IE42756B1 - Improvements in or relating to electromechanical filters - Google Patents

Abstract

1531759 Electrochemical filters SIEMENS AG 6 Nov 1975 [8 Nov 1974] 45986/75 Heading H3U An electrochemical filter comprises parallel rodshaped resonators R1-R10 which are interconnected by at least one coupling wire LK, the input and output transducers EM being connected to resonators other than the end resonators, those resonators outside the signal path between the transducers providing attenuation poles in the transmission characteristic of the filter. The resonators are supported at nodal points by wires S welded thereto and mounted in grooves in base G, which is either of metal or synthetic resin. As shown, the resonators are bending oscillators and electrostrictive transducers are mounted on flat faces of resonators R2, R9; the other resonators also have flat faces by which they are coupled to wire LK. In a modification resonators R2, R9 each have a portion with a second flat face perpendicular to the first one and for coupling them to wire LK. Each transducer is connected to a respective coil, which together with the self-capacitance of the transducer forms a resonant circuit tuned to the centre of the filter pass band but with a greater band width. The position of the poles introduced by resonators such as Rl, R10 may be displaced by varying the masses of these resonators or the length or thickness of the adjacent part of wire LK. Further poles may be provided by coupling wires UK which bridge over one or more intermediate resonators. The resonators may be torsional oscillators in which case they may be supported by wires HD, Fig. 7, attached to projections A of base Gl; alternatively they may each be supported by a single respective wire connected between a node and the base (Fig. 5, not shown) or all the resonators may be supported by two common wires HD mounted in chips (SH, Fig. 6; not shown) extending from the base. In each case the resonators are coupled by a wire KD.

Description

The invention relates to the type of electromechanical filter employed in communications equipment, where the problem frequently occurs that in the attenuation characteristic, the transition from a pass band into a blocking band is required to exhibit as steep as possible a gradient and/or that in specific regions of the blocking bands extremely high blocking attenuations are required. In electrical filters these requirements may be fulfilled by the provision of additional resonant circuits to produce attenuation poles in the filter characteristic at predetermined frequencies. Another partial remedy is to block the transmission by compensation at the appropriate frequencies by means of a so-called over-coupling. Both methods are known not only in electrical filters with lumped component circuit elements, such as coils and capacitors, but also in so-called electro-mechanical filters, where the physical coupling of mechanical resonators is such that the formation of attenuation poles by means of over-coupling necessitates a relatively high outlay when several attenuation poles are concerned, because this inevitably leads to difficulties regarding the tolerances to be adhered to during production. Also, if there are several over-couplings, disturbing subsidiary resonances ean occur. Therefore, in order to form attenu25 ation poles in electromechanical filters, it is known to provide additional resonators coupled to the input and output of the actual filter portion, as described for - 3 example in British Patent Specification No. 959,393, and the work published prior to this patent in Proceedings, 11th Annual Symposium on Frequency Control, 7th-9th May 1957, p 535 to 555, see in particular Figure 4 on page 547 and the associated text on page 537.

One object of the present invention is to provide a construction of an electromechanical filter in which attenuation poles produced by additional resonators are located outside the actual filter portion, but the difficulties referred to in the British Patent and in the abovementioned publication are substantially eliminated.

These difficulties encountered in the prior art are that the attenuation poles cannot be freely selected in terms of frequency, and also additional mechanical resonators considerably affect the quality of the coupling to the mechanical resonators located at the filter input and output, and also the tuning of the latter.

The invention consists in an electromechanical filter in which a plurality of parallel, rod-shapedmechanical resonators in a common plane are connected in a cascade by means of at least one common coupling wire, and in which the supply and the withdrawal of electric signals is effected by transducer resonators which lie within the cascade, and those resonators of the cascade outside the signal path between the transducer resonators provide attenuation poles in the transmission characteristic of the filter, the individual resonators being mounted on support wires which are separate from said or each coupling wire and are firmly secured to a common base plate which bears the entire electromechanical filter system. The invention is based on the recognition that in order to overcome the aforementioned difficulties 43756 - 4 it is necessary that the individual resonators are operated as four-terminal devices, in contrast to the arrangement as described in British Patent Specification No. 959,393, so the is necessary to fully separate the support facilities of the resonators from their coupling device, and these requirements can be realised particularly simply when the individual resonators of the filter are of rod-shaped design and are arranged axially parallel next to one another in one place.

Advantageously the respective support wires, which are preferably to be in the form of pins, each engage onto the individual associated resonators in a mutually similar way, and preferably each in the region of a nodal point of the individual associated resonator.

Advantageously, the resonators possess a generally circular cross-section, the diameter of which is equal for all the resonators. It is advisable to provide the circular'cross-section of at least those mechanical oscillators serving as pure resonators with a flattened portion which serves as base and as fixing point for the minimum of one coupling wire. A flattened portion of this kind is also advantageous in the case of the resonators which have transducers attached, since the flattened surface can then serve to support and fix eleotrostrietive transducer material which is attached to the resonator, and is orientated at right angles to the plane determined by the resonators in such manner that the minimum of one coupling wire which is secured to the resonator is excited to longitudinal oscillations. For the fixing of the minimum of one coupling wire, expediently a resonator having an attached transducer is provided with a further flattened portion, the surface of which is parallel to the plane determined by the individual resonators, and which also encloses the coupling wire. In the case of the transducers it is advisable to additionally assign each of these a coil which, together with the transducer capacitance, forms a resonant circuit whose band width is considerably greater, for example from 2 to 5 times greater, than the width of the pass band of the mechanical filter portion of the electromechanical filter. Such a filter design permits the fundamental transit time of the filter between its input and output terminals to be kept low in a simple fashion, in spite of a high flank gradient and high blocking attenuation. In particular when the filter is employed as a channel filter in a carrier frequency level with premodulation, it is advisable to ensure that between the input and output of the filter, i.e. between the transducers, the number of resonators provided in the cascade is sufficiently low to ensure that the fundamental transit time of the signals which are to pass through lies is not significantly greater than 0.8 ms. With a pass band width of between 3 and 4 kHz at an operating frequency of 50 kHz, using resonators in the form of bending-mode oscillators provided with longitudinal coupling, in order to adhere to this special dimensioning requirement it is sufficient for between 8 and 10 resonators, including those with transducers to be arranged in the signal path, and one respective resonator located outside the signal path at each end of the cascade for the production of the attenuation poles below and above the pass band. In the case of eight resonators it is advisable to use two additional resonators to provide two attenuation poles in the blocking band which lies below the pass band frequency and by means of at least one over-coupling which serves to bridge one 48756 - 6 resonator in the aforementioned signal path to provide one attenuation pole in the blocking band which in frequency lies above the pass band. In many cases it is also advantageous to provide a multiple resonant system for the resonators which are located outside the signal path portion of the cascade, as a result of which a plurality of attenuation poles qan be achieved.

The invention will now be described with reference to the drawings, in which:10 Figure .1 is an explanatory graph? Figure 2 is a schematic perspective view of the embodiment to which Figure 1 relates? Figure 3 is a further explanatory graph? Figure 4 schematically illustrates a further exem15 plary embodiment of the invention? and Figures 5 to 7 are detail views of alternative constructions.

To define an exemplary embodiment it has been assumed that a theoretical attenuation characteristic shown in Figure 1 as a shaded profile is required for a filter that is to be used in a particular carrier frequency systan in which such filters are used as channel filters in the premodulation section. The frequency of the electric signals is plotted in kilohertz on the abscissa, and the pass attenuation or operating attenuation a^ is plotted in decibels on the ordinate. It is also assumed that a fundamental transit time of less than 0.8 ms is required in the pass band. The actual characteristic obtainable with an exemplary embodiment of the invention is also shown. A schematic plan view of the filter is included in Figure 1, and the details of this embodiment are more clearly shown in Figure 2, which indicates the parallel disposition of ten rod-shaped resonators Rl to RIO that operate in the bending mode. Each resonator is supported on a base plate or filter base G by a respective pair of relatively stiff metallic pins S which engage its oscillatory nodes. The pins, which are secured by welding to the individual resonators are then glued, cast or soldered into an associated groove in the filter base. Expediently, the filter base consists of sheet metal or a synthetic resin material, such as a polycarbonate resin. The individual resonators are connected to one another to form a cascade by a common coupling wire LK which engages the centre of each said resonator and acts as longitudinal coupler, which possesses a constant cross-section over its entire length. The coupling wire LK is firmly secured to the individual filter resonators by welding. Those resonators which are not transducers are provided with a flattened portion which serves to ensure a well-defined coupling with the coupling wire in a simple manner. Two resonators, R2 and R9 have eleCtrostrictive material fixed thereto, to form transducers EM the electrostrietive material being secured to another flattened portion by soldering. On the side facing away from the solder, the electrostrietive material possesses an electrically conductive coating, consisting e.g. of gold which serves as counter-electrode, to which a respective connecting lead A connects the transducer to an associated coil Sp, one end of which is connected to reference potential, to form input and output circuits, as shown in the plan diagram included in Eigure 1. The inductance of each coil Sp is selected to be such that it acts together with the self capacitance of the electrostrietive material to form a parallel resonant circuit which is tuned to - 8 43756 the middle frequency of the pass band, but whose band width is considerably greater than the width of the pass band, which in the exemplary embodiment extends from 48.2 to 51.5 kHz.

The individual resonators of this filter are connected in a cascade by the common coupling wire which serves as longitudinal coupler, but only a portion from R3 to R8 of this cascade of ten resonators lies between the transducer resonators R2 and R9, and the two resonators Rl and RlO which lie outside the signal path from R2 to R9 each serves to produce an attenuation pole at a frequency of approximately 47.6 kHz. Without these, the attenuation curve of the filter would have a first pole located at approximately 47 kHz, after which it would continuously rise for lower frequencies, but the susceptance represented by the externally coupled resonators causes a dip DE in the attenuation curve which can be selected to be such that it enhances the relationship between the actual characteristic and the theoretical requirement. In the filter described, this dip in the attenuation occurs at approximately 42.6 kHz, and by further displacement can be rendered ineffective to the extent that in the lower frequency blocking band the transmission attenuation scarcely falls below 70 decibels. There are various possibilities of effecting this displacement. The coupling and/or the mass of the relevant end oscillator Rl and/or RIO may be changed, for example. In order to increase the coupling factor it is either possible to make the coupling wire thicker and/or to reduce the length of the coupling wire between two resonators.

In addition, in order to steepen the gradient of the transition from the pass band into the upper blocking - 9 band two further poles are provided at a frequency of approximately 51.6 kHz by means of two mechanical overcouplings UK.

Figure 3 shows the curve of the group transit time -TB in dependence upon the frequency f for the filter shown in Figures 1 and 2.

It can be seen from the graph that in the major part of the pass band which is of interest, the group transit time lies between 0.6 and 0.7 ms, whereas a filter not having the aforementioned attenuation poles, but fulfilling the same pass attenuation characteristic and exploiting the entire cascade circuit plus two additional resonators for the characteristic formation, possesses a group transit time of approximately 1.2 ms.

It should be noted that a variation of the coupling between consecutive resonators along the coupling wire forming the common main coupler beneath the resonators is effected by altering the distances between the resonators.

It should also be mentioned that the attenuation poles can be brought about by means of additional resonators above and/or below the pass band frequency, either in conjunction with or without the additional attenuation poles produced by over-coupling. In addition, the number of resonators can differ in dependence upon the requirements imposed by any particular required filter characteristics.

Figure 4 shows one alternative example of a filter constructed in accordance with the invention, with ten resonators in a signal path between transducer resonators R2 and Rl3, and additional resonators Rl and R14.

The other details of the filter shown In Figure 4 correspond to those described with reference to Figure 2. - 10 Figure 4 also shows a section through a resonator e.g. Rl which is not loaded by electrostrictive material, and also a section through a transducer resonator e.g. R2, which is loaded with electrostrictive material. The resonator diameter d is equal for all the resonators in this case.

A flattened portion is provided to effect different tuning of the individual resonators. Another feature of this exemplary embodiment is that considered towards the middle of the mechanical filter, the symmetrically spaced resonators are respectively tuned to substantially similar resonance frequencies, e.g. fQ, f^....fg, and ίθ', f^'.... fg‘. The selection of the frequencies determines the behaviour of the filter in the pass characteristic which can, in known manner be selected for example to possess the maximum degree of flatness or to comply with a Tsohebyscheff function.

The cross-section of the loaded transducer resonator shown in Figure 4 has a metal body whose cross-section has two flattened portions, in order to obtain another parameter for the setting of the resonant frequency of the resonator, allowing for the mass that is to be used to drive the resonator.

It should also be mentioned that the use of torsional oscillators in a cascade as shown in the preceding example with the additional resonators has proved advantageous. Torsional oscillators can be supported either as shown in Figure 5, by means of a support wire HD in the region of a node NZ of a rod-shaped torsional resonator R, or as shown in Figure 6 by means of the engagement, at this point of a common support using two support wires HD in one or more clip-like support components SH. However, it is advantageous if, as in Figure 7, a longitudinal 43756 - 11 support wire is provided at the ends of the individual torsional resonator, these two support wires HD being arranged to run at least approximately in the direction of the torsional axis or a surface line of the associated rod-shaped torsional resonator. These support wires can then advantageously be secured by adhesion, or soldering in a projection A of appropriate shape which extends on each side of a filter base plate G‘. However, it is advisable for this support to be practically identical for all the resonators and to be separate from the actual coupling, which in a torsional filter also takes place by means of at least one longitudinal coupling wire KD. Only in this way is it possible to ensure that the break in the blocking attenuation which is caused by the addit15 ional mechanical susceptance is located in a non-eritieal region inside the blocking band.

Claims (16)

1. CLAIMS:1. An electromechanical filter in which a plurality of parallel, rod-shaped mechanical resonators in a common plane are connected in a cascade by means of at least one common coupling wire, and in which the supply and the withdrawal of electric signals is effected by transducer resonators which lie within the cascade, and those resonators of the cascade outside the signal path between the transducer resonators provide attenuation poles in the transmission characterictic of the filter, the individual resonators being mounted on support wires which are separate from said or each coupling wire and are firmly secured to a common base plate which bears the entire electromechanical filter system.

2. A filter as claimed in claim 1, in which each said support wire engages its associated resonator in mutually identical fashion.

3. A filter as claimed in Claim 1 or Claim 2, in whieh said support wires are in the form of pins secured to the individual resonators by welding.

4. A filter as claimed in any preceding Claim, in which said support wires engage the individual resonators in the region of the nodal points of said resonators.

5. A filter as claimed in any preceding Claim, in which said individual resonators are of generally circular cross-section whose diameter is equal for all the resonators.

6. A filter as claimed in Claim 5, in which at least those resonators not serving as transducer resonators have a flattened portion which serves as a fixing for said or each coupling wire. 4S

7. S6 - 13 7. A filter as claimed in Claim 6, in which each said transducer resonator is provided with a first flat portion which serves as support for eleotrostrietive transducer material, this flat portion being orientated at right angles to the common plane containing the resonator axes in such manner that the or each said coupling wire is excited to longitudinal oscillations.

8. A filter as claimed in Claim 7, in which each said transducer resonator possesses a second flat portion provided for the support and fixing of the or each said coupling wire, said second flat portion extending at right angles to the first flat portion and having its surface parallel to said common plane.

9. A filter as claimed in any preceding Claim, in which each said transducer resonator is connected to a respective coil which acts together with the transducer self-capacitance to form a resonant circuit tuned to the middle frequency of the electromechanical filter, but having a band width considerably greater than the width Of the electromechanical filter.

10. A filter as claimed in any preceding Claim, in which the number of resonators provided in said cascade between the transducer resonators is restricted to that at which the fundamental transit time of the electric signals between the input and the output of the filter is not significantly greater than 0.8 ms.

11. A filter as claimed in any preceding Claim, in which attenuation poles at a frequency below the pass band are formed by means of resonators located outside the portion of the cascade enclosed by the transducer resonators, and attenuation poles at a frequency above the pass band are formed by additional coupling wires which 43756 - 14 serve to couple resonators which are not directly adjacent to one another.

12. A filter as claimed in Claim 11, in which each said additional coupling wire crosses over only one iriter5 mediate resonator.

13. A filter as claimed in any preceding Claim, in which the resonators located outside the portion of the cascade enclosed by the transducer resonators consists, at least at one end, of a plurality of resonators forming 10 a multiply resonant system, so that a plurality of attenuation poles are produced at that end.

14. A filter as claimed in any preceding Claim, in which said resonators are bending oscillators.

15. A filter as claimed in any one of Claims 1 to 15 13, in which said resonators are torsional oscillators.

16. A filter substantially as described with reference to Figures 1 to 3, or Figure 4, or as described with reference to Figures 1 to 3 or to Figure 4 as modified with reference to any one of Figures 5 to 7 in each