GB2167243A - Electronically tunable band pass filter - Google Patents

Abstract

An electronically tunable narrow band tuned cavity filter includes a housing (2) which bounds a tuned cavity (3), a plurality of resonator bars (7) secured to a wall (5) of the housing, extending across the tuned cavity and having respective extensions (9) that pass through respective openings (6) in the opposite wall (4) of the housing to the exterior of the tuned cavity, and a set of tuning capacitor plates (17) arranged around the extension of each of the resonator bars. The tuning capacitor plates associated with the respective resonator bar are arranged outside the tuned cavity, preferably in a compartment (15) surrounding the opening, together with respective switching networks (18) driven by an electronic drive means (23) and operative for selectively including the capacitor plates in or excluding the plates from electric circuit with the respective resonator bar, whereby to tune the filter. <IMAGE>

Description

SPECIFICATION Electronically tunable band pass filter The present invention relates to band pass filters in general, and more particularly to a narrow band pass filter which is electronically tunable over a predetermined frequency range.

There are already known various filter constructions capable of passing a relatively narrow band of frequencies. Such filters have a high degree of selectivity, which is important for establishing communications with a minimum of interference. This is of special significance in the field of military communications, such as between aircraft or between aircraft and ground, where a plurality of colocated transmitters and receivers operating within the same frequency range such as between 225 and 400 MHz may be operating simultaneously.

The desirability of use of such narrow band pass filters with a high degree of selectivity has already been recognised and various filters of this type have been developed. So, for instance, there are known tuned cavity filters of the Butterworth and Chebyshev types which have relatively narrow pass bands, such as of the order of 2 percent of the transmission frequency, and relatively low insertion losses such as, for instance, around 1 dB at and around the tuned frequency. Such filters achieve excellent results so long as they are being operated around the tuned frequency.

However, tuning of such filters to a given frequency is relatively time-consuming, so that conventionally constructed tuned cavity filters of this type are not suited for use in communication links utilising frequency-hopping techniques even at relatively low frequencyhopping rates.

To make filters of this type usable in arrangements utilising frequency hopping, it was previously attempted to electronically tune the filter at any given instant of time to the respective narrow frequency band which includes the transmission frequency band. To this end, it was proposed to use a tuned cavity filter with tapped resonators, wherein a set of tuning capacitors was arranged in the tuned cavity at a tapping region of the respective resonator bar, each of the capacitors being individually switchable by an electronic switching circuit between its active and inactive states so as to contribute or not to contribute to the total capacitance of the respective set and thus to the resonance frequency of the tuning capacitor-resonator bar circuit. The filter of this construction was of the combline type and was designed for use in low power receiver front ends.

Experience with tuned-cavity combline filters of this type using the electronically switchable capacitors arranged within the tuned cavity around the tapping regions of the resonator bars has shown that they have excessive insertion losses and other disadvantageous properties which make this filter hardly usable in high-power transmissions, if at all. The problems encountered in this particular filter construction are at least partially attributable to the accommodation of the tuning capacitors and the associated electric circuitry in the tuned cavity where they are influenced by the electromagnetic field permeating the tuned cavity.

The present invention seeks to provide an electronically tunable narrow band pass filter which does not possess the disadvantages of known filters of this type, which is simple in construction, relatively inexpensive to manufacture, easy to use and reliable in operation.

The present invention may permit construction of a band pass filter which permits very rapid, very precise, and very frequent tuning to relatively narrow bands including the respective frequencies of transmission. The filter may be constructed in a form which can be used in a transmission system using frequency-hopping techniques at moderate to high hopping rates.

According to the invention there is provided a narrow band pass filter electronically tunable over a predetermined frequency range by signals issued by an electronic driver, characterised in a housing bounding a tuned cavity and having two opposite walls one of which has a plurality of through openings, a plurality of resonators each extending from the other of said opposite walls toward one of said through openings and each having an extension projecting through a different one of the openings to the exterior of the tuned cavity, a set of tuning capacitors for each of the resonators, including capacitor plates arranged around an extension of the resonator externally of the tuned cavity, and means for selectively activating and deactivating the capacitor plates in dependence on the signals to thereby include or exclude the tuning capacitors of each set in an electric circuit with their particular resonator, and thus to vary the tuned frequency band of the fiiter.

A particular advantage of this construction is that, since the tuning capacitors are located outside the tuned cavity, there is no interference between the electromagnetic field existing in the tuned cavity and the tuning capacitors and/or the means which selectively activates or deactivates the individual tuning capacitors. Moreover, the extension of the resonator advantageously forms one of the capacitor plates of each of the tuning capacitors of the respective set, so that there is obtained an improved incorporation of the tuning capacitors into the resonating circuit containing the respective resonator as another member thereof, and the risk of failure due to loose or broken electrical connections is then reduced.

Advantageously, the respective sets of capacitor plates may be accommodated, together with the respective extensions of the resonators and the associated switching circuitry which constitutes the selectively activating and inactivating means in separate compartments of the housing which surround the aforementioned openings. The activating and deactivating means for each of the tuning capacitors may include an electric switching circuit which switches the respective capacitor plate in or out of electric circuit with the resonator.An additional capacitor plate may be mounted on a cover wall of the housing that spans the respective compartment, for movement toward and away from a capacitor plate portion of the extension of the respective resonator to form an initial tuning capacitor therewith, by means of which the respective resonating circuit can be tuned to an initial resonating frequency which is then influenced by the aforementioned tuning capacitors of the respective sets.

The capacitance of the respective tuning capacitor may be made adjustable by providing either in the respective capacitor plate or, preferably, in the resonator extension, a recess into which there extends, to a greater or lesser extent, a slug that contributes to a greater or lesser degree, depending on the extent of its projection into the recess, to the total capacitance of the respective tuning capacitor.

The capacitances of the individual tuning capacitors may be so selected that the tuned bands of the filter partially overlap one another and that the insertion loss is within an acceptable range of, for instance, 2 dB even at the point of intersection of the neighbouring tuned frequency bands. To this end the tuning capacitors of the respective sets may have different capacitances to provide a plurality of capacitative steps of different magnitudes. Advantageously, the capacitative steps may be related to one another in a geometric progression, each succeeding capacitative step advantageously being twice the respectively preceding capacitative step.It has been established that, to cover the range of frequencies between 225 and 400 MHz with insertion loss within the aforementioned limit, it is advantageous to use six capacitative steps related to one another in the before-mentioned fashion.

If the extension of the resonator is square or rectangular in cross section, two capacitor plates of the respective set can be juxtaposed with each of two of the circumferential faces of the extension, while one capacitor plate is juxtaposed with each of the remaining circumferential faces. However, to reduce capacitative cross-coupling between the capacitor plates of the respective sets to a minimum, if not eliminate the same altogether, it is especially advantageous to give the extension of the resonator a hexagonal cross section. In this case, each of the capacitor plates of the respective set is juxtaposed with one of the six circumferential faces of the extension.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which: Figure 1 is a partly diagrammatic sectional view of a filter constructed in accordance with the present invention, Figure 2 is a diagrammatic representation of a single capacitor switching circuit for use in the filter of Figure 1, Figure 3 is a graphic representation of the relationships between insertion loss and frequency for the filter of Figure 1, Figure 4 is an enlarged perspective view of a detail A of Figure 1, Figure 5 is a sectional view of a detail B of Figure 4, Figure 6 is an enlarged top plan view of the area of the detail A of Figure 1 showing a modified construction, Figure 7 is a perspective view of the detail A of Figure 1 but showing the modified construction of Figure 6, and Figure 8 is a cross-sectional view taken on line Vlil-Vill of Figure 6 but showing a further modification.

Referring now to the drawirigs in detail, and firstly Figure 1, it may be seen that the reference numeral 1 has been used to identify a combline-type filter in its entirety. The filter 1 includes a housing 2 that boupds a tuned cavity 3. The housing 2 includes, as considered in the position illustrated in Figure 1, a top wall 4 and a bottom wall 5. The top wall 4 has a plurality of through openings 6 therein.

The filter 1 further includes a plurality of resonator bars 7 each of which is secured to the bottom wall 5 of the housing 2 and extends therefrom toward one of the openings 6. Each of the resonator bars 7 has a resonator section 8 which extends across the tuned cavity 3, and an extension 9 which extends to the exterior of the tuned cavity 3 through a respective opening 6.

The tuned cavity 3 further receives an input transformer 10 and an output transformer 11.

Electric leads 12 are connected to the input transformer 10 and are operative for energising the latter in dependence on the electric signal to be filtered. The output transformer 11 has electric leads 13 connected thereto, the leads 13 carrying electric signals representative of the filtered information as obtained from the filter 1. The construction and operation of a tuned-cavity combline-type filter 1 as described so far but without the extensions 9 s so well known from its use as a narrowband fixed-tuned filter that no elaboration thereon is needed here. Suffice it to say that the filter 1 used in the present invention preferably is constructed as a three-pole Butterworth filter with the resonator bars 7 arranged in a combline array.

The extensions 9 are arranged at the highimpedance ends of the respective resonator sections 8. As illustrated in Figure 1, the housing 2 is provided with a superstructure 14 bounding a plurality of compartments 15.

The compartments 15 are arranged around respective openings 6, and the extensions 9 of respective resonator bars 7 project into respective compartments 15. Each of the compartments 15 also accommodates a set of tuning capacitor arrangements 16 distributed around respective extensions 9 in a manner which will be discussed in detail later on. Each of the arrangements 16 includes a tuning capacitor plate 17 and a switching network 18 which is shown only in block form in Figure 1.

As shown in Figure 1, the compartments 15 have upwardly facing open ends. These open ends are closed by a cover wall 14' which is shown to be separate from the superstructure 14. For obvious reasons, the cover wall 14' will be secured to the superstructure 14 by any conventional means, such as by screws.

The cover wall 14' is provided with a plurality of internally threaded bores 19 which are respectively aligned with the extensions 9 in the mounted position of the cover wall 14' on the superstructure 14. Each of the threaded bores 19 accommodates an externally threaded plug 20 which is so constructed as to be able to serve as an additional capacitor plate. In this respect, the plug 20 cooperates with a capacitor plate portion 21 of the extension 9. The capacitor plate portion 21 has been illustrated in Figure 1 as being of a different configuration than the rest of the extension 9, for instance, cylindrical in conformity with the cylindrical configuration of the plug 20. However, the capacitor plate portion 21 may also have the same cross-section as the remainder of the extension 9 and thus be visually indistinguishable therefrom.The plug 20 may have such a diameter as to itself serve as a capacitor plate; however, it could also have a relatively small diameter and be connected to a capacitor plate 22 proper, as illustrated in the right-hand portion of Figure 1 which also indicates the indistinguishable configuration of the capacitor plate portion 21 of the extension 9 of the resonator bar 7. The capacitor plate portion 2 1 and either the capacitor plate 22 or the plug 20 configured to form the capacitor plate together form an initial tuning capacitor. By turning the plug 20 in the threaded bore 19 of the cover wall 14', it is possibie to initially tune the resonator circuit including the resonator bar 7 inclusive of its extension 9 and the initial tuning capacitor to a desired resonant frequency such as, for instance, 400 MHz.

Finally, Figure 1 also illustrates, in a diagrammatic fashion, a diode driver 23 which is connected by individual leads 24 with the respective tuning capacitor arrangements 16.

One lead 24 is provided for each of the arrangements 16. The diode driver receives information as to the instantaneous frequency to which the filter 1 is to be tuned and translates this information, in a known manner which need not be discussed here, into driving signals which are then applied via respective leads 24 to respective tuning capacitor arrangements 16. As illustrated in Figure 2, a respective lead 24 is connected to an electric switching circuit 25 which includes, as its main components, a switching diode 26, a grounded shunting diode 27, and a tuning capacitor 28. The switching circuitry or network 25 further includes a capacitor 29 interposed between the lead 24 and the ground, and two resistors 30 and 31 respectively arranged in parallel with the switching diode 26 and with the shunting diode 27.

To correlate the electric circuit of Figure 2 to the filter 1 depicted in Figure 1, it is to be realised that the capacitor plates of the tuning capacitor 28 of Figure 2 respectively correspond to the capacitor plate 17 and to a portion of the extension 9 of the respective resonator bar 7 of Figure 1, while the remainder of the electric circuitry 25 interposed between the lead 24, the ground, and the capacitor plate 17 constitutes a part of the circuitry of the switching network 18 diagrammatically indicated in Figure 1. A lead 32 from the capacitor region of the extension 9 of the tuning capacitor 28 represents the electrical connection to the section 8 of the resonator bar 7 as shown in Figure 1.

The switching diode 26 and the shunting diode 27 are preferably so-called PIN or NIP diodes with an intrinsic region interposed between the p and n regions. Such diodes exhibit low forward drop at high currents. The diodes 26 and 27 shown in Figure 2 are NIP diodes. Trigger signals suppiied from the diode drive 23 through a respective line 24 to a switching diode 26 will turn the diode on, thereby including tuning capacitor 28 in the electric circuit with the extension 9 and the resonator section 8 with attendant lowering of the resonating frequency of the resonating circuit including the resonator bar 7. In the absence of such signals, the switching diode 26 is turned off and the tuning capacitor 28 is excluded from the electric circuit with the resonator 7.By selectively activating and deactivating the respective tuning capacitors 28 of the respective set, that is, by selectively turning the switching didodes 26 associated therewith on and off, it is possible to tune the resonating frequency of the resonating circuit of the respective resonator bar 7 to any desired narrow frequency band within the frequency range.

In accordance with a currently preferred aspect of the present invention, the tuning capa citors 28 of the respective set all have different capacitances so as to provide a plurality of capacitance steps. Obviously, the size of the smallest capacitance step will determine the minimum spacing between the adjacent bands. The characteristic response of the filter 1 constructed in accordance with Figure 1 to various frequencies in terms of insertion loss is depicted in Figure 3 for two of such adjacent bands. A curve 33 is representative of the response of the filter to one combination of tuning capacitancessupplied by the tuning capacitors 28 of the respective set, while a curve 34 is representative of the response for an adjacent band for a combination of tuning capacitances differing from the combination of the curve 33 by the smallest capacitive step.

The curves 33 and 34 intersect at a crossover point 35. It may be seen that the curves 33 and 34 have relatively steep slopes at the upper and lower regions of the respective frequency bands, and a substantially flat plateau in the central region ofthe band. The insertion loss at the centre frequency of the band is in the vicinity of 1 dB, while the insertion loss at the cross-over point 35 is about 2 dB, which satisfies the design criteria. The width of the band is about 2 percent of the central frequency at the 3 dB level. It will be appreciated that the shapes of the curves 33, 34 and those of the remaining bands throughout the frequency range to be covered, such as between 225 and 400 MHz, in conjunction with the maximum acceptable insertion loss at the cross-over point 35, will determine the number and size of the capacitive steps of the tuning capacitors 28 of the respective set.It was established that, to obtain the situation depicted in Figure 3, it is advantageous to use six of the tuning capacitors 28 in each of the sets. The capacitances of these tuning capacitors 28 are advantageously related to one another in a geometric progression, the size of any succeeding capacitive step being advantageously twice that of the immediately preceding step. In this manner, there is obtained a binary-type progression which is easy to implement and use.

Turning now to Figure 4 of the drawing, it may be seen that it shows two of the tuning capacitor arrangements 16 arranged at one circumferential face of the extension 9 of the resonator bar 7. The switching networks 18 of these arrangements 16 are mounted on a common metallic support plate 36 which, in turn, is mounted on the top wall of the housing 2 within the compartment 14 of the superstructure 15 by means of respective mounting bolts 37. The metallic support plate 36 simultaneously serves as a heat sink and as an electric ground. The switching networks 25 are constructed by using printed circuit technology. As shown, the bypass capacitor 29, the switching diode 26 and the resistor 30 are all arranged on a substrate 38 which is connected to or integral with the support plate 36. On the other hand, the shunting diode 27 is directly mounted on the support plate 36.

A dielectric spacer and support plate 39 is secured to the support plate 36 and extends therefrom toward the extension 9 of the resonator bar 7. Two capacitor plates 17, each associated with a different one of the switching networks 25, are mounted adjacent to one another on the dielectric spacer and support plate 39. The capacitor plates 17 define respective air gaps 40 with an associated planar peripheral face 41 of the extension 9 of the resonator bar 7. In this manner, the capacitor plates 17 and the extension 9 together form the respective tuning capacitors 28. The extension 9 is shown to have a rectangular or square cross-section, thus presenting four of the peripheral faces 41 for juxtaposition of the capacitor plates 17 therewith.Since, as mentioned previously, each set of the tuning capacitor arrangements 16 includes six of such arrangements, it will be apparent that two of the arrangements 16 will have to be associated with each of two of the peripheral faces 41. The remaining two arrangements 16 can then be arranged one at each of the remaining peripheral faces 41 of the extension 9.

Figure 4, and in even greater detail Figure 5, illustrates another expedient used in accordance with the present invention. This expedient is based on the recognition of the fact that it is needed to fine-tune the capacitances of the respective capacitors 28. Coarse adjustment of the capacitances is achieved by properly adjusting the sizes of the respective air gaps 40. To achieve the fine capacitance tuning, the extension 9 is provided, at a region thereof which is juxtaposed with the respective capacitor plate 17, with a recess or window 42. A trimmer slug 43 partially extends into the recess 42, thus contributing to the total capacitance of the respective tuning capacitor 28. The extent to which the trimmer slug 43 extends into the recess 42 determines the amount of contribution of the trimmer slug 43 to the total capacitance of the respective capacitor 28. To be able to vary the amount by which the trimmer slug 43 contributes to the total capacitance, the trimmer slug 43 is externally threaded and is received in an internally threaded bore 44 which opens onto a top end face 45 of the extension 9 at one of its ends and passes through and slightly beyond the recess 42 at its other end. Thus, turning the trimmer slug 43 about its axis will result in upward or downward movement of the trimmer slug 43 in the bore 44, whereby the extent to which the trimmer slug extends into the recess 42 and thus its contribution to the total capacitance are varied.

Experience with the construction as illustrated in Figure 4 has shown that the adjacently mounted capacitors 17 may influence one another due to stray capacitances. This problem is avoided in the construction shown in Figure 6, where the extension 9 has a hexagonal cross-section, thus presenting six of the peripheral surfaces 41 for the juxtaposition of the six capcitor plates 17 of the set with such peripheral surfaces 41. As shown in Figure 7, the resonating section 8 preferably has rectangular or square cross section, and it is only the extension 9 which has the hexagonal cross section. To achieve this result, the extension 9 is provided with additional facets at the corners of the rectangular or square contour of the resonating section 8 to form the respective peripheral faces 41.Of course, the extension 9 need not have the cross section of a regular hexagon; rather, the angles at which the aforementioned facets are inclined may be so selected as to obtain different areas for the respective peripheral faces 41, correspondingly to the areas needed for the formation of the respective tuning capacitors 28.

The plates 36 are shown in Figure 6 to be mounted by means of mounting bolts 46 rather than by the mounting bolts 37 shown in Figure 4. Spacers 47 of dielectric material may be interposed between the capacitor plates 17 and the surfaces 41 of the extension 9. These spacers 47 serve for holding the extension 9 in position relative to the capacitor plates 17. Referring now back to Figure 1, it may be seen that the resonator bars 7 are secured only to the bottom wall 5 of the housing 2, so that they constitute cantilevers.Now, should the filter 1 be subjected to mechanical vibration, as is often the case in airplanes or at other intended locations of use of the filter 1, the cantilevered resonator bars 7 could vibrate or oscillate in response to such mechanical vibrations, especially if the frequency of the mechanical vibrations were in the vicinity of the natural resonance frequency of the resonator bars 7 or multiples thereof, whereby the sizes of the air gaps 40 would change with attendant changes in the tuned frequencies of the tuning capacitors 28. This situation is avoided or at least significantly improved by the use of the spacers 47.

In order to keep the resonator bar 7 from vibrating, the spacers 47 have to be relatively rigid. However, most highly rigid dielectric materials are also very brittle. To avoid the possibility of crushing the spacers 47 by the forces with which the capacitor plates 1 7 and the extension 9 act thereon, while still at least damping or suppressing the vibrations of the resonator bars 7 and maintaining the sizes of the air gaps 40 of the tuning capacitors 28 constant, it is proposed to so mount the capacitor plates 17 on the housing 2 as to be able to move relative to the housing 2 and thus to eliminate undue stressing of the spacers 47. This possibility is illustrated in Figure 8.

As shown in Figure 8, the housing 2 is provided with an internally threaded bore 48 which meshingly receives an externally threaded shank portion 49 of a respective bolt 46. The shank portion 49 passes through an opening or slot 50 of the support plate 36, the opening 50 being larger at least in the directions indicated by the double-headed arrow than the corresponding section of the shank portion 49, so that the support plate 36 can move in the directions of the doubleheaded arrow so long as the bolt 46 is loose and is clamped in position by a head portion 51 of the bolt 46 upon tightening. When the bolt 46 has been tightened, the support plate 36 is stationary relative to the housing 2.

The support plate 36 is shown to have a recess 52 which accommodates a portion of the dielectric spacer and support plate 39 for movement therein in the direction of the double-headed arrow. At least one spring 53 is accommodated in the recess 52 and rests against the bottom of the latter and acts on the spacer and support plate 39 to urge the same toward the extension 9. The extent of movement of the spacer and support plate 39 relative to the support plate 36 is limited by a travel limiting rivet 54 or a similar element which passes through an opening or slot 55 of the spacer and support plate 39. The opening 55 again has a dimension exceeding the corresponding dimension of the rivet 54 at least in the directions indicated by the doubleheaded arrow.Thus, the spacer and support plate 39 is free to perform movement within a range limited by the rivet 54 and the slot 55 relative to the stationary support plate 36 in the directions of the double-headed arrow, with attendant compression or relaxation of the spring or springs 53. The pre-tension of the spring or springs 53 is adjusted by selecting the desired position of the support plate 36 with the bolt or bolts 46 loosened, and is then maintained by tightening the bolt or bolts 46 and thus clamping the support plate 36 in position. It is also shown in Figure 8 that the capacitor plate 17 is mounted on the spacer and support plate 39 by a rivet 56 or a simiiar fastening element. Thus, the force exerted by the respective spring 53 on the spacer and support plate 39 will be transmitted through the rivet 56 and the capacitor plate 17 to the spacer 47 and ultimately to the extension 9, so that mechanical oscillations or vibrations of the latter relative to the housing 2 will be damped by the action of the spring or springs 53. Yet, the resiliency imparted to the mounting arrangement by the presence of the spring or springs 53 will prevent undue stressing of the material of the spacer 47 which could lead to destruction of the latter and which could result either from mechanical vibrations or from thermal expansion of the extension 9 and/or of the mounting arrangement.

Claims (30)

1. A narrow band pass filter electronically tunable over a predetermined frequency range by signals issued by an electronic driver, characterised in a housing bounding a tuned cavity and having two opposite walls one of which has a plurality of through openings, a plurality of resonators each extending from the other of said opposite walls toward one of said through openings and each having an extension projecting through a different one of the openings to the exterior of the tuned cavity, a set of tuning capacitors for each of the resonators, including capacitor plates arranged around an extension of the resonator externally of the tuned cavity, and means for selectively activating and deactivating the capacitor plates in dependence on the signals to thereby include or exclude the tuning capacitors of each set in an electric circuit with their particular resonator, and thus to vary the tuned frequency band of the filter.

2. A filter as claimed in claim 1, characterised in that the housing further bounds a plurality of compartments situated externally of the tuned cavity around the respective through openings, the extensions each extending into a different compartment, and each of the compartments accommodating at least the capacitor plates of a particular set.

3. A filter as claimed in claim 2, characterised in that each of the compartments each accommodates the selectively activating and deactivating means for its particular set of capacitor plates.

4. A filter as claimed in claim 2 or 3, characterised in that the housing includes at least one cover wall extending across the respective compartment transversely of and with an axial spacing from the extension of the respective resonator; wherein the extension has a capacitor plate portion facing the cover wall, and further comprising an additional capacitor plate juxtaposed with the capacitor plate portion and forming an initial tuning capacitor therewith that is permanently included in electric circuit with the respective resonator.

5. A filter as claimed in claim 4, characterised in that the additional capacitor plate is mounted on the cover wall for movement toward and away from the capacitor plate portion to thereby vary the capacitance of the initial tuning capacitor.

6. A filter as claimed in claim 5, characterised in that the cover wall has a threaded bore therein that is aligned with the capacitor plate portion and wherein the additional capacitor plate includes a threaded plug meshingly received in the threaded bore.

7. A filter as claimed in claim 6, characterised in that the additional capacitor plate includes a circular portion juxtaposed with the capacitor plate portion.

8. A filter as claimed in any one of the preceding claims, characterised in that the activating and deactivating means includes, for each of the capacitor plates, an electric switching circuit controlled by the electronic driver and operative for switching a particular capacitor plate in the electric circuit with the resonator in response to the signal and out of the electric circuit in the absence of the signal.

9. A filter as claimed in claim 8, characterised in that the electric switching circuit includes a switching diode operative for closing the electric circuit in response to the signal.

10. A filter as claimed in claim 9, characterised in that the switching diode is an NIP diode.

11. A filter as claimed in claim 9, characterised in that the electric switching circuit further includes a shunting diode arranged in series with the switching diode and between the capacitor plate and ground.

12. A filter as claimed in claim 11, characterised in that the shunting diode is an NIP diode.

13. A filter as claimed in any one of the preceding claims, characterised in that each of the resonators includes a resonator bar and the extension has a plurality of capacitor plate regions respectively juxtaposed with the capacitor plates to constitute the tuning capacitors therewith.

14. A filter as claimed in claim 13, characterised in means for adjusting the capacitance of the respective tuning capacitor, including an adjusting section of one of the capacitor plate and the associated capacitor plate region of the extension, having a recess therein, and at least one adjustment part at least partially received in the recess and operative for contributing to the total capacitance of the tuning capacitor.

15. A filter as claimed in claim 14, characterised in that the adjusting section has a bore that opens into the recess and wherein the said part is a slug received in the bore for movement therein between different positions in which it extends to a greater or lesser degree into the recess to contribute more or less to the tuning capacitance of the tuning capacitor.

16. A filter as claimed in claim 15, characterised in that the bore has an internal thread; and wherein the slug is externally threaded to mesh with internal thread and to conduct the movement between the different positions in response to the turning of the slug.

17. A filter as claimed in claim 14 or 15, characterised in that a recess is provided in the associated region.

18. A filter as claimed in any one of claims 13 to 17, characterised in that the extension of at least one of the resonator bars is crosssectionally polygonal to present a plurality of flat zones to serve as the associated regions.

19. A filter as claimed in claim 18, characterised in that at least one of the capacitor plates is juxtaposed with each of the flat zones to form the respective tuning capacitor with the associated region.

20. A filter as claimed in claim 19, wherein each of the sets of capacitor plates includes six of the capacitor plates.

21. A filter as claimed in claim 20, characterised in that wherein the extension is hexagonal in cross section to form six of the flat zones and wherein each of the capacitor plates is juxtaposed with one of the six flat zones to form the respective tuning capacitor with the associated region thereof.

22. A filter as claimed in any one of claims 13 to 21, characterised in that the resonator bar is secured to the other wall and forms a cantilever including the extension and further comprising means for at least suppressing mechanical oscillations of the resonator bar relative to the capacitor plates with attendant capacitance variations of the tuning capacitors.

23. A filter as claimed in claim 22, characterised in that the suppressing means includes spacer means interposed between the capacitor plates and the extension.

24. A filter as claimed in claim 22 or 23, characterised in means for mounting the capacitor plates on the housing for movement toward and away from the extension and wherein the suppressing means includes means for urging the capacitor plates toward the extension.

25. A filter as claimed in claim 24, characterised in that the urging means for each of the capacitor plates includes at least one spring.

26. A filter as claimed in claim 22, characterised in that the capacitor plates are stationarily mounted on the housing and wherein the suppressing means includes spacing means interposed between the housing and the resonator bar at the through opening.

27. A filter as claimed in any one of the preceding claims, characterised in that the tuning capacitors are air-gap parallel-plate tuning capacitors.

28. A filter as claimed in any one of the preceding claims, characterised in that the capacitances of the tuning capacitors differ from one another.

29. A filter as claimed in claim 28, wherein the capacitances of the tuning capacitors are in geometric progression relative to one another.

30. A narrow band pass filter substantially as described herein with reference to the drawings.