DE2657606C2 - Mechanical filter that has rod-shaped resonators and transducers - Google Patents

Description

The invention relates to a mechanical filter comprising rod-shaped resonators and transducers which Execute bending vibrations with two vibration nodes and each for itself in at least one Vibration nodes are stored.

Such a mechanical filter is, for example Described in DE-AS 15 41975. In the DE-AS 15 41 975 is mentioned that because of the small cross-section of the resonators and transducers of the series after connecting the coupling element, unwanted secondary vibrations are practically not transmitted.

In addition, the requirement is made there that the bending vibrations occurring perpendicular and parallel to the resonator flattening should differ with regard to their frequency position by at least one percent. This would e.g. B. at a lower cutoff frequency of the filter of about 48 kHz mean that the secondary waves of the resonators occur at about 500Hz below the lower cutoff frequency of the pass band.
However, it has been shown clearly in practice that the conditions are not sufficient in all cases to prevent undesired attenuation drops in the filter blocking areas.

The main cause of the parasitic vibrations is to be found in the transducers. The frequency of their secondary vibrations Due to a strong flattening, z. B. about 4 kHz above the Durchiaßbereiches.

Another cause of strong side wave collapses In the filter blocking areas, the secondary vibrations of the two transducers are direct neighboring resonators.

It is known that the retaining pins of the transducers and resonators act as coupled resonators and influence their resonance frequency.
In the article "The resonance frequency measurement of mechanical filter resonators with novel electromechanical transducers" by Chr. Give In Hochfrequenztechnik und Elektroakustik 76 (1967), pp. 142 to 147, the theoretical course of the resonator detuning for a compression resonator and bending coupler is given. This article deals with an arrangement for measuring purposes, in which a bracket coupler is firmly attached to a transducer resonator on one side and loosely pressed against the resonator to be measured on the other side and stimulates this to coinpressions oscillations by means of a thrust force.

From DE-PS 8 80 324 it is already known in Rod-shaped resonators made of piezoelectric material, which have a rectangular cross-section and which are dimensioned for laying vibrations with two nodes, disturbing vibrations by the Attachment of appropriately matched small resonance bodies at the nodes of the useful vibration and to suppress at the antinode of the disturbance oscillation. The mediated by DE-PS 8 80 324 Teaching is for the elimination of useful vibrations orthogonal spurious oscillations are not provided.

The object of the invention is, with mechanical Specify filters of the type mentioned at the beginning, such as attenuation drops in the blocked areas of these filters can be limited to a permissible level without affecting the transmission behavior of the filter.

This object is achieved according to the invention by the Im

characterizing part of claim 1 cited Features solved.

Advantageous further developments of the invention are described in the subclaims.

The invention is explained in more detail below. It shows the
1 shows the design of a multi-circle mechanical filter of known design,

Flg. 2a a transducer that is only in one vibration node stored and in the area of the node axis of the other vibration node

additional mechanical resonator is attached,

2b shows the direction of the useful oscillation F _n and the disturbance oscillation J ₀ on a transducer,

F i g. 3 a transducer, which is mounted in both vibration nodes and a holding wire on one of them additional mechanical resonator is attached,

Fig. 4 Attenuation curves above the pass band in the auxiliary wave range of the converter,

F i g. 5 the dependence of the resonance frequencies of the useful oscillation J _n or the orthogonal secondary oscillation J _{0 of} a transducer or resonator on the length of the additional resonator, in this case the length of an attached piece of wire,

Fig. 6 attenuation curves below the pass band in the spurious range of the resonators. .

The mechanical filter according to Flg. 1 consists of the resonators 1 and the end resonators 2, which at the same time act as transducers and which are fastened to the base plate 3 by means of the retaining wires 4 in a solder sump 5. The end resonators 2 are referred to below as "converters". The flexural vibrations of the transducers and resonators are transmitted by means of the coupling wire 6, which is fastened here in the middle of the transducers and resonators. The excitation of the mechanical vibrations and the conversion back into electrical vibrations takes place with the help of a piezoceramic plate 7, which is applied to the flattened area of the first and last resonator. In the Nutzschwingung J, _n is parallel to the longitudinal axis of the filter, the support wires are excited mainly to torsional 4, wherein the Nutzschwingung / _v orthogonal spurious J ₀ but to flexural and longitudinal vibrations, the uncoupled to the base plate 3, and these are transmitted to the filter output. This results in the attenuation notches shown in FIGS. 4 and 6 in the blocking areas of the filter.

Fig. 5 shows the dependence of the frequency of. Useful oscillation J _n , as well as the secondary oscillation orthogonal to this, of the length of a piece of wire 8, which is attached along one node axis of the transducer 2 in the catfish shown in FIG. 2a. The frequency / 1 results when either both retaining wires are soldered into the solder sump 5 or only one of the two and the other is completely missing. As the length of the piece of wire 8 nlmnu increases, the frequency J ₀ initially drops, then suddenly jumps to higher frequencies and then decreases again.

Provided that the from Flg. 2 a visible piece of wire 3 only bending vibrations executes, whereby the thrust at the free end is equal to zero, one obtains for zeroing the detuning the condition

ν is the speed of sound in the piece of wire
D is the diameter of the piece of wire.

Curve A of FIG. 4 is normally obtained when all holding wires of both transducers are firmly soldered in and the transducers almost match in terms of the frequency of their orthogonal secondary oscillation. _{If the length / 2} is selected for the piece of wire 8 of a transducer, its interference frequency shifts to / ₂ , ie downward and a damping curve according to curve B of FIG. 4 is obtained. For a length / ₃ , there is a further downward shift , and one obtains a course of attenuation according to curve C of FIG. 4. In curve D of FIG. 4, a length l _{A was} selected, resulting in a strong shift towards higher frequencies. The influence of the damping curve is even more pronounced if both converters are used to detune the interference frequency and a length / j is set for one and a length U for the other. As a result, one of the converter interference frequencies is shifted downwards, the others upwards, and curve E.

In FIG. 6, the influencing of the secondary ripple of the resonators according to the P-incip according to the invention is shown. The curve F \ n F i g. 6 shows the frequency response of a known type of filter in the lower stop band, while curve G shows the splitting of one minimum into several smaller ones with the aid of the attachment of pieces of wire 8 according to the invention. The way it works is the same as that shown above for the converters. The strong dips only come from the two resonators directly adjacent to the transducers.

A similar influence is obtained by attaching a piece of wire 9 to one of the holding wires 4 soldered into the two test according to FIG. 3.

Instead of shortening a piece of wire 8 or 9 on the finished filter, you can do it before the Shorten the assembly of the filter structure to such a constant length that the Preservation requirements are met. The attenuation behavior in the pass band of the filter does not change in any of the cases described.

For this purpose 3 sheets of drawings

k- I = (An-I) -

55

and for poles

k- I = (In- I)
Diirin mean further
k = Vs nf

-withn = 1,2,3 ,,,

ν D

I the length of that piece of wire
f the frenzy

60

65

Claims (5)

Patent claims: 1. Mechanical filter, which has rod-shaped resonators and transducers, the bending vibrations perform with two vibration nodes and each of which is stored in at least one vibration node, marked by the use of at least one additional resonator (8 or 9) that suppresses parasitic vibrations and which, through its attachment, taking into account the position of the nodes of the useful resonator oscillation (f _N ) and / or the useful transducer oscillation Ολ,) has no effect on the resonator - Usable vibrations or on the converter useful vibrations
with the proviso, that the additional mechanical resonator (8 or 9) or each of the additional mechanical Resonators (8 or 9) on selected transducers (2) and / or resonators (1) in the area of one of the two node axes of the converter useful vibrations or the resonator useful vibrations are provided Is and that by the additional mechanical resonator or for the additional mechanical Resonators selected tuning the interfering vibrations orthogonal to the useful vibrations of the Selected transducers (2) and / or resonators (1) are prevented in the blocking areas of the filter. 2. Mechanical filter ave claim 1, thereby characterized in that individual selected transducers (2) and / or resonators (1) nr ~ in one of the both vibration nodes are supported and in the area of the node axis of the other vibration node the additional mechanical resonator (8) is provided. 3. Mechanical filter according to claim 1, characterized in that all transducers and / or resonators in two points given by your nodal axes through to the direction of the useful oscillation vertical retaining wires (4) are stored and each of a retaining wire (4) selected transducers (2) and / or resonators (1) the additional mechanical resonator (9) is attached. 4. Mechanical filter according to claim 2, characterized in that the additional mechanical Resonator on the transducer (2) and / or resonator (1) perpendicular to the oscillation plane of the useful oscillation attached piece of wire (8) is (Fig. 2a). 5. Mechanical filter according to claim 3, characterized in that the additional mechanical The resonator is a piece of wire (9) attached to the holding wire (4) and running perpendicular to it (Fig. 3).