Filter And Method For Its Manufacture

Abstract

An electromechanical filter having a first and a second transducer resonator and a plurality of additional resonators, each resonator having first and second end surfaces. The resonators are arranged substantially parallel to one another. Two longitudinally vibrating thin, coupling wires are respectively coupled to the first end surfaces and to the second end surfaces. A plurality of thin, metal mounting strips are fastened to the resonators. The thickness d, and length 1, of each of the mounting elements corresponds approximately to the equation: d/l.sup.2 = .omega..sub.o .alpha. .sqroot..rho./E Such an electromechanical filter is manufactured by fixing each of the resonators to mounting elements, and removing material as required from at least one of the resonators to effect tuning.

Description

BACKGROUND OF THE INVENTION

This invention relates to an electromechanical filter and to a method of making the filter. The present invention relates, more particularly, to an electromechanical filter having resonators of the flexural vibration type and longitudinally oscillating coupling elements as well as piezoelectrically acting transducer resonators at its input and its output.

Mechanical filters with a frequency of about 50 kHz and a bandwidth of about 3 kHz have recently gained particular importance primarily because a carrier frequency system is about to be introduced which uses channel filters having this frequency capability. Thus an eminently important situation has developed for electromechanical filters.

While thus far performance qualities were of foremost importance and principal research in the electromechanical field has been directed thereto, the emphasis in the field of these filters has now shifted to production techniques and substantial automatization.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electromechanical filter which can be very economically fabricated.

It is another object of the present invention to provide a simple economical method of fabricating electromechanical filters.

One purpose is to eliminate production steps, if necessary at the sacrifice of higher quality, if this were to exceed, to too large an extent, the minimum requirement for the filter tolerances.

It has been found, in practicing the present invention, that there are a series of previously unknown practices and constructions which permit simplification of the fabrication process and even an improvement in quality to some extent. Thus it is possible to lower quality demands at other points in the fabrication process. The present invention therefore resides in a combination of features which optimizes the total filter design in the desired manner.

The invention, in its apparatus aspect, resides in an electromechanical filter having an input arrangement formed by a first piezoelectrically acting transducer resonator, having first and second end surfaces. An output arrangement is formed by a second piezoelectrically acting transducer resonator, having first and second end surfaces. A plurality of intervening resonators of the flexural vibrating type, each having first and second end surfaces, are provided. The transducer resonators and the intervening resonators are arranged substantially parallel to one another. Two longitudinally vibrating coupling elements in the form of thin coupling wires, are provided. One of the coupling elements is coupled to each of the first end surfaces and the other coupling element is connected to each of the second end surfaces. A plurality of mounting elements in the form of respective, thin, metal strips are fastened to the resonators of the plurality of resonators and to the transducer resonators. The thickness and the length of the mounting elements correspond approximately to the equation:

d/1.sup.2 = .omega..sub.o .alpha..sqroot..rho./E

where d is the thickness of the metal strips, 1 is the length of the metal strips, .omega..sub.o is the angular (radial) frequency of the filter, .rho. is the density of the metal strips, E is the modulus of elasticity of the metal strips and .alpha. is a constant.

The end surfaces to which two of the coupling wires are fastened are preferably the frontal faces of the intervening resonators and transducer resonators so that the succession of end surfaces throughout the filter are connected together by at least one continuous wire.

The constant .alpha. preferably has a value of 0.62, for .lambda./4 type behavior; 0.115 for 3.lambda.14 type behavior; and 0.046 for 5.lambda./4 type behabior.

The invention in its method aspect, involves the making of an electromechanical filter which includes providing a first transducer resonator, a second transducer resonator, and a plurality of intervening resonators, fixing each of the intervening resonators and each of the transducer resonators to mounting elements, and removing material as required from at least one of the resonators to effect tuning thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of an electromechanical filter constructed in accordance with the present invention, some of the resonators being absent, for the sake of clarity, and one resonator being shown in section.

FIG. 2 is a perspective view of a plurality of resonators coupled together by a wire.

FIG. 2A is a diagrammatic illustration of a resonator and an associated coupling wire of the single coupling wire type, with the coupling wire displaced from the center of the resonator.

FIG. 3 is a graphical representation of the coupling ratio (K/K.sub.max) plotted against the ratio of the distance that the coupling wire, as shown generally in FIG. 2A, is from an end of the resonator to the half length of the resonator (1.sub.2 /1.sub.1).

FIG. 4 is a graphical representation of the frequency characteristics of an electromechanical filter constructed in accordance with the present invention.

FIG. 5 is a diagrammatic, perspective view of a mounting element suitable for use as part of an electromechanical filter constructed in accordance with the present invention.

FIG. 6 is a perspective view of a second embodiment of a mounting element suitable for use as part of an electromechanical filter constructed in accordance with the present invention.

FIG. 7 is a perspective view of a mounting arrangement composed of a plurality of mounting elements formed by a common metal piece.

FIG. 8 is a perspective view of a transducer resonator suitable for use as part of an electromechanical filter constructed in accordance with the present invention.

FIG. 9 is a plan view of a plurality of intervening resonators and a transducer resonator coupled together by wires in accordance with a preferred embodiment of the present invention.

FIG. 10 is a plan view of a plurality of resonators and a transducer resonator coupled together by wires in accordance with a further preferred embodiment of the present invention.

FIG. 11 is a perspective view of one embodiment of an electromechanical filter constructed in accordance with the present invention comprising additional coupling wires between nonadjacent resonators.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an embodiment of an electromechanical filter constructed in accordance with the present invention. The economic and technological advantage of the present invention is to be explained with the aid of FIG. 1. As shown in FIG. 1, the electromechanical filter according to the present invention includes electromechanical transducers and a mechanical filter structure which transmits purely mechanical waves, in its transmission range, and reflects them, in its blocking range. In order to achieve this, a series of intervening resonators 2 which generally are made of a special metal alloy are disposed between an input piezoelectrically acting transducer resonator 1 and an output transducer resonator 1'. For the sake of clarity, only three of the resonators 2 are shown in FIG. 1, one in section, so as to show more clearly other parts of the electromechanical filter. However, in the illustration, eight intervening resonators are to be provided.

The housing includes a cover 6 which is shown broken away to expose elements 1-4, connected, by conventional means, to a mounting plate 5 on which the mounting elements 4 are fixed. The mounting elements 4 are, as shown in FIG. 1, thin metal strips formed from parts of a thin metal member which is fixed, for example by welding, to the mounting plate 5. A piezoelectric member 8, preferably in the form of a ceramic member, is fixedly positioned on a surface of the second transducer resonator 1'. A lead 7', on which the output signal from the filter appears, is connected to an electrode of the piezoelectric member 8. Another similar piezoelectric member (not visible in FIG. 1) is fixedly attached to a surface of the first transducer resonator 1, a lead 7 being provided to an electrode of this transducer to serve as the signal input lead of the filter. 66

All of the resonators 1, 1' and 2, as shown in FIG. 1, are connected in succession by means of coupling elements 3, e.g. thin wires preferably connected to the resonator end faces. Nonadjacent ones of the resonators may be additionally coupled together in any of many possible ways. In the first case, the electrical equivalent circuit diagram exhibits the characteristics of a polynomial filter. With the additional coupling, it is possible to produce attenuation peaks at real and complex frequencies. It is also of importance that the coupling elements 3 have a given length which is related to the wavelength of the oscillation sought to be passed through the electromechanical filter.

A very decisive role is played by a plurality of substantially identically dimensioned, mechanical mounting elements 4 for the resonators 1, 1' and 2. Two of the mounting elements 4 are provided for each of the resonators 1, 1' and 2, and are fixed to spaced points of the respective resonators 1, 1' and 2 inwardly of their end surfaces, to which the coupling elements 3 are fixed.

The mounting elements 4 serve to decouple the filter as completely as possible from the housing 5, 6. The mounting elements 4 must be stable, can be produced inexpensively, and must not detune the resonators 1, 1' and 2 or only detune them in a consistently repeatable manner, i.e. by the same amount from one production unit to the next. The resonators 1, 1' and 2 must not have additional resonant points (ancillary waves) as a result of their association with the mounting elements 4; any existing undesired resonances must be either attenuated or displaced into noninterfering frequency regions.

The cover 6 is of importance because of the ancillary waves, if flexural resonators are used whose sound irradiation cannot be neglected, particularly in the range of about 50 kHz. Generally proven structures can be dependably used.

This leads to the subject of the problems which must be particularly considered in connection with a filter operating at a frequency of about 50 kHz. Since the dimensions of known longitudinal and torsion resonators would become much too large, lengths of about 50 mm or about 28 mm being required, only flexural resonators can realistically be considered. In this regard, there exists a prior proposal, as shown generally in FIG. 2, to couple the resonators 10 of an electromechanical filter by means of a single wire 11 which vibrates longitudinally. This known technique is disclosed in German Patent No. 1,100, 834 and in German Laid Open Patent Applications Nos. 1,922,551 and 1,541,975. At this point, dual criteria may be applied, which lead to a first partial understanding of the theory of the invention described herein. Since the .lambda./4 length of the longitudinal vibration in the coupling wire is, at a frequency of 50 kHz, 25 mm, the coupling length, in order to reduce the overall structural length, must be <<25mm, i.e. practically a length of 5mm.

If the influence of the temperature coefficient of the longitudinal speed of sound on the position of the filter edges is calculated for such a filter, it can be found that the only coupling wire suitable for this purpose is formed from a special nickel-iron alloy, such as is sold under a number of trade names, e.g. Ni-Span-C, Thermelast or the like, after it has been subjected to a special heat treatment. Ni-Span-C is a Ni-Fe alloy consisting of 42,5 % Ni, 5 % Cr, <1 % Al and Ti, remainder Fe. The components of Thermelast are as follows: 42 % Ni, 9 % Mo, <1 % Be, remainder Fe.

If .lambda./4 coupling were used, it would have been possible to do without the special alloys mentioned above. In practice this means, over and beyond the heat treatment which can be easily mastered, that wires with very small diameter tolerances, about .+-.1 micron, must be made from a charge. This is not possible, however, with sufficient uniformity throughout the fabrication process using the alloys mentioned above compared with wires of a well-known nickel-iron-molybdenum alloy, which in the tube art are called A-wires and which have served extraordinarily well for .lambda./4 coupling. A-wire material consists of Ni, Fe and Mo. It is, therefore, a preferred feature of the present invention to employ two coupling wires which are so combined, from the different existing drawing charges, that the sum of the cross sections of the two wires corresponds to the total coupling wire cross section required for the value of the total coupling which is to be accurately maintained.

When only one coupling wire 11 is used for coupling, it initially appears to be of advantage to attach this wire 11 at the center of the flexural resonators 10, as shown in FIG. 2. However, if the efficiency of such a coupling is considered, it will be found that in order to obtain a certain band width in the center of the resonators 10, the coupling wire 11 must have a cross section which is greater by the factor 1/0.37 = 2.7 (FIG. 3) as can be learned from an article in the publication AEU 16 (1962) (Archiv der Elektrischen Ubertragung), S. Hirzel Verlag, Stuttgart, West Germany, pages 355-358. Experience has shown that in a channel filter operating at a frequency of about 50 kHz and with the use of a single wire 11 in the center of the resonators 10 (1.sub.2 = 1.sub.1 in FIG. 2a), the diameter of the coupling wire 11 is about 0.45mm. If two wires serving as the coupling members 3 (FIG. 1) are used instead and connected at the two end surfaces of the resonators 1, 1' and 2, in accordance with the present invention, the diameter of these coupling wires reduces to 0.45 .sup.. 1/.sqroot.2.sup.. 1/.sqroot.2.7mm = 0.19mm. This reduction in the cross section is very decisive in another respect. As can be seen in FIG. 3, the coupling ratio decreases as the single coupling wire 11 is moved away from the center point, reaching zero at a 1.sub.2 /1.sub.1 ratio of about 0.3 and thereafter reaching 1.0 when 1.sub.2 is zero; i.e., the wire 11 is at the end of the resonator 10.

FIG. 4 shows graphically how strongly the frequencies shift at the upper (f.sub.o) and lower (f.sub.u) edges of the band as well as at the center band frequency f.sub.M when the length of a coupling member 1.sub.K differs from .lambda./4. The frequencies are standardized to the prealignment frequency f.sub.R of the resonators inherent to them before the coupling members or wires are welded on. K is the coupling factor of the resonators. The steep rise of the curves directly shows that for f.sub.o and f.sub.M at 1.sub.K <<.lambda./4 the maintaining of an accurate coupling length is very critical. For a 50kHz channel filter with 1.sub.K = 5mm (.congruent..lambda./20) the permissible tolerances for the filter result in a manufacturing accuracy of the length 1.sub.K of .DELTA.1.sub.K .congruent. .+-.4.mu..

The position of the welding point for welding the coupling wire to the flexural resonator must be correspondingly accurate. The advantage of using two coupling wires, in accordance with this invention, each having a diameter of 190.mu. instead of one wire with a diameter of 450.mu. becomes immediately evident. In principle, a wire of 190.mu. can be welded much more accurately than a wire of 450.mu.. Added to this advantage is a further advantage when the proposed two coupling wires are welded to the two frontal faces of the resonators, in accordance with the present invention. The position of a coupling wire and its thickness are decisive, with regard to the degree (strength) of the coupling effected by the coupling wire, but not its accurate positioning on the frontal face of the resonators. When only one coupling wire is used, which is attached to the center of the resonators, the accurate positioning of this point is also decisive as can be seen from FIG. 3 (with 1.sub.2 .apprxeq. 1.sub.1). The value K = K.sub.max is thus dependably achieved in the arrangement of the present invention over the entire frontal face. This is of importance for the manufacture of the filters.

An electromechanical filter according to FIG. 1 is to be manufactured in such a way that first all parts of the filter shown in FIG. 1, except for the coupling elements (wires) 3, are connected together, for example, by electrical spot welding, soldering or cementing. Then the individual resonators 2 and the transducer resonators 1 and 1' are accurately frequency tuned. The welding jig in which the resonators 2 and the transducer resonators 1 and 1' are welded to the mounting elements 4 need not meet any extreme precision requirements. A spacing tolerance of about .+-. 0.1mm in the alignment of these parts to one another is sufficient for the type of filters according to the present invention. The very decisive process of welding on the coupling elements (wires) 3 of the prescribed length 1.sub.K .+-. 4.mu. can then be effected with the utmost precision, for example with special precautions in an accurate, adjustable jig. It is then, however, not necessary to observe simultaneously the accurate positioning of the welding point on the resonators 2 and on the transducer resonators 1 and 1'.

Since the importance of the arrangement of the resonators 1,1' and 2 and the coupling elements (wires) 3 in FIG. 1 has now been made clear, a further point which brings practical success to this arrangement is the mounting of the resonators 2 and transducer resonators 1 and 1'.

The difficulty here again is the attainment of relatively short mounting elements 4. In principle, ultrasonic vibrations will always propagate along the mounting elements 4. Depending on whether the propagating energy is reflected or absorbed, the mounted resonators 1, 1' and 2 are detuned or attenuated. The magnitude of these effects can be reduced by providing a cross section for the mounting element 4 which just suffices for mounting purposes, so that the coupling of the mounted resonators 1, 1' and 2 with the cover 6 and the mounting plate 5 is reduced in that the acoustically effective length of these mounting elements 4 is made about .lambda./4. This is possible in an arrangement such as shown, for example, at 4 in FIGS. 1 and 2 of the German Laid Open Patent Application No. 1,022,550, only when the length of the holding pins is about 14mm. The torsional vibrations transmitted from the holding pins would be optimally decoupled from the housing.

In the design of the filter according to FIG. 1 hereof a novel approach is taken which will be explained with the aid of that figure. The mounting elements 4 in FIG. 1 each substantially consist of an actual supporting part which is shown, in an enlarged view, in FIG. 5 and which has a length L, a width b and a thickness d. This element is torsionally stressed by that one of the resonators 1, 1' and 2 with which it is associated, and performs flexural vibrations in the plane of the resonators 1, 1' and 2. If this mounting element is imagined to be separated into parallel sections along the dashed lines shown in FIG. 5, the individual sections perform flexural vibrations. Nothing changes in the mechanism of these vibrations in a first approximation, i.e. at a low amplitude, if the separation is not made. The thickness value d can be determined, for example, with the aid of an experiment set forth in the periodical AEU 15 (1961), Issue No. 4, pages 175-180, equations (1a) and (20a) of which are of interest, when the length of the mounting element is given and there is a .lambda./4 type coupling. The following applies:

.fourthroot.p .sup.. q/E .sup.. J .omega..sub.o.sup.2 = 2.365/v.sub.L (1)

where v.sub.L = E/.rho. .apprxeq. 5 .sup.. 10.sup.5 cm/sec, the speed of sound for longitudinal waves. E is the modulus of elasticity. The symbol .rho. is the density of the material. The symbol .omega..sub.o is the angular frequency; i.e., .omega..sub.o = 2.pi.f.sub.o, f.sub.o being the center frequency of the filter, and

q/J = (.DELTA.b .sup.. d/1/12.DELTA.b .sup.. d.sup.3) = 12/d.sup.2 (2)

where J is the geometrical moment of inertia of the cross-sectional mounting element shown in FIG. 5 with the width .DELTA.b and the thickness d about an axis in the direction of the edge having the length .DELTA.b. It can be seen that the value b, or .DELTA.b, is derived from the calculations of the .lambda./4 length for mounting elements and this again justifies the assumption that a mounting element according to FIG. 5 can be separated, for analytical purposes, into segments in the manner shown by the dashed lines.

The above equations thus result in:

d/1.sup.2 = .omega..sub.o .sqroot..rho./E .sup.. 0.62 (3)

for .lambda./4 type coupling, the units for d and 1 being in cm.

If l = 0.3cm is selected, the following applies at a frequency:

f.sub.o = .omega..sub.o /2.pi.= 50 kHz .fwdarw. d = 0.035cm (4)

In an entirely analogous manner, the corresponding formulas result for the relationships, for example, at 3.lambda./4 and 5.lambda./4 type coupling, i.e.

d/1.sup.2 = .omega..sub.o .sqroot..rho./E .sup.. 0.115 (5)

for 3.lambda./4 type coupling and

d/1.sup.2 = .omega..sub.o .sqroot..rho./E .sup.. 0.046 (6)

for 5.lambda./4 type coupling.

An advantageous further embodiment of a mounting element suitable for use in the present invention is shown in FIG. 6. The mounting element consists, at least in its portion which is torsionally stressed, of a frame having approximately the following inner dimensions:

L' = L and b' = 1/2L' to 1/3 L'.

With such a configuration of the mounting element it is possible to obtain even better decoupling between the resonators 2 or transducer resonators 1 and 1', and the housing cover 6 and the support plate 5.

The width b is adapted to that of the resonators 2 or that of the transducer resonators 1 and 1'. In practice, b will be approximately 2mm. Thus the entire arrangement has suitable stability in addition to optimum decoupling.

Advantageously the .lambda./4 long mounting elements 4 (FIG. 1), as more specifically shown in FIGS. 5 or 6, are to be provided using conventional principles employed for the construction of filters and the arrangement of the resonators 1, 1' and 2 thereon. The mounting elements 4, as shown in FIG. 1, are of a common metal piece, are to be made, for example, by stamping, and are to be bent in such a manner that an integral body results, as shown in FIG. 7. This produces good stability during the first fabrication steps, and results in a stable, sturdy mounting arrangement.

This stability is particularly important since, after assembly of the parts of the filter shown in FIG. 1, or before welding on the coupling wires 3, the resonators 2 and the transducer resonators 1 and 1' must be turned. Either because of their precise fabrication or as a result of a first prealignment before being welded to the mounting elements 4 they already have an approximately correct resonant frequency and band width. However, the first welding step produces such stray phenomena that subsequent tuning, in the range of a few Hertz, is generally needed at this point. The magnetic fields produced as a result of electric welding, if such a technique is used, have an influence on the inherent frequency which must be eliminated by a demagnetization process.

The actual tuning process can be effected by grinding with a fine high-speed grinding disc or the like. It is best to grind the end of the resonators and thus increase the frequency. This retains the characteristic impedance Z of the resonators 1, 1' and 2 and thus also the degree of coupling between the adjacent resonators at a given cross section of the coupling wires 3.

It should be mentioned at this point that the coupling between the resonators must be able to meet the requirements of filter theory; generally the coupling decreases from the outer to the inner resonator pairs by a factor of 2, or more. With a constant spacing 1.sub.K between the resonators and a constant cross section of the coupling wires 3, this can be accomplished only by varying the cross section of the resonators 1, 1' and 2.

It is much more practical, however, to slightly vary the lengths 1.sub.K and thus set the coupling between the resonators 1, 1' and 2. The tuning frequencies between the individual resonators 2 and between the transducer resonators 1 and 1' are then also slightly different. Generally the resulting difference in length is so slight (a few tenths of a millimeter), that it need not be considered in the welding process, particularly when the coupling wires are welded on. Only with transducer resonators 1 and 1', it may happen that they become noticeably shorter since the piezoelectric member 8 preferably a ceramic transducer which serves to excite and to transmit the mechanical vibrations, substantially reduces the inherent frequency when the length is the same as that of the normal resonators. To compensate for this, the metal portions of the transducer resonator 1', as well as the transducer resonator 1, must be shortened by up to about 2mm.

In order to prevent difficulties with the preferably substantially straight coupling wires 3, it is advantageous to employ a transducer resonator as shown in FIG. 8. Protrusions 9, as shown in FIG. 8, cause the total length of the transducer resonator 1' to become approximately equal to that of the resonators 2. The shape of these protrusions is almost arbitrary. Of course, the transducer resonator 1 is provided with similar protrusions. In practice, the metallic portion of the transducer resonators 1 and 1' is provided, for example, as an integral piece which has been machined to provide the protrusions 9, but these protrusions may also be welded on. The advantage is then that the welded-on protrusions need not necessarily be made of a temperature compensating material.

The length of the transducer resonators 1 and 1' may, however, also be selected to equal that of the intervening resonators 2, if at the same time the diameter of the transducer resonators 1 and 1' is increased. With the appropriate selection of length and diameter, tne inherent flexural resonant frequency remains substantially the same; however, the distance of the transducer resonators 1 and 1' from the adjacent resonators 2 must be reduced due to the decrease in coupling.

The adaptation of the length may also be effected by a slight curvature of coupling wires 13 fastened to the end surfaces of resonators 14 and 15, as shown in FIG. 9, or by a variation of the lengths of a first transducer resonator 17, as well as of intervening resonators 18 and a second transducer resonator (not shown), so that the filter has the approximate construction shown in FIG. 10, coupling wires 16 being essentially straight, but not parallel to one another.

FIG. 11 shows another embodiment of an electromechanical filter constructed in accordance with the present invention. All of the resonators 1, 1', and 2 are connected in succession by means of couplingelements 3, e.g. thin wires preferably connected to the resonator end faces. Nonadjacent ones of the resonators are coupled together by additional coupling wires 3'.

The length of these wires corresponds approximately to the equation

(2 n + 1) l.sub.k + .lambda./2

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims

1. An electromechanical filter having a signal input means and a signal output means comprising, in combination:

1. a first piezoelectrically acting transducer resonator having first and second end surfaces and serving as said input means, a plurality of intervening resonators of the flexural vibrating type each having first and second end surfaces, and a second piezoelectrically acting transducer resonator having first and second end surfaces and serving as said output means, said resonators being arranged substantially parallel to one another;

2. at least two thin longitudinally vibrating coupling wires, one of said coupling wires being coupled to said first end surface of each said resonator and another of said coupling wires being coupled to said second end surface of each said resonator; and

3. a plurality of thin metal mounting strips fastened to said resonators, the thickness and the length of each said mounting strip corresponding approximately to the equation:

d/1.sup.2 = .omega..sub.o .alpha..sqroot..rho./E

where d is the thickness of the metal strips, 1 is the length of the metal strips, .omega..sub.o is the angular frequency of the filter, .rho. is the density of the metal strips, E is the modulus of elesticity of the material of the metal strips, and .alpha. is a constant having a value selected from the group consisting of values of substantially 0.62; 0.115;

2. An electromechanical filter as defined in claim 1, wherein the constant .alpha. is substantially 0.62 and said filter exhibits .lambda./4 - type

3. An electromechanical filter as defined in claim 1, wherein the constant .alpha. is substantially 0.115, and said filter exhibits 3.lambda./4 -

4. An electromechanical filter as defined in claim 1, wherein the constant .alpha. is substantially 0.046, and said filter exhibits 5.lambda./4 -

5. An electromechanical filter as defined in claim 1, wherein said first end surfaces and said second end surfaces are frontal faces of said

6. An electromechanical filter as defined in claim 1, wherein the total

7. An electromechanical filter as defined in claim 1, wherein each of said resonators has a respective longitudinal axis, and wherein said coupling elements are arranged substantially parallel to one another and approximately at a right angle to said longitudinal axis of said

8. An electromechanical filter as defined in claim 1, further comprising additional coupling wires attached between nonadjacent resonators thereby

9. An electromechanical filter as defined in claim 1, wherein each of said metal strips is in the shape of a frame at least in that area which is

10. An electromechanical filter as defined in claim 9, wherein each said frame has a respective opening having a height approximately equal to the length of each of said metal strips and having a width of between about

11. An electromechanical filter as defined in claim 1, wherein all of said resonators have the same resonant frequency and different lengths and said coupling wires are curved so as to adapt them to the differing lengths of

12. An electromechanical filter as defined in claim 1, wherein all of said resonators have substantially the same resonant frequency and have substantially the same length and said two coupling wires are each

13. An electromechanical filter as defined in claim 12, wherein said first and said second end surfaces of each said transducer resonator are each provided with a respective axially extending protrusion having a cross section smaller than the cross section of each said transducer resonator,

14. An electromechanical filter as defined in claim 12, wherein each said resonator is substantially circular in radial cross section, the diameters of said transducer resonators being greater than the diameters of said

15. An electromechanical filter as defined in claim 12, wherein each said resonator is substantially circular in radial cross section, said first transducer resonator, said intervening resonators and said second transducer resonator being arranged in succession, the lengths and diameters of said resonators increasing progressively from one resonator

16. An arrangement as defined in claim 1, wherein said filter constitutes a

17. A method of making an electromechanical filter comprising: providing a first transducer resonator, a plurality of intervening resonators and a second transducer resonator each resonator having two end surfaces; fixing each resonator to a mounting element; fine tuning the resonators by mechanically removing material from at least one resonator after said step of fixing each resonator to a mounting element; and, after said step of

18. A method of making an electromechanical filter comprising: providing a first transducer resonator, a plurality of intervening resonators and a second transducer resonator each resonator having two end surfaces; forming mounting elements as thin metal strips of material forming the edge portions of a common piece of metal so as to form the mounting elements as parts of a single piece of metal; fixing the resonators to the mounting elements; fixing coupling wires to the resonator end surfaces; and fine tuning the resonators by mechanically removing material from at least one resonator after said step of fixing each resonator to a mounting

19. A method as defined in claim 18, wherein said step of forming the mounting elements in the form of thin metal strips is effected by stamping out portions therebetween from the common piece of metal and subsequently

20. A method as defined in claim 18, wherein said step of forming the mounting elements in the form of thin metal strips is effected by etching out portions therebetween from the common piece of metal and subsequently

21. A method as defined in claim 17, wherein the step of fixing the resonators is effected by spot welding, soldering, or cementing.