass filter using two resonators illustrated in Figure 1;
• - Figure 3, an embodiment of a bandpass filter with two poles at variable central frequency according to the invention;
• - Figure 4, a representation of the response curves of a filter as shown in Figure 3, with different supply voltages on the varicap diodes.

In Figure l, a ceramic resonator is shown parallelepiped dielectric as mentioned above and described in the French patent application cited above. It comprises a parallelepipedic body 4 of dielectric ceramic coated on two of its large opposite faces with a metal layer 1 and 2 having an electrode function, and on one of its small faces, and in the extension of the electrodes of the large faces 1 and 2 opposite each other, an electrode metallization 3 ensuring operation in a quarter-wave circuit.

Figure 2 shows the arrangement of two resonators as described in Figure 1 to achieve a bandpass filter with two poles. It comprises a parallelepipedic housing 15 comprising a rectangular central cavity 16 made of a material having little electromagnetic loss. Two resonators 17 and 18 are made integral with the bottom of the housing 15 in the manner illustrated in this FIG. 2: the electrodes of the large faces respectively 1 and 2, 11 and 12 are arranged parallel to each other, 2 and 11 being distant from a distance d, while the electrodes of the small faces 3 and 13 are placed in the extension of one another. All these electrodes are located in the planes perpendicular to the plane of the cavity 16. The distance d between the resonators 17 and 18 determines the coupling coefficient between these two resonators, this in a manner known per se. The signal is input by Il substantially at the intersection of electrodes 1 and 3, while the signal output is by SI substantially at the intersection of electrodes 12 and 13.

FIG. 3 represents an embodiment diagram of a dielectric filter with variable central frequency, with two poles. It consists of two resonators RESI and RES2 arranged as shown in Figure 2. The input of the signal on the filter is done in 1 with an electromagnetic connection or coupling in It arranged as explained in Figure 2. The output of the filter is carried out in S with a connection or a coupling SI carried out at the place indicated in figure 2. Near the electrodes 1 and 2, on the side of the small non-metallized parallelepiped face 4, are connected, to the electrode 1, the first end of a capacitor C1, the other end of which is connected on the one hand to a resistor RI and on the other hand, to the cathode of a varicap diode Dl whose anode is connected to the end of electrode 2, on the side of the small non-metallized face. The second end of the resistor R1 is connected to a variable DC voltage source V. In the same way, the electrode 12 of the resonator RES2 is connected to a first end of the capacitor C2, the second end of which is connected on the one hand to the cathode of the varicap diode, the other end of which is connected to the electrode 11 near its end on the side of the small non-metallized face of the resonator, and on the other hand to the resistor R2, the other end of which is also connected to the variable DC voltage source V. In parallel between the input I and the ground C is placed a shock inductor L1, while at the output is also placed a shock inductor L2. Of course, these chokes can have variable values depending on the application envisaged and are not properly part of the filter according to the invention. They make it possible to guarantee the polarization of the diode with variable capacity, by their resistance having an adapted value.

FIG. 4 shows the passbands of the filter as a function of the voltage V applied to the varicap diodes for a filter having the parameters mentioned in the example below. The OdB level corresponds to the level of the input signal. There is a signal at the output, the loss of which is not greater than 3dB. The bandwidths of the - 3dB filter are mentioned numerically in the figure, making it possible to note the excellent results obtained using the filter according to the invention.

Of course, it is possible to produce filters comprising a plurality of poles using a plurality of resonators. For this, each resonator is placed at a distance d from the neighboring resonator (see FIG. 2) according to the desired coupling coefficient in a manner well known to those skilled in the art. Each circuit, such as CI, Dl and RI of each resonator such as RESI is connected in the same way to said resonator, the end of each resistor, such as RI, being connected directly to the voltage source V. Of course, the connections Il and SI are respectively made on the first filter of the plurality and on the last filter of the latter. In all cases, it will however be necessary to provide polarization means suitable for each diode with variable capacitance.

• RI, R2 = 10k _Q
• Dl, D2 = diode à capacité variable BA 149 (6pF à 4 volts) C1, C2 = 1,5 pF
• V = source de tension continue, variable de 0 à 30 volts.

Example: the electrical diagram is that illustrated in Figure 3 with the following numerical values:

• RI, R2 = 10k _Q
• Dl, D2 = BA 149 variable capacitance diode (6pF at 4 volts) C1, C2 = 1.5 pF
• V = DC voltage source, variable from 0 to 30 volts.

The resonators RES1, RES2 have the arrangement shown in FIG. 2. They are bonded to an alumina substrate, at a distance d = 7 mm. Their dimensions are 16.5 x 7.5 x 7.5 (in millimeters). The electrode metallizations are made of silver, have a thickness of approximately 50 µ, and are deposited using a lacquer by screen printing.

The entire resonator is placed in an aluminum housing, the distance between the faces of the resonators and the upper and lower walls of the housing being of the order of 3 mm.

Throughout the description, the term electrode has been used to define the conductive walls of the resonant cavity.

Claims (6)

1. Dielectric filter with variable central frequency comprising at least one dielectric resonator (RES !, RES2) consisting of a body (4, 14) of dielectric ceramic coated with electrodes (1, 2, 3, 11, 12, 13) as well as signal input and output electromagnetic coupling means (I1, SI) on the filter, characterized in that it comprises a variable capacity (Dl, D2) connected between the electrodes (1, 2, 11, 12) of the resonator (RESI, RES2), in a capacitive zone of said resonator, making it possible to continuously vary the central frequency of said filter. 2. Dielectric filter according to claim 1, characterized in that it comprises a plurality of dielectric resonators (RESI, RES2) arranged close to each other and coupled together electromagnetically by air. 3. Dielectric filter according to claim 1 or 2, characterized in that each resonator (RES1, RES2) is of the quarter-wave type, comprising a parallelepiped bar having four large faces and two small faces, metallized on two large faces ( 1, 2, 11, 12) and on a small face (3, 13) located in the extension of the two large faces. 4. Filter according to one of claims 2 or 3, characterized in that the resonators are integral with a support having a low loss factor, by one of their large non-metallized faces, the small metallized faces (3, 13) being located on the same side of the faces (1, 2, 11, 12). 5. Filter according to one of claims 3 or 4, characterized in that the input and / or output of the signal takes place on an electrode placed on a large face (1, 2, 11, 12), in one point close to the small metallized face (3, 13), while the variable capacity (Dl, D2) is connected between the ends of the electrodes (1, 2, 11, 12) placed on the large faces, near the small non-metallic side. 6. Filter according to one of claims 1 to 5, characterized in that the variable capacity is a variable capacity diode (DI, D2) associated with corresponding polarization means (LI, L2).

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