EP0261634A1 - Tunable band filter - Google Patents

Abstract

Adjustable band filter comprising a conductive shielding body (10, 11), formed of two parts (10 and 11) joined to each other on either side of a separation plane (13), the interior of which is a cavity (12) containing a resonant line (15) in half-wavelength carried by a first face (16) of a substrate (14) suspended, coupled at the end, and embedded in notches (17) made in the walls of the first part (10). The first face (16) of the substrate (14) divides the cavity (12) into two non-symmetrical volumes so as to allow a modification of the bandwidth of said filter. Application to the microwave domain.

Description

The invention relates to an adjustable band filter used at high frequencies.

It is known to carry out:
- circular or rectangular cavity filters (in guide) with a high overvoltage coefficient greater than 3000 for example,
- coaxial filters (cylindrical or rectangular) of the end coupling type (of length ≦ αµρ¨ λ / 2; λ being the wavelength of the guided wave) or with coupled lines (length ≦ αµρ¨ λ / 4 ) with an overtension coefficient less than or equal to 1000;
- micro-strip filters (on a dielectric support but with very low overvoltage coefficients less than 200 and significant insertion losses;
- filters with dielectric resonators with intermediate overvoltage coefficients between 1000 and 3000.

However, the production of such filters at high frequencies remains difficult and costly.

Another possibility currently being implemented resides in the use of resonant lines in λ / 2 carried by a suspended substrate and coupled at the end.

An article in "International journal of infrared and millimeter waves" (vol.6, No7, 1985) entitled "Design and performance of millimeter wave end coupled bandpass filters" describes filters of this type in which the resonant lines are formed by sequences periodic discontinuities located along transmission lines to form series of resonators which are coupled together.

The object of the invention is to provide a device having the advantages of this type of filter, that is to say:
- good reproducibility due to the implementation of a chemical photoengraving process;
- low cost due to the simplicity of assembly;
- lack of adjustment.

But it also allows a modification of the bandwidth and it makes possible the integration of a band stop function with absorption or rejection.

The device of the invention makes it possible to work in a frequency range extending from 1 GHz to 100 GHz.

The invention proposes, for this purpose, an adjustable band filter comprising a conductive shielding body, formed of two parts joined to one another on either side of a separation plane, inside which is a cavity containing a half-wavelength resonant line formed of resonators in series coupled together by their ends and carried by a first face of a suspended substrate, coupled at the end, and embedded in notches made in the walls of one of the two parts, characterized in that the first face of the substrate divides the cavity into two non-symmetrical volumes so as to allow a modification of the bandwidth of said filter, and in that the substrate is located in the first part of the shielding body and has its first face located in the separation plane.

The advantage of such a filter is that it uses a simple technique which allows adjustment without the addition of a dielectric or metallic screw in front of its elements.

In addition, the invention makes it possible to produce bandwidth filters of a few% to a few tens of% from 11 to 15 GHz while the principle of filters with end coupling remains vested in a few% of band.

In a first type of embodiment, the invention proposes a filter in which the first face of the substrate is located in the separation plane, and in which the volume of the cavity, located inside the second part, is greater than the one inside the first part.

In another type of embodiment, the invention provides a filter in which the first part is offset with respect to the second part in a transverse direction located in the separation plane, so as to create a kings discontinuity of the cavity in this plane.

Advantageously, the invention proposes a filter in which the two parts of the shielding body have their walls in contact in which transverse notches are machined.

Advantageously to produce an adjustable filter, according to the invention, the second part of the shielding body forms a sheath in which the first part can slide so as to modify the position of the substrate in the cavity.

The characteristics and advantages of the invention will become apparent from the description which follows, by way of nonlimiting example, with reference to the appended figures in which:
FIG. 1 represents the filter according to the invention,
FIG. 2 represents a part of the filter illustrated in FIG. 1,
- Figures 3 and 4 are views in cross section and in longitudinal section of the filter according to the invention in which have been shown the different electrostatic capacities.
- Figures 5 to 8 illustrate several variants of the filter according to the invention.

The filter of the invention, shown in Figure 1, comprises a shielding body (10, 11) of parallelepiped shape inside which is located a cavity 12 of the same shape.

The shielding body is formed by a first part 10 and a second part 11 located on either side of a plane 13 called "separation".

A substrate 14 carrying a resonant line 15 in half wavelength, on a first face 16 is embedded in two notches 17 made in said first part 10, so that its first face 16 is located in the separation plane 13 .

, k étant une constante et λ la longueur d'onde de l'onde guidée.As shown in FIG. 2, this line is for example a line of microstrip type formed by a series resonator coupled by their ends (series capacitive coupling). The length of these resonators is less than or equal to λ / 2 or almost equal to k

, k being a constant and λ the wavelength of the guided wave.

With such a type of filter the excitation of the line is done "at the end", and is a longitudinal excitation.

To obtain a bandwidth adjustment, according to the invention, an asymmetry (proportional to the height of the dielectric) of the upper part is introduced: ie H> Hʹ.

To balance the electromagnetic field, the rules of the art would like that: H> Hʹ.

It can be observed that the capacity distribution of the line is asymmetrical from the electrostatic point of view for the metallizations opposite with, as shown in FIGS.
- Cf angle and leakage capacity;
- CHʹ box / line capacity (lower);
- CH box / line capacity (upper).

It is thus possible, by means of the asymmetry, according to the invention, to act on the bandwidth of the filter, by action on the distribution of these capacities, to thus obtain a widening thereof, while retaining a good adaptation. and limiting losses to a minimum.

avec
- ϑ j : longueur électrique du résonateur
- Bj, j+1 : susceptance de la capacité Cc
- yo : impédance de la ligne.A resonator is defined by its impedance and its coupling to the other lines with the formula:

with
- ϑ j: electrical length of the resonator
- Bj, j + 1: susceptance of the capacitance Cc
- yo: line impedance.

Therefore the modification of H leads to a variation of CH and Cf, therefore of yo. there is therefore an increase in the coupling Cc in proportion to

In FIG. 5, according to a variant of the invention, an asymmetry is introduced by shifting (19) the first part 10 relative to the second part 11 in a transverse direction situated in the separation plane 13, but in this case we can have: H ≧ Hʹ.

To promote the adaptation of line 15, a dielectric tongue 21 which is superimposed on the line can be inserted (22), as shown in FIG. 6, into a small notch 20 placed in the second part 11 of the shielding body. 15 so as to favor the couplings, in particular at the ends thereof.

Similarly, a vertical metallic or dielectric screw could be used, placed in an opening above the circuit 15 which, by adjusting the height of its end, would make it possible to promote said couplings.

It is also possible, as shown in FIG. 7, due to the radial distribution of the electric field, to associate the filter of the invention with a band-stop function by the addition of a guide on at least one of the sides of the shielding body. In this FIG. 7, these guides are produced by transverse notches 23 machined in the walls in contact with the two parts 10 and 11 of the shielding body. If these notches are closed, they form rejection stop bands; if they are partially filled with an absorbent material, they form absorption stop bands.

These notches 23 have only been shown on the first part of the shielding body 10, but they are arranged in the same way on the second part 11.

These notches could just as easily have been made on the wall of the second part 11 opposite the substrate 14.

These notches 23 can be spaced regularly or not.

Such stop-band filtering makes it possible to carry out particular rejection, in particular to weaken certain harmonics.

FIG. 8 represents a housing for an adjustable band filter in which the second part 11 comprises a sheath 24 which envelops the first part 10 which allows a modification 25 of the dimensions of the cavity 12 and of the dimension H relative to the dimension Hʹ as shown in Figure 1.

By way of nonlimiting example, a filter has been produced, as shown in FIG. 1, having the following dimensions, considering the body arranged vertically:
- width of the cavity: 5.6 cm
- height of the cavity (H + Hʹ): 2.85 cm
- thickness of the dielectric: 0.254 c,
- metallization thickness: 17 µm
- length of the notches (17): 0.5 cm.

A filter centered on 15 GHz is thus obtained with a passband of 2.7 GHz.

For better operation, the shielding body has a very low roughness state.

It is understood that the present invention has only been described and shown as a preferred example and that its constituent elements can be replaced by equivalent elements without, however, departing from the scope of the invention.

Thus one could thus have metallizations on each side of the substrate.

Likewise, the body could have a shape other than a parallelepiped.

Claims (9)

1 / Adjustable band filter comprising a conductive shielding body (10, 11), formed of two parts (10 and 11) joined to each other on either side of a separation plane (13) , inside which is a cavity (12) containing a half-wavelength resonant line (15) formed of resonators in series coupled together by their ends and carried by a first face (16) of a suspended substrate (14), coupled at the end, and embedded in two parts, characterized in that the first face (16) of the substrate (14) divides the cavity (12) into two non-symmetrical volumes so as to allow a modification of the bandwidth of said filter, and in that the substrate (14) is located in the first part (10) of the shielding body and has its first face (16) located in the separation plane (13). 2 / Filter according to claim 1, characterized the volume of the cavity (12) located on the side of the first face (16) of the substrate is larger than the other. 3 / Filter according to claim 1, characterized in that the first part (10) is offset with respect to the second part (11) in a transverse direction located in the separation plane (13), so as to create a discontinuity of the walls of the cavity in this plane (13). 4 / Filter according to any one of the preceding claims, characterized in that the cavity (12) is of parallelepiped shape, the separation plane (13) dividing it into two volumes of the same shape. 5 / Filter according to any one of the preceding claims, characterized in that the two parts (10 and 11) of the shielding body have their walls in contact in which are machined notches (23) transverse. 6 / Filter according to claim 5, characterized in that these notches (23) are regularly distributed in the longitudinal direction. 7 / Filter according to any one of claims 5 or 6, characterized in that these notches (23) are partially filled with an absorbent material. 8 / Filter according to any one of the preceding claims, characterized in that the second part (11) has a notch (20) in which a dielectric strip (21) can slide which is thus positioned in the vicinity of the first face ( 16) of the substrate (14) so as to improve the coupling. 9 / Filter according to any one of the preceding claims, characterized in that the second part (11) forms a sheath (24) in which the first part (10) can slide so as to modify the position of the substrate (14) in the cavity (12).

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