US3710275A

US3710275A - Low frequency oscillator employing a pair of u-shaped mechanical vibrators

Info

Publication number: US3710275A
Application number: US00087305A
Authority: US
Inventors: T Tanaka; K Bansho
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-08-24
Filing date: 1970-11-05
Publication date: 1973-01-09
Anticipated expiration: 1990-01-09
Also published as: CH511067A; GB1229871A; CH1273968A4; CH554552A; FR1577131A; DE1797167B2; DE1797167A1; US3659230A

Abstract

A U-shaped mechanical vibrator having a pair of strip-like vibratory reeds of substantially the same configuration, and a base portion coupling together the pair of vibratory reeds at one end as a unitary structure, the width of each reed being selected greater than the thickness thereof, the vibratory reeds being arranged in a single plane including their surfaces in the widthwise direction in parallel and side-by-side relation, and the pair of vibratory reeds vibrating in anti-phase relation to each other at right angles to the single plane.

Description

United States Patent 1191 Tanaka et a1. 1451 Jan. 9, 1973 [54 LOW FREQUENCY OSCILLATOR 2,942,205 6/1960 McShan ..331/116 M EMPLOYING A PAIR OF U-SHAPED 3,269,249 8/1966 Dailey .310/25 x MECHANICAL VIBRATORS 3,477,223 11/1969 Tilse etal. ..58/23 TF 3,517,230 6/1970 Lewis et al.... ..84/409 X [75] Inventors: Tetsuro Tanaka, Kyoto; Kiyoshi Bmsho Tokyo both of Japan Primary Examiner-Roy Lake [73] Assignee: Shlgeru Knkubari, Tokyo, Japan Assistant E iner-Siegfried H. Grimm Filed: Nov. 5 19-70 Attorney-H111, Sherman, Merom, Gross & S1mpson [21] Appl. No.: 87,305 [57] ABSTRACT Related U.S. Application Data A U-shaped mechanical vibrator having a pair of striplike vibratory reeds of substantially the same configu- [60] ggg f f it zg i a ggbr gg ig ration, and a base portion coupling together the pair 1970 3,659,230 of vibratory reeds at one end as a unitary structure, 1 the width of each reed being selected greater than the 52 U.S. c1 ..331/37, 58/23 TF, 318/128, thickness thereof, the vibratory reeds being arranged 331 1 M, 331 15 in a single plane including their surfaces in the [51] Int. Cl. ..H03b 5/30 wi hwise direction in parallel and side-by-side rela- [58] Field of Search ..331/37, 41, 116 M, 156; tion, and the pair of vibratory reeds vibrating in anti- 310/25; 318/128; 84/409; 58/23 TF; 333/71 phase relation to each other at right angles to the single plane. [5 6] References Cited 3 Clalms, 26 Drawing Figures UNITED STATES PATENTS 3,522,554 8/1970 Tanaka et a1. ..331/1l6 M l "1 /6l3 /ll%1l} /03 1 105,, l28-= l L J 134 #15 1 4 I26 II I32 1 1 1 1 I 1 1 1 1 I g I: 1

H3

115, 1 7 I I 1 1 l a, I "J l /m I '1 I i i 1 1 1 1' 1 1 1 /20 l 1 1 1 l 1 1 1 PATENTED JAN 9 I975 SHEET 03 0F 12 PATENTED JAN 9 973 SHEET UUUF 12 lii.

in Zempemlure PATENIEUJAH 9192s sum 05 0F 12 Iii. 14

PATENTED JAN 9 I975 3,710.2 75

SHEET lEUF 12 LOW FREQUENCY OSCILLATOR EMPLOYING A PAIR OF U-SHAPED MECHANICAL VIBRATORS REFERENCE TO RELATED APPLICATIONS This application is, a division of an application entitled U-Shaped Mechanical Vibrator, Ser. No. 754,416, filed Aug. 21, 1968, and now abandoned in favor of a continuation application of that same title, Ser. No. 88,507, filed Nov. 10, 1970, which issued as U.S. Pat. No. 3,659,230 on Apr. 25, 1972.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a vibrator for use in oscillators, mechanical filters or the like, and more particularly to a vibrator which is small in size, easy to manufacture and suitable for mass production.

2. Description of the Prior Art In the prior art the so-called tuning fork has been proposed as a vibrator but fabrication of such a conventional tuning fork involves an appreciable amount of time and high precision cannot be expected so that the prior art tuning fork is not suitable for mass production. Further, miniaturization is very difficult.

SUMMARY OF THE INVENTION The principal object of this invention resides in the provision of a novel mechanical vibrator which is free from the drawbacks experienced in the prior art.

The mechanical vibrator of this invention can be produced by punching process or etching of a thin sheet metal, and hence is easy to manufacture, high in precision and suited for mass production. Further, the present invention allows ease in the production of extremely miniaturized vibrators.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective viewshowing a conventional tuning fork;

FIG. 2 is a plan view showing one example of a planar tuning fork type mechanical vibrator produced according to this invention;

FIG. 3 is a side view of the vibrator depicted in FIG. 2;

FIG. 4 is a perspective view showing the manner in which the vibrator of FIG. 2 vibrates;

FIG. 5 is a graph showing loss-frequency characteristics relative to the position of electromechanical transducer elements;

FIG. 6 is a graph showing loss-frequency characteristics relative to the size of the electromechanical transducer elements;

FIG. 7 is a connection diagram illustrating one example of an oscillator circuit employing the vibrator of this invention;

FIG. 8 is a graph showing its frequency variation rate relative to temperature change;

FIG. 9 is a plan view illustrating, by way of example, one process for the manufacture of the vibrator of this invention;

FIG. 10 is a plan view showing still another example of the vibrator of this invention;

FIG. 1 l is a side view of the vibrator depicted in FIG. 10;

FIG. 12 is a plan view illustrating one example of a planar compound vibrator consisting of two vibrators of this invention assembled in side-by-side relation;

FIG. 13 is a side view of the planar compound vibrator exemplified in FIG. 12;

FIG. 14 is a connection diagram illustrating one example of the planar compound vibrator as applied to an oscillator;

FIG. 15 is a graph showing its frequency variation rate relative to temperature change;

FIGS. 16A and 16B are side views respectively illustrating other examples of the planar compound vibrator of this invention;

FIG. 17 is a plan view showing one example of a filter employing two planar vibrators of this invention;

FIG. 18 is a side view of the filter shown in FIG. 17;

FIG. 19 is a graph showing loss frequency characteristic curves of the filter of FIG. 17 with the coupling degree of its vibrators being as a parameter;

FIG. 20 is a graphical representation of the relationship of the coupling degree to frequency deviation;

FIG. 21 is a schematic diagram showing the connections of a signal source, the filter and output terminals;

FIG. 22 is a graph showing loss-frequency characteristic curves with an external resistance being as a parameter;

FIG. 23 is a plan view illustrating another example of the filter of this invention;

FIG. 24 is a perspective view illustrating still another example of the filter of this invention;

FIG. 25 is a plan view illustrating one example of a frequency selector device employing a plurality of vibrators of this invention; and

FIG. 26 is a side view of the frequency selector device depicted in FIG. 25.

DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1 there is schematically illustrated one example of a conventional mechanical vibrator, commonly referred to as a tuning fork, which consists of a pair of vibratory reeds 1a and lb arranged in opposing and predetermined spaced relation and a common base portion 2 interconnecting the reeds. Such a tuning fork is usually produced by machining of a metal block. However, the machining process presents a problem as it requires an appreciable amount of time and introduces a difficulty in obtaining the tuning fork with high precision and hence the prior art tuning fork is not suited for mass production with uniform characteristics. In particular, this imposes a severe limitation on the construction of small-sized tuning forks.

A detailed description will hereinafter be described in connection with a U-shaped mechanical vibrator of this invention which is free from the drawbacks encountered in the prior art.

In accordance with this invention the U-shaped mechanical vibrator consists of a pair of

vibratory reeds

3a and 3b formed of resilient metal sheets in substantially the same configuration and arranged to extend in parallel with each other in the same plane, while being spaced a predetermined distance d and a common plate-like base portion 4 joined contiguously to the vibratory reeds'3a and 3b and lying in the same plane as the reeds, as illustrated in FIGS. 2 and 3. On the

vibratory reeds

3a and 3b there are fixedly mounted in close end portion of the coupling portion 6' is bent substantially at right angles to be parallel with the

reeds

3a and 3b,.thus providing a mounting portion 7. The mounting portion 7 is secured to a base plate 6 by means of, for example, a screw 8 by which the U-shaped vibrator is mounted on the base plate 6. The U-shaped vibrator may be formed, for example, by punching a thin-metal sheet to have the

vibratory reeds

3a and 3b, the base portion 4, the coupling portion 6 and the mounting portion 7 as a unitary structure. The vibrator may be made of a resilient material having small coefficient of thermal expansion such, for example, as ELINVAR. With such a vibrator (hereinafter referred to as a planar tuning fork) it has been ascertained that the

vibratory reeds

3a and 3b vibrate opposite in phase in a direction at right angles to their surfaces (in this case the width W, of the vibratory '

reeds

3a and 3b is selected well greater than the thickness t thereof). Namely, the

vibratory reeds

3a and 3b vibrate in such a mode that they first swing away from their common reference plane in opposite directions and then back to the reference plane, as depicted in FIG. 4, in which manner the

vibratory reeds

3a and 3b repeatedly vibrate. r

The resonance frequency f of the planar tuning fork is given by the following equation:

f=(1/21r)=(a,,2/F) (z/V 12) /(E/p) (1) where am is a coefficientvarying with vibrating conditions of the tuning fork and is 1.8751 in the case of basic vibration, namely m being equal to l, l is the length of the

vibratory reeds

3a and 3b in their longitudinal direction (refer to FIG. 2), t their thickness (refer to'FlG. 3) p the density of their material and E its Youngs modulus.

For example, in the case where lis 18.5 mm., t is 0.5 mm., the width W, of the

vibratory reeds

3a and 3b is 1.8 mm. and the distance d, between the vibratory reeds is 1 mm., the resonance frequency f of the tuning fork is 1,015 c/s from the above equation 1.

Now, a discussion will be made in connection with the influence exerted on the vibration of the

vibratory reeds

3a and 3b by the position of the

piezoelectric elements

5a and 5b. With the distance d, from the

piezoelectric elements

5a and 5b to a demarcation line 9 between the

vibratory reeds

3a and 3b and the base portion 4 being 1 mm., and 1 mm., the elements a and 5b each being formed of PZT to have a length L of 3.0 mm., a width W of 1.7 mm. ancla thickness of 0.25 mm., the insertion loss'vs. frequency characteristics are respectively given as indicated by

curves

10, 11 and 12 in FIG. 5 in which the ordinate represents loss in dB and the abscissa frequency in c/s and d, is employed as a parameter. As will be apparent from the graph, when d, 1 mm. the resonance frequency f is 1,015.5 c/s nearly equal to the aforementioned calculated value and when d, 0 and 1 mm. the resonance frequencies are respectively as high as 1,041.5 c/s and 1,044.5 c/s.

This is considered to result from the fact that the

piezoelectric elements

5a and 5b act as stiffness on the vibratory reeds 3a and'3b, not as mass. The Qs of the planar tuning fork in the above three cases are 1,128 in the case of d, being 1 mm., 1,160 in the case of d, being 0 and 1,158 in the case of d, being -1 mm., and the Q in the first case is a little lower than the others. This implies that the Q lowers as the piezoelectric elements approach the side of the

vibratory reeds

3a and 3b of large amplitude, that is, their free ends. Consequently, the in- 'sertion loss is most minimized with d, being 1 mm. The ratio of an output voltage to an input voltage, that is, the coupling factor is a little greater when d, 1 mm. The foregoing numerical values are given in the following table 1.

TABLE 1 Position Resonance Q Insertion Coupling (1, mm frequency (c/s) loss (dB) factor A, "1015.5 1128 0.7 0.029 B 1041.5 1160 2.0 0.027 C-l 1044.0 1158 2.0 0.027

FIG. 6 is a graph showing the influence exerted upon the insertion loss by changing the size of the

piezoelectric elements

5a and 5b, in which the ordinate represents the loss in dB and the abscissa frequency in c/s. In the figure the curve 13 indicates a case of the piezoelectric elements each having a length of 3 mm. and a width of 1.7 mm. and the curve 14 a case of the elements having a size of a length of 1.3 mm. and a width of 1.2 mm. It appears from the graph that the resonance frequencies are approximately equal to 1,015.5 c/s and Qs are also substantially equal to 1,128 but that the insertion losses are 0.7 dB and 3 dB. Further, the coupling factors are 0.027 and 0.025, namely the smaller piezoelectric elements slightly lower the coupling factor.

It is preferred that the distance d between the

vibratory reeds

3a and 3b be smaller than the width W of each reed, for example, less than one-half thereof. This is because of the fact that with the distance d being greater than the width W of the

reeds

3a and 3b, torsion or twist is yielded in the base portion 4, that is, the coupling portion of the reeds which is likely to cause a variation in the resonance frequency. In addition, the width W, of the vibratory reeds is selected smaller than the length l of the base portion 4 in the lengthwise direction of the vibratory reeds and the length I; is selected, for example, more than 1.5 times as great as the width W The reason is that the length I, smaller than the width W, causes an increase in vibration rendered to the mounting portion 7 through the coupling portion 6 from the base portion 4. Where the

piezoelectric elements

5a and 5b are deposited on the same side of the

vibratory reeds

3a and 3b, an input signal fed to either one of the piezoelectric elements and an output signal obtained from the other piezoelectric element are opposite in phase. Consequently, an output signal in phase with the input signal may be obtained by the use of piezoelectric elements of opposite polarities or by depositing the piezoelectric elements on different surfaces of the vibratory reeds. I FIG. 7 illustrates one example of a self-excited oscillator employing the planar tuning fork of this invention described above, which may be provided by the same connections as a self-excited oscillator using a conventional tuning fork except the use of the planar tuning fork of the present invention, as depicted in the figure. Namely, an amplifier 21 consisting of

transistors

19 and 20 of cascade connection is provided and connections are made such that one portion of the output from the output terminal of the amplifier 21 is applied to the piezoelectric element, for example 5a of the vibratory reed 3a of the planar tuning fork and an output obtained by the other piezoelectric element 5b of the vibratory reed 3b is fed to the input side of the amplifier 21, that is, to the base of the transistor 19, thus providing an oscillator having an oscillation frequency determined by the resonance frequency of the planar tuning fork. In FIG. 8 there is depicted the frequency variation rate Af/f relative to temperature change (C) in the above case, the ordinate representing the frequency variation and the abscissa temperature. This graph" was obtained in the case where only the tuning fork was subjected to temperature change. From the graph it appears that the frequency variation is 1 X 10 deg. in a range of about 19C to 59C, which indicates an excellent characteristic.

Although the foregoing has stated that the planar tuning fork of this invention may be produced by press work (punching process), it may also be made of one base plate by means of chemical etching techniques. That is, grooves 16 are formed in a thin sheet of ELINVAR by chemical etching in a manner to leave the

vibratory reeds

3a and 3b, the base portion 4 and the coupling portion 6', as shown in FIG. 9. In this case, it is preferred to form a groove 17 by chemical etching at a distance from the coupling portion 6' on the opposite side from the vibratory reeds. With photoetching techniques used in the manufacture of semiconductor devices, smalIed-sized tuning forks can be produced with high precision, and the planar tuning fork thus produced is suitable for use with, for example, semiconductor integrated circuit devices.

In the foregoing example the piezoelectric elements are used as electromechanical transducer elements but may be substituted with electromagnetic transducer elements. Namely, instead of mounting the

piezoelectric elements

5a and 5b on the planar tuning fork, electromagnetic units 18a and 18b each consisting of a core and a coil wound thereon are disposed opposite the

vibratory reeds

3a and 3b in the vicinity of the free ends where their amplitude of vibration is great. With an exciting signal being fed to, for example, the electromagnetic unit 18a, the vibratory reed 3a is driven to vibrate and the vibratory reed 3b is thereby driven through the base portion 4, by which an electric signal of the same frequency as the signal fed to the unit 18a is obtained from the electromagnetic unit 18b.

As has been described in the foregoing, this invention enables mass production of planar tuning forks of high precision and uniform characteristics without requiring such a troublesome machining of a metal block.

In FIG. 12 there is illustrated another example of this invention in which a plurality of, for example, two planar tuning forks such as depicted in FIG. 2 are jointed together at their base portions with their surfaces being flush with each other. A detaileddescription will hereinbelow be given of this example.

Reference numerals

101 and 102 indicate two planar tuning forks, which are assembled together in the following manner. That is, vibratory reeds 103a, l03b and 103a,, 103b, are disposed substantially in parallel at moderate intervals with their planar surfaces being substantially flush with one another, and coupling portions 106' and 106, extending from the

base portions

104 and 104, on the side remote from the vibratory reeds in parallel relation thereto are secured to a common support 113, thereby providing an assembly of the tuning forks. In this case the

base portions

104 and 104, of the

planar tuning forks

101 and 102 are jointed together through a joint portion 109.

Reference numerals

105a, 105b, 105a and l05b designate piezoelectric elements fixed mounted on the vibratory reeds of the planar tuning forks, which are identical with those exemplifiedin FIG. 2. The planar tuning forks thus assembled together are mounted on a base plate 106 by clamping their common support 113 onto the base plate 106 by means of a screw 116 inserted through a hole 115 bored in the support 113. It is a matter of course that such a compound tuning fork consisting of the two

planar tuning forks

101 and 102 depicted in FIG. 12 may be produced from a sheet metal by means of punching or etching in the same manner as that described above with FIG. 2.

The resonance frequency of each tuning fork is determined by the equation 1 as previously described but the frequencies of the two planar tuning forks are rendered different from each other by selecting the length of the vibratory reeds of either one of the tuning forks to be smaller or greater than that of the reeds of the other. In FIG. 12 the

vibratory reeds

103a, and 103b, of the tuning fork 102 are shorter than those 103a and l03b of the other tuning fork 101. It is preferred that the vibratory reeds of the same tuning forks, that is, 103a and 1031; or 103a, and 103b are identical in shape with each other.

Assuming that the widths of the vibratory reeds of the

planar tuning forks

101 and 102 are taken as a, and a the distance between the vibratory reeds 103a and 103k is b,, the distance between the

reeds

103a and 103b, is b and the lengths of the

reeds

103a, 1013b and 103a,, 103b, are I, and I respectively, a 11,, a, and b are selected such that a b, and a b This is because of the fact that b, or b exceeding a or a produce torsion in the base portion 104 or 104' to cause a change in the resonance frequency. Further, if the lengths of the

base portions

104 and 104 in the lengthwise direction of the vibratory reeds are taken as c, and c (c 0 in the figure), c and c are selected greater than the widths a and a of the vibratory reeds. With being smaller than a, or a,, vibration of the

base portion

104 or 104, in the direction of the

coupling portions

106 and 106 increases, which is undesirable as set forth above. In order to provide the coupling portion 109, a slit 116' is formed in the jointed portion of the

base portions

104 and 104, on the side of the vibratory reeds, in which case the depth W of the slit is selected great enough to permit the

tuning forks

101 and 102 to function independently from each other.

For instance, a typical size is such that a, a, 1.8 mm., b, b, 1.0 mm., W 1.6 mm., the thickness t of the

vibratory reeds

103a, 103b and 103a,, 103b; 0.5 mm. and the distance d between the adjacent

vibratory reeds

103b and 103a, 1.0 mm. In such a case, the

resonance frequencies f; and f, of the

planar tuning forks

101 and 102 are 1,377 c/s and 1,299 c/s respectively. To electrically drive the tuning

forks

101 and 102 to-obtain electric signals therefrom, piezoelectric elements 105a,-105b and 1050,, 105b, of, for example, PZT may be deposited by an adhesive binder on the

vibratory reeds

103a, 103b and 103a, and 103b, on the side of the base portions 104 and 104 g In the example shown in FIG. 12 the

planar tuning forks

101 and 102 are formed as a unitary structure but each of them performs the function of substantially an independent tuning fork. Consequently, it is possible that separate oscillators having the tuning forks as the reference frequency sources are provided and adapted to obtain an output of a frequency corresponding to a difference in their oscillation outputs, so that an oscillation output of low frequency can be obtained with relatively small tuning forks. FIG. 14 is a connection diagram illustrating one example of such construction. In the figure reference numeral 124 indicates an amplifier consisting of transistors 119a and 119b, the transistor 119a having its collector connected to a power source 120 and its emitter grounded through a resistor 121 and connected to the base of the transistor 119b and the transistor 11% having its collector connected to the power source 120 through a resistor 122 and its emitter grounded through a resistor 123. Reference numeral 125 designates an oscillator circuit having incorporated therein the amplifier 124. Namely, the input end of the amplifier 124, that is, the base of the transistor 119a is connected to, for example, the piezoelectric element 105b of the vibratory reed 103b of the planar tuning fork 101, and the output side of the amplifier, that is, the collector on the transistor 1 19b is connected to the piezoelectric element 105a of the vibratory reed 103a, by which the vibratory reed 103a is driven to drive the vibratory reed 1013b and the transistor 11% is driven by an output of the piezoelectric element 105b of the vibratory reed 103b to permit oscillation of the oscillator circuit 125 at the resonance frequency of the planar tuning fork 101. In a similar manner an amplifier 130 is constituted with

transistors

126a and 126b, and the input side of the amplifier 130, that is, the base of the transistor 126a is connected to the piezoelectric element 105b, of the other tuning fork 102 and the output side of the amplifier, that is, the collector of the transistor 126b is connected to the piezoelectric element-105a,, thus providing an oscillator circuit 126 oscillating atthe resonance frequency of the planar tuning fork 108. Further, these

oscillator circuits

125 and 126 are interconnected and their outputs are fed to a frequency converter. In the figure the

oscillators

125 and 126 are coupled together by the mechanical coupling of the

planar tuning forks

101 and 102 through the coupling portion 109, under which conditions when the oscillator, for example, 126 is main, the oscillation frequency f, of the oscillator 126 isamplitude-modulated at the oscillation frequency f,

of the oscillator 125. Accordingly, the output of the oscillator 126, that is, the collector output of the transistor 126b is fed through a low-pass filter 131 to an amplifier 133 consisting of a transistor 132 of emittergrounded connection, from an output terminal 134 of which amplifier can be obtained a signal f, -f, =f corresponding to a difference in the oscillation frequencies of the oscillators and 126. It is also possible in this case that the respective outputs of the

oscillators

125 and 126, that is, the output of the transistor 119b and the collector output of the transistor 126b are separately applied to the common frequency converter circuit to obtain a beat frequency therebetween. In order to facilitate coupling of the planar tuning forks through the coupling portion 109 for obtaining an amplitudemodulated signal, it is preferred that in the planar tuning fork 102 of the main oscillator the outer vibratory reed 103b, (on the opposite side from the planar tuning fork 101) is a drive side and the inner reed 103a, a pickup side and that in the other planar tuning fork 101 the inner vibratory reed l03b is a drive side and the outer reed 103a a pickup side. The

planar tuning forks

101 and 102 for producing the reference frequencies of the

oscillators

125 and 126 for obtaining a beat signal are produced as a unitary structure by'punching of a resilient sheet of metal, forexample, ELINVAR. Consequently, the frequency vs. temperature characteristics of the two oscillators are substantially the same and further since a difference frequency is obtained, a

beat frequency output remarkedly stable in tempera-- ture can be produced. In FIG. 15 there is illustrated frequency change ratio Af/f relative to temperature change in the case where only the

planar tuning forks

101 and 102 are subjected to temperature change. It appears from the graph that the temperature coefficient is substantially zero in a temperature range of 19C to 65C. In the illustrated example f, 1,377 c/s, f 1,299 c/s and f 78 c/s at a temperature of 20C. Since a signal corresponding to the difference in the oscillation frequency between the two oscillators is taken out as described above, even if the

planar tuning forks

101 and 102 are miniaturized, a low-frequency signal can be obtained. In the prior art, a tuning fork oscillating, for instance, at 78 c/s is bulky and is difficult to drive. In the example depicted in FIG. 14 the piezoelectric elements are employed as electric transducer elements but they may be replaced with, for instance, electromagnetic transducer units or electrostatic transducer units, as will hereinbelow be described with FIG. 16. That is, as depicted in FIG. 16A an electromagnetic unit consisting of a magnetic member and a coil wound thereon is disposed opposite the vibratory reed of the planar tuning fork, or fixed electrode 136 is placed as the electrostatic transducer unit in opposed relation to the vibratory reed, as

illustrated in FIG. 16B, in which case a high DC voltage source 137 is applied between the electrode 136 and the vibratory reed while at the same time applying or taking out an AC signal. It will be apparent that all or tuning forks, in which case a difference between the vibration frequencies of two tuning forks is first obtained and then a difference between the resulting difference frequency and the vibration frequency of the remaining tuning fork is obtained. Further, it is easy to produce an oscillator having oscillation frequencies corresponding to the differences in the vibration frequencies of more than four tuning forks.

In FIGS. 17 and 18 planar tuning forks of this invention such as depicted in FIG. 2 are mechanically coupled together in the same manner as in the example shown in FIGS. 12 and 13. The similar parts to those in FIGS. 12 and 13 are identified by the similar reference numerals and no detailed description will be repeated for the sake of brevity. In this case the

vibratory reeds

1030, 103b, 103a and 103b, of the

planar tuning forks

101 and 102 are substantially equal in length I to one another. As driving and detecting elements of a filter, piezoelectric elements of, for example, PZT are used but in the present example the piezoelectric elements are mounted on the two outer

vibratory reeds

103a and 103b, of the

tuning forks

101 and 102 in proximity to the

base portion

104 and 104 thereof, as indicated by 105a and 105b,.

The resonance frequency f of these

planar tuning forks

101 and 102 is given by the aforementioned equation 1;

For example, where L, 1.85 cm. and t= 0.05 cm., the resonance frequency f is 10,150 c/s.

Applying an electrical signal to the piezoelectric element 105a of the planar tuning fork 101, the vibratory reed 103a is thereby vibrated, which leads to driving of the vibratory reed 103b in anti-phase relation to the reed 1030, thus rendering the planar tuning fork in its driven condition. This applies vibration to the planar tuning fork 102 through the coupling portion 109 to cause its vibratory reeds 103a, and 1031b to be driven at the same time, with the result that an electrical signal is taken out from the piezoelectric element 105b, of the vibratory reed 103b, of theplanar tuning fork 102. In such a case, the

planar tuning forks

101 and 102 are caused to vibrate only by a signal having a particular frequency equal to their resonance frequency, and they hardly vibrate at other frequencies. Therefore, a filter having a pass band corresponding to. the oscillation frequency of the planar tuning forks can be provided.

By the way, if the width of the vibratory reeds of each of the

planar tuning forks

101 and 102 is taken as a and the distance between adjacent vibratory reeds is b, b is selected to be about one half of a. When b is greater than a pseudo-vibration is caused in the

base portions

104 and 104 to shift the resonance frequency. In addition, the length c of the

base portion

104 or 104, in the lengthwise direction of the vibratory reeds is selected greater than the width a of the vibratory reeds, for example, about 1.5 times as great as the width 0. This is because of the fact that when is smaller than a vibration is much transmitted to the support 113 through the coupling portions 106' and 106,. It is preferred to locate the

piezoelectric elements

105a and 105b, a little further to the free end of the vibratory reeds than the demarcation between the reeds and the

base portions

104 and 104,. In this case the resonancefrequency of the planar tuning forks becomes approximately equal to the aforementioned equation 1. There is a tendency that shifting of the piezoelectric elements toward the free ends of the vibratory reeds causes their resonance frequencies to deviate higher. Now, the coupling portion 109 will be discussed. If the distance from the coupling portion 109 to the demarcation between the vibratory reeds l03b and 103a, and the

base portions

104 and 104, is taken as W, a loss characteristic curve with W varying as a parameter is as shown in FIG. 19, in which the ordinate represents loss in dB and the abscissa frequency in c/s. That is, curves 12, 13 and 14 respectively indicate the cases of W being 1.50 mm., 1.56 mm. and 1.62 mm. From the graph it appears that a decrease in W tightens the coupling of the two

planar tuning forks

101 and 102 and causes the characteristic curve to be double-humped and widens its pass band, for example, up to about 8 c/s in the graph. In the case of the curve 13 the pass band is approximately 5 c/s. With the lowering of the coupling of the planar tuning forks, that is, with an increase in W, the characteristic curve becomes to be substantially single-humped and its pass band width becomes 2.5 c/s. The depths of the troughs of the

curves

12, 13 and 14 are respectively 8 dB, 5 dB and 1 dB. It will be understood from this that the band width can be widened by decreasing W to increase the coupling of the

planar tuning forks

101 and 102 and that the band width can be narrowed by increasing W to decrease the coupling. In the above example the values of a, b, ..t are those previously mentioned and the piezoelectric elements of PZT are employed and are of a size of 3 X 2 mm Further, it will be seen that the relation between the frequency difference of the peaks of the characteristic curves and W is such that an increase in W causes a linear decrease in the frequency difference as shown in FIG. 20, the abscissa representing W in mm. and the ordinate the frequency difference in c/s.

The filter characteristic of the filter such as depicted in FIGS. 17 and 18 can be improved by input and output resistances. That is, as shown in FIG. 21, a signal is applied to the filter 215 of this invention from a signal source through a resistor 217, namely the signal is fed to the piezoelectric element 105a of the filter 215, and a resistor 218 is connected between the output side or the piezoelectric element 105b, and ground, and output terminals 219 are led out from the both ends of the resistor 218. This provides a maximum output when the resistance value R of the resistor 218 on the output side is (1/21rfc c, being the capacitive component of the piezoelectric element PZT. Where the resonance frequency is 1,000 c/s and the capacitive component 0 of PZT is 2,000 PF, R l megohm is a maximum output. In FIG. 22 there is illustrated a graph showing loss frequency characteristic curves obtained with the resistance values of the

resistors

217 and 218 being altered, the ordinate representing loss in dB and the abscissa frequency in c/s. The curve 20 in the graph indicates the case where the resistance value R, of the resistor 217 is zero and R is infinite, in which case the depth of the trough between peaks of the curve 20 is 6.5 dB and the insertion loss at the peaks is approximately zero. When R, 0 and R 1 M9, the insertion loss at the peaks is about 9 dB but the characteristic curve becomes nearly single-humped and the depth of the trough is 3 dB. Further, when R, is 1 M0, the insertion loss is the same as in the above but the characteristic curve becomes further single-humped and the depth of the trough is about 1 dB. The band width does not vary with the resistance value R but lowers from 5 cls to 4.5 cls when R, is altered from to l MQ.

As has been described above, this invention permits of fabrication of the tuning fork filter by means of punching a sheet metal and allows ease in mass production of miniature and highly precise filters. The band width can be adjusted by controlling the coupling degree of the coupling portion 109 of the two

planar tuning forks

101 and 102.

The filter described above exhibits excellent temperature characteristic such that the frequency change ratio relative to a temperature change is -l X '10 or so. In addition, the cut-off characteristic of the filter is also excellent, as will be seen from the aforementioned loss characteristic.

The cut-off characteristic can be enhanced by further connecting the planar tuning forks in side-byside relation, as exemplified in FIG. 23. In the figure a planar tuning fork 103 identical with the

forks

101 and 102 is interposed therebetween, and these tuning forks may be produced by punching of a sheet metal. In this case all the planar tuning forks are coupled integral with the support 113 through a

coupling portion

106, 1 extending from a base portion 104, of the intermediate structure of the planar tuning forks. With the three

planar tuning forks

101, 102 and 103 being coupled together in side-by-side relation, a filter of excellent selectivity-can be obtained. It is possible to connect more planar tuning forks in side-by-side relation.

In the above example, the planar tuning forks are arranged with their vibratory reeds lying substantially in one plane but such arrangement is not always necessary. It is sufficient only to dispose a pair of vibratory reeds of at least one planar tuning fork in one plane. For example, it is possible that a plurality of planar tuning forks are assembled together by suitable coupling member in a manner to arrange their pairs of vibratory reeds one over another in spaced relation, as exemplified in FIG. 24. In the figure, four

planar tuning forks

301, 302, 303 and 304 each having their two vibratory reeds in one plane are employed and these planar tuning forks are assembled together to arrange their pairs of vibratory reeds one over another in opposed and moderately spaced relation, and

base portions

304, 304 304, and 304 of the

planar tuning forks

301, 302, 303 and 304 are extended in a direction remote from the vibratory reeds. The extended base portions are respectively put between block members 30b,-30b,, 30b, and 30b, and bonded together, and coupling members 309 309, and 309, are each interposed between adjacent planar tuning forks at a place on the

base portions

304, 304 304, and 304 or on the vibratory reeds close to the portion, thus interconnecting the planar tuning forks. In this case it is preferred that the planar tuning forks are each produced by punching of a sheet metal and assembled together so as to ensure uniformity of their temperature characteristic. Also in this case piezoelectric elements are mounted on the vibratory reeds of the uppermost and lowermost tuning forks.

In the examples above described with FIGS. 17 23 and 24 the filters employ the plate-like planar tuning forks and hence they can be miniaturized in construction. Further, the planar tuning forks of high precision and uniform characteristics can be mass produced by punching process to ensure the fabrication of excellent filters. The use of the photoetching techniques employed in the manufacture of semiconductors allows ease in the production of small-sized planar tuning forks,'which leads to further miniaturization of the filters.

In the above examples the piezoelectric elements are used as driving and detecting elements of the filters but they may be replaced with electromagnetic or electrostatic elements. It is of course possible to employ these three electromechanical transducer elements in combination.

FIGS. 25 and 26 illustrates a frequency selector unit which is applicable to an alarm device, a frequency analyzer or the like and in which a plurality of planar tuning forks of this invention are employed and signals are selectively picked up according to their frequencies from many input signals of different frequencies.

In the

figure reference numerals

401, 402, 403, 40n respectively designate planar tuning forks such as exemplified in FIG. 2. The

tuning forks

401, 402, ..40n are respectively provided with a pair of

vibratory reeds

401a and 401b, 402a and 402b, 40m and 40nb, in exactly the same manner as in the foregoing examples. All the

planar tuning forks

401, 402, ..40n are assembled together as a unitary structure at their

base portions

404,, 404 404,, through

coupling portions

409,, 409 ..409,, in such a manner that their vibratory reeds 401a, 401b, ..40na, 40nb may lie in the same plane at certain intervals. Free ends of

coupling portions

406,, 406 ..406,,' extending from the central portions of the base portions of the tuning forks in a direction opposite to the vibratory reeds are bent substantially at right angles and are secured to a support 413 parallel to a base plate 406 which support 413 extends substantially in parallel with the vibratory reeds. Further, electromechanical transducer elements such, for example, as piezoelectric elements 4015a and 4015b, 4025a and 4025b, ..40n5a and 40n5b are deposited, by means of an adhesive binder, on the

vibratory reeds

401a and 401b, 402a and 402b, .....40na and 40nb of the

planar tuning forks

401, 402, ..40n in proximity to the

base portions

404,, 404,, ..404n thereof. The

planar tuning forks

401, 402, 403, ..40n are different in length of the vibratory reeds so as to obtain different resonance frequencies. The resonance frequencies f of the planar tuning forks are given by the aforementioned equation 1 as in the foregoing examples:

In FIG. 25 the

planar tuning forks

401, 402, 403, .....40n are adapted such that their resonance frequencies f f f ..f,,, gradually lower. Namely, the lengths of the vibratory reeds of the planar tuning forks are rendered sequentially greater. In-each tuning fork the width a of its vibratory reeds is selected greater than the space b between the reeds, for example, ((1 12) z b With I); being greater than a; torsional vibration is yielded in the base portion to change the resonance frequency f and hence a stable resonance frequency cannot be obtained. Further, the length 0,, of the base portion in the lengthwise direction of the vibratory reeds is selected greater than the width a for example, l.5a z When c,, is smaller than a vibration of the vibratory reeds is much transmitted from the coupling portion to the support to cause loss. As in the foregoing examples, the vibratory reeds of each planar tuning fork vibrate in anti-phase relation to each other at right angles to their plane. That is, they vibrate in such a manner that they first go away from their'plane in opposite directions and then back to the plane.

In FIG. 25 the

coupling portions

409,, 409 .....409,, are formed contiguous to adjacent ones at the ends of the base portions of the planar tuning forks remote from the vibratory reeds but coupling members may be formed contiguous directly to the vibratory reeds in proximity to the base portions thereof. These planar tuning forks, their coupling portions and the support may be produced by punching process of a sheet of a resilient alloyed metal such as ELINVAR. That is, they are produced successively from one material without interruption. Where the piezoelectric elements of PZT are employed as the electromechanical transducer elements, it is preferred to locate them on the vibratory reeds of the planar tuning forks in close but definitely spaced relation to the base portions so as to obtain resonance frequencies nearly equal to calculated values. Further, it is also preferred to form the

coupling portions

409,, 409 409 ..409,, as small as possible to ensure the elimination of interference between adjacent planar tuning forks. The support 413 for mounting the planar tuning forks on the same base plate 406 may be formed in common to all the tuning forks but may be provided separately for each of them.

The electromechanical transducer elements 4015a, 4025a, 4035a, ..40n5a, each being one of the elements in pairs, are connected to a common signal source 411. Namely, signals are impressed from the common signal source 41 1 to the planar tuning forks in parallel relation. While, signals are separately led out from the other electromechanical transducer elements 4015b, 4025b, 4035b, ..40n5b. In the figure transistors 412a, 412b, 412e, ..412n are provided each for each of the

planar tuning forks

401, 402, ..40n and are connected, for example, in am emitter grounded manner, the collectors of the transistors being respectively connected to oneend of a power source E through resistors 4130, 413b, 413c ..4l3n and the bases being respectively connected to the transducer elements 4015b, 4025b, ..4025n of the output side of the planar tuning forks. In addition,

output terminals 4140, 414b, 4140, ..414n are connected to the collectors of the transistors.

With such an arrangement as shown in FIG. 25, upon application of a signal having a frequency f from the signal source 41 1 to the selector, only the planar tuning fork 401 vibrates to thereby feed a signal to the transistor 412a, providing a signal of the frequencyf at the output terminal 414a. In a similar manner, only the planar tuning fork'having a resonance frequency equal to the frequency of a signal fed from the signal source vibrates to provide a signal output at the terminal corresponding to the vibrated planar tuning fork, by which the frequency of the signal from the signal source 411 can be known. Even if a plurality of signals of different frequencies are simultaneously applied from the signal source 411 to the selector, only the planar tuning forks corresponding to the particular frequencies vibrate to produce signals at the corresponding output terminals. Thus, the selector unit of this example enables detection of the frequencies of the signals from the signal source 411 at the output terminals. Consequently, the unit can be used as a frequency analyzer unit, an alarm device or the like. In the case of the alarm device, a plurality of alarm signal sources placed at different locations are adapted to raise alarm signals of different frequencies, by which detection of an output produced at any of the output terminals 414a, 414b, 4140, ..414n enables location of the source of the alarm signal being raised.

As described above, the selector unit employing the planar tuning forks of this invention enables discrimination of signal frequencies and in this case the

planar tuning forks

401, 402, 403, ..40n are formed as a unitary structure with their vibratory reeds lying in substantially the same plane, so that the entire structure can be produced by punching process of one sheet metal to facilitate the fabrication. Further, these planar tuning forks are of substantially the same temperature characteristic and hence a selector unit stable in temperature can be obtained. Even if the temperature characteristic is a little altered, the alterations of the planar tuning forks are rendered uniformly in the same direction and hence can easily be compensated for. If the punching template of high precision is made, highly precise selector unit can be mass produced.

In the foregoing the piezoelectric elements are used as the electromechanical transducer elements but they may be substituted with electromagnetic or electrostatic transducer elements. In addition, a high precise selector unit can be produced as described above, so that if input signal frequencies are extremely close to one another, the insertion loss can be held low because of sharp resonance frequency characteristic of the planar tuning forks. For example, when a, 1.8 mm., b 1.0 mm., c 1.35 mm., d,,= 18.5 mm. and t 0.5 mm., the resonance frequency is 1,015.0 c/s and Q is more than 1,100 and insertion loss can be held lower than 0 to several dB.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.

We claim as our invention:

l. A low frequency oscillator comprising two U- shaped mechanical vibrators, each including a pair of strip-like vibratory reeds of substantially the same configuration and a base portion coupling together the pair of vibratory reeds at one end as a unitary structure, the width of each of the vibratory reeds being selected fully greater than the thickness thereof, the pair of vibratory reeds being arranged in spaced and parallel relation to each other with their surfaces lying in substantially the same plane and vibrating in anti-phase relation to each other at right angles to the plane which includes their surfaces, the length of the vibratory reeds of one of said

Claims

1. A low frequency oscillator comprising two U-shaped mechanical vibrators, each including a pair of stRip-like vibratory reeds of substantially the same configuration and a base portion coupling together the pair of vibratory reeds at one end as a unitary structure, the width of each of the vibratory reeds being selected fully greater than the thickness thereof, the pair of vibratory reeds being arranged in spaced and parallel relation to each other with their surfaces lying in substantially the same plane and vibrating in anti-phase relation to each other at right angles to the plane which includes their surfaces, the length of the vibratory reeds of one of said U-shaped mechanical vibrators being selected shorter than that of the other vibrator so that said vibrators vibrate at the respective frequencies f2 and f1, a first electromechanical transducer element fixedly mounted on one of the vibratory reeds of said one U-shaped mechanical vibrator and a second electromechanical transducer element fixedly mounted on the other vibratory reed of said one U-shaped mechanical vibrator, a first amplifier having an input connected to said first transducer element and an output connected to said second transducer element, a third electromechanical transducer element fixedly mounted on one of the vibratory reeds of said other Ushaped mechanical vibrator, a fourth electromechanical transducer element fixedly mounted on the other vibratory reed of said other U-shaped mechanical vibrator, a second amplifier having an input connected to said third transducer element and an output connected to said fourth transducer element, a mechanical coupling connecting said base portions of said vibrators to effect amplitude modulation of the frequency f2 at the frequency f1, and a low pass filter connected to said output of said first amplifier to provide the low frequency f0 f2-f1.

2. A low frequency oscillator according to claim 1, wherein each of said transducer elements is a piezoelectric element.

3. A low frequency oscillator according to claim 1, comprising a third amplifier connected to said low pass filter for receiving and amplifying oscillations at the low frequency f0.