EP0053986A2 - Bandpass filter tunable to a predetermined number of discrete frequencies in a broad frequency band - Google Patents

Abstract

Coaxial filter of which at least one of the internal (3) and external (1) conductors is divided into sections (S ₁ to S ₄ ) by cuts (11 to 14) formed by annular slots of very small thickness relative to the wavelength.

Each section S ₁ to S ₄ comprises at least one reactive tuning element (21 to 24) such as a portion of open coaxial line (21) or short-circuited (22 to 24) and at least one switching element (31 to 34) arranged in the vicinity of the corresponding cut-off (11 to 14) to selectively short-circuit this cut-off or to insert the corresponding reactive tuning element in response to electronic control means.

Application to telecommunications.

Description

Bandpass filter tunable over a predetermined number of discrete frequencies distributed over a wide frequency band.

The subject of the present invention is a bandpass filter tunable over a predetermined number of discrete frequencies distributed over a wide frequency band, comprising at least one coaxial resonant cavity defined by an external conductor and an internal conductor.

We already know many examples of bandpass filters made from at least one resonant cavity constituted by a coaxial line comprising an internal conductor and an external conductor and terminated by a short circuit, input coupling means or outlet being associated with the cavity.

Generally, the passband of such a filter is very narrow and the tuning means on a determined frequency of this passband are constituted by mechanical adjustment means.

Microwave bandpass filters have also been produced constituted by a single coaxial line divided into several adjacent resonant cavities separated by coupling members such as pistons, a tuning device being provided for each resonator and a coupling adjustment device being associated with each coupling member between two successive resonant cavities.

Such a type of filter can thus constitute a standard element which can be adapted to slightly different frequencies in a frequency band, by manual adjustments of the position of the pistons forming coupling members for example, or by action on other parameters of the geometry of the system.

We also know hyper filters tunable rod frequencies comprising waveguide sections divided into several adjacent resonators by curtains of conductive rods and comprising a mechanical tuning device for each resonator such as an adjustable position dielectric stick and a coupling setting for each curtain of rods, such as a metal screw.

With such a filter, it is possible to adjust the characteristics of a standard element from the outside, by manual action on mechanical adjustment elements, and thus to adapt the bandwidth to a predetermined channel inside. 'a range of frequencies. However, it is not possible to carry out, during operation, the modification of the characteristics of the filter to effect the instantaneous passage from a discrete frequency to another discrete frequency defined within a wide band of frequencies d 'one or more octaves.

The present invention specifically aims to make it possible to produce a band-pass filter, essentially constituted by a coaxial cavity, the practically instantaneous tuning of which is possible for a certain number of discrete frequencies distributed in a wide frequency band.

According to the invention, a coaxial bandpass filter is thus essentially characterized in that at least one of the internal and external conductors of the coaxial cavity is divided into sections separated by cuts constituted by annular slots whose thickness is very small compared to the wavelength corresponding to the average frequency of the pass band and in that each section comprises at least one reactive tuning element and at least one switching element arranged in the vicinity of the cut of the section corresponding to selectively short-circuit this cut or insert the reactive tuning element of the corresponding section in response to electronic control means.

Each tuning reactive element consists of a portion of open or short-circuited coaxial line produced in an internal conductor or external conductor section.

The cavities of revolution formed in sections of internal or external conductor for producing an open or short-circuited coaxial line have transverse dimensions much greater than the thickness of a cut but small compared to the wavelength corresponding to the average bandwidth frequency.

Thus, according to the present invention, thanks to a coaxial resonant cavity with predetermined geometric characteristics, it is possible to achieve, without manual action on mechanical adjustment elements, an almost instantaneous tuning of the cavity on a discrete tuning frequency chosen from a large number of predetermined frequencies distributed in a wide frequency band, thanks to a selective action for each of the switches associated with the breaks, which by an all-or-nothing command allow the closing of a break or its opening with the commissioning of the associated reactive element. It follows that with N cuts, the number of discrete frequencies for which it is possible to achieve a tuning of the resonant cavity amounts to 2 ^N , taking into account the possible combinations of states (open or closed) of the N cuts established using associated switches controlled by digital electronic tuning means.

Advantageously, for each section, several switching elements supplied with parallel are regularly distributed along the cut to achieve the short-circuiting of said cut or the insertion of a reactive element.

According to one embodiment, the switching elements consist of electromagnetic relays whose dimensions are small with respect to the average operating wavelength, and which are arranged in the immediate vicinity of the annular slots constituting the cuts between sections.

According to another preferred embodiment, the switching elements are constituted by PIN diodes arranged in the immediate vicinity of the annular slots constituting the cuts between sections.

The control of the switching elements is effected by direct current by means of insulated conductor wires decoupled from the high frequency and incorporated in the internal or external conductors.

The realization of a bandpass filter. According to the invention is facilitated if the internal and external conductors of the coaxial cavity are composed of superimposed metallic elements screwed into each other.

According to a particularly interesting embodiment, the dimensions of the coaxial cavity, those of the reactive elements of the N sections and the position of the N cuts are determined to define an approximately constant law Af, where f denotes any one of the 2 ^N discrete frequencies agreement obtained by the selective insertion of the N reactive elements and & .f represents the average of the differences between this frequency f and the adjacent frequencies among the 2N possible frequencies.

The bandpass filter according to the invention can be used in the most diverse applications but is advantageously applied to the connection to a single aerial, of several transmitters or receivers working on different frequencies.

• - la Fig. 1 est une vue schématique en coupe axiale d'un premier mode de réalisation d'un filtre passe-bande selon l'invention,
• - la Fig.2 est une vue schématique en coupe axiale d'un second mode de réalisation d'un filtre passe-bande selon l'invention, et
• - la Fig 3 est une vue partielle en coupe axiale montrant l'assemblage possible de différents éléments constitutifs des'différentes sections d'un filtre selon l'invention.

Other characteristics and advantages of the invention will emerge from the description which follows on from particular embodiments, given by way of nonlimiting examples with reference to the appended drawings, in which:

• - Fig. 1 is a schematic view in axial section of a first embodiment of a bandpass filter according to the invention,
• FIG. 2 is a schematic view in axial section of a second embodiment of a bandpass filter according to the invention, and
• - Fig 3 is a partial view in axial section showing the possible assembly of different components of the different sections of a filter according to the invention.

Reference will first be made to FIG. 1 which represents the basic configuration of a bandpass filter according to the invention divided, by way of example, into four sections S ₁ to S4 ^.

The coaxial cavity of the filter of Fig 1 has an external body or conductor 1 closed by a cover 2 which is screwed onto the body 1. The internal conductor 3 is divided into sections (Si to S ₄ ) separated by cuts ₁ 1 to 14 having the form of annular slots whose thickness e is very small compared to the wavelength corresponding to the average frequency of the passband of the filter. This configuration is possible if the internal conductors 3 and external 1 are for example produced in several superposed sections such as 301,302,303 or 101,102,103 respectively (Fig 3) screwed into each other. Thus, a part 101 of external conductor can be connected to a part 102 by a screw connection lOla, 102b, the part 102 superimposed on a part 103 being itself connected to the latter by a screw connection 102a, 103b. Similarly, .the 301,302,303 sections of inner conductor 3 may be joined together by portions 3 ₀ 1a, 302b and 302a, 303b are screwed into each other. In this case, the positions of the joint planes are only determined by machining conveniences and only intervene in the operation with respect to electromagnetic waves the location of cuts such as 12,13,12 ', 13' as well as the forms cavities such as 22,23, 22 ', 23'. Consequently, in FIGS. 1 and 2, the lines of separation between the various parts constituting the internal 3 and external 1 conductors of the coaxial cavity have not been shown, but only the cuts such as 11 to 14, which define the differential _your sections _S1 to ^S4.

The external 1 and internal 3 metal conductors can be made, for example, of brass. An insulating cylinder 5, for example made of polytetrafluoroethylene, which bears on the cover 2, ensures the centering of the internal conductor 3 relative to the body 1 while exerting suitable compression on the various sections constituting the internal conductor 3.

In general, the coaxial cavity behaves like a quarter wave line at resonance: substantially for the average frequency of the pass band of the filter. As can be seen in FIG. 1, each section S ₁ to S ₄ comprises a tuning reactance 21 to 24 incorporated in the internal conductor 3 and which can be put into service at the level of the corresponding cut 11 to 14. A reactance okay, can be constituted by a short-circuited coaxial line (reactances 22, 23, 24) which is equivalent to a self inductance or by an open line (reactance 21) which is equivalent to a capacity. In FIG. 1, the open coaxial line 21, which constitutes the tuning reactance of the first section S ₁ , is centered by means of a cylindrical sleeve 4 made of a dielectric material. as such as polytetrafluoroethylene, and bearing on the metal bottom 6 of the coaxial cavity which realizes the short circuit of the external conductor 1. The tuning reactances. second, third and fourth sections S ₂ , S ₃ , S ₄ are themselves constituted by three portions of coaxial lines in the air short-circuited, defined by annular cavities whose shapes and dimensions can be diverse. However, in order for the coaxial cavity of the filter to have sufficiently high no-load overvoltage coefficients, it is necessary to reduce the losses of the circuits and in particular those of the open or short-circuited transmission lines including the input impedances identify with tuning reactances. The transverse dimensions of the cavities 21 to 24 must therefore have non-negligible transverse dimensions, although always small compared to the wavelength to avoid parasitic modes of the TE or TM type.

As will be explained in more detail below, at least one switch 31, 32, 33, 34 is associated with each cut 11, 12, 13, 14 and makes it possible either to short-circuit this cut by closing a contact. 41,42,43,44, that is to allow the insertion of the tuning reactance 21,22,23,24 included in the section S ₁ , S ₂ , S ₃ , S ₄ corresponding to this cut-off. It can thus be seen that the effective tuning frequency of the filter depends on the state of the switches 31 to 34 and can be chosen from a large number of discrete frequencies even for a relatively small number N of cuts, since the number of discrete frequencies possible agreement depends on the combination of different states possible of the different cuts and is thus worth 2 ^N , each cut allowing the putting into service or not of a tuning reactance.

Figure 1 shows a cavity model which corresponds to N = 4 and has 2 ⁴ = 16 discrete tuning frequencies. For example, such a cavity can operate in the UHF band (225 to 400 MHz).

The dimensions of the coaxial cavity constituting the filter, the position of the cutouts 11 to 14 and the dimensions of the reactances 21 to 24 can be optimized to achieve an approximately constant law Δf, f denoting any of the N tuning frequencies and Δf being the average of the differences between this frequency and the adjacent tuning frequencies.

By way of example, we give below numerical values which, applied to a filter with four sections such as that shown in FIG. 1, make it possible to obtain such an approximately constant law Δf: ^f

For an internal conductor 3 of radius a _o = 32 mm, an external conductor 1 whose internal radius is bo = 41, lmm, cavity lengths ρ _t = 245.8 mm and ρ ' _t = 32mm (see Fig 1) , an input capacity of the open coaxial line (7) c _e = 2.91 p ^F and an end capacity 8 of the capacitive reactance 21 reduced to the first cut 11 of c _{el =} 2.8 pF, the table I gives the position d (k) relative to the bottom of the cut-out cavity of rank "k", the dimensions a _k and b _k of the corresponding tuning reactance (ie the radii of the coaxial faces of the annular cavity defining the tuning reactance) and the capacity Cp (k) of the cutoff "k" in pF which allows the theoretical and experimental frequencies to be adjusted as best as possible.

Table II gives the list of the discrete tuning frequencies obtained with the example of a four-section filter defined above, as a function of the state of the cuts 11 to 14. The letter F represents a short-circuited closed cut while that the letter 0 represents an open cut ensuring the insertion of a tuning reactance. For each different combination of the cutoff states 11 to 14, Table II indicates in MHz the theoretical frequency Ft, the frequency F _e obtained experimentally and the difference F _e - Ft.

If FIG. 1 shows an example of a filter with four sections S1 to S ₄ and four annular slots forming cuts 11 to 14, the invention naturally takes into account filters of this type having a different number of sections, and thus having a number different from tuning frequencies. Thus, there is shown in FIG. 2 a coaxial filter comprising seven cuts 11 to 14 and 12 'to 14' which make it possible to define 27 = 128 discrete frequencies, for example in the same frequency band as that taken into account in the example given above.

Figure 2, like Figure 3 further show that the cuts can be made both in the external conductor .1 in the internal conductor 3 of the coaxial cavity. Furthermore, the annular cavities 22 ', 23', 24 'formed in the external conductor 1 to produce coaxial lines forming the tuning reactances associated with the cuts 12', 13 ', 14' respectively are not necessarily identical to. those (12,13,14) formed in the corresponding internal part of the internal conductor 3. Furthermore, the cavities 12 ′, 13 ′, 14 ′ could also be used alone, independently of the cavities 11 to 14 of the internal conductor 3, these last remaining for example short-circuited.

The switches 31 to 34, 32 'to 34' actuating the contacts 41 to 44, 42 'to 44' to define the open or closed state of a cut 11 to 14, 12 'to 14' can be electromagnetic relays. However, preferably small PIN diodes are used. In general, the dimensions of the switching elements 31 to 34, 32 'to 34' must be small with respect to the operating wavelength and these switching elements must be placed in the cavity as close as possible of the slot ring to short-circuit.

The switches (31 to 34, 32 'to 34') are controlled by direct current by means of insulated conductors (not shown) arranged either in the axis of the central element 3 or in the wall of the element outside 1 of the coaxial cavity of the filter. A system for decoupling conductors supplied with direct current, consisting of capacitors, makes it possible to isolate the high frequency from the outside.

The switches (31 to 34, 32 'to 34') are preferably each composed of a plurality of elements regularly distributed along the cut (11 to 14, 12 'to 14') corresponding and supplied in parallel.

Indeed, for each cut such as 11, a plurality of short-circuit elements such as 31, 41 arranged symmetrically (for example four elements 31, 41 distributed at 90 ° from one another) and supplied in parallel ensures a better distribution of currents.

The electronic circuits making it possible to determine the selective supply of the various switches 31 to 34, 32 'to 34' fixing the state of the cuts 11 to 14, 12 'to 14' according to the discrete frequency of tuning chosen, can be composed of very common logic circuits.

The connections of the filter to the input and the output (transmitter and receiver) are also carried out in a conventional manner by inductive or capacitive couplings. FIG. 2 shows an example of an input or output coupling, produced by a small antenna 9 (capacitive coupling) connected to a coaxial base 1 ₀ .

The bandpass filter according to the invention makes it possible in particular to make the connection of several transmitters, or receivers working on different frequencies, at a single air. The field of application of such a filter is nevertheless much wider in the field of telecommunications, and another application may lie, for example, in the suppression of parasitic noise.

Claims (7)

1. Bandpass filter tunable over a predetermined number of discrete frequencies distributed over a wide frequency band, comprising at least one coaxial resonant cavity defined by an external conductor (1) and an internal conductor (3) characterized in that at at least one of the internal and external conductors of the coaxial cavity is divided into sections (S ₁ to S ₄ ) separated by cuts (11 to 14, 12 'to 14') constituted by annular slots whose thickness is very low compared to the wavelength corresponding to the average frequency of the passband and in that each section (S ₁ to 5 ₄ ) comprises at least one reactive tuning element (21 to 24, 22 'to 24' ), constituted by a cavity of revolution formed in sections of internal or external conductor for producing an open coaxial line (21) or short-circuited (22 to 24, 22 'to 24') having transverse dimensions very much greater than the thickness of a cut, but small compared to the length r wave corresponding to the average frequency of the pass band, and at least one switching element (31 to 34, 32 'to 34') arranged in the vicinity of the cutoff (11 to 14, 12 'to 14') de- the corresponding section to selectively short-circuit this cut-out or to insert the reactive tuning element of the corresponding section in response to electronic control means. 2. Filter according to claim 1, characterized in that for each section several switching elements (31 to 34, 32 'to 34') supplied in parallel are regularly distributed along the cut-out to short-circuit the said cut (11 to 14, 12 'to 14') or the insertion of a reactive element (21 to 24, 22 'to 24'). 3. Filter according to any one of claims 1 or 2, characterized in that the switching elements (31 to 34, 32 'to 34') are constituted by electromagnetic relays whose dimensions are small vis-à-vis the average operating wavelength, and which are arranged in the immediate vicinity of the annular slots constituting the cuts between sections. 4. Filter according to any one of claims 1 to 3, characterized in that the switching elements (31 to 34, 32 'to 34') are constituted by PYN diodes arranged in the immediate vicinity of the annular slots constituting the cuts between sections . 5. Filter according to any one of claims 1 to 4, characterized in that the control of the switching elements (31 to 34, 32 'to 34') is effected by direct current by means of isolated conductive wires decoupled from the high frequency. and incorporated in the internal or external conductors. 6. Filter according to any one of claims 1 to 5, characterized in that the external (1) and internal (3) conductors of the coaxial cavity are composed of metallic elements (101, 102; 301, 302, 303) superimposed screwed into each other. 7. Filter according to any one of claims 1 to 6, characterized in that the dimensions of the coaxial cavity, those of the reactive elements of the N sections and the position of the N cuts are determined to define an approximately constant law Δ f, where f denotes any f of the 2 ^N discrete tuning frequencies obtained by the selective insertion of the N reactive elements and Δ f represents the average of the differences between this frequency f and the adjacent frequencies among the 2 ^N possible frequencies.

Images