US3686593A

US3686593A - Electromechanical resonator

Info

Publication number: US3686593A
Application number: US10409A
Authority: US
Inventors: Hisham Mohamed Saadail Zakaria
Original assignee: International Standard Electric Corp
Current assignee: STC PLC
Priority date: 1969-03-07
Filing date: 1970-02-11
Publication date: 1972-08-22
Anticipated expiration: 1989-08-22
Also published as: DE2010196A1; GB1277614A; CH526246A; FR2037731A5

Abstract

An electromechanical resonator is provided incorporating a vibrating reed driven by electrostatic forces between itself and an output electrode. The output is represented by capacitance changes between the reed and an output electrode. Multiple resonators may be used to form band-pass filters and the like.

Description

United States Patent Zakaria ELECTRQMECHANICAL RESONATOR [72] Inventor: Hisham Mohamed Saadallah Zakaria, London, England 73 Assignee: International Standard Electric Corporation, New York, NY.

221' Filed: Feb.l1, 1970 21 Appl.No.: 10,409

[30] Foreign Application Priority Data March 7, 1969 Great Britain ..12,218/69 [52] US. Cl. ..333/71, 333/72, 310/82, 317/249, 200/181, 310/6 Traub ..333/76 1451 Aug. 22, 1972 3,020,455 2/1962 Reifel ..317/250 3,283,226 11/1966 Umpleby et al ..317/249 R 2,542,611 2 1951 Zuck ..317/250 x 3,192,456 6/1965 Reifel e161. ..317/250 3,166,696 1/1965 Furman ..317/250 3,413,497 11/1968 Atalla ..200/181 x FOREIGN PATENTS 0R APPLICATlONS 321,151 10/1929 Great Britain ..333 71 865,093 4/1961 01661131161111 ..333/71 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Att0mey-C. Cornell Remsen, Jr., Walter J. Baum, Percy P. Lantzy, .1. Warren Whitesel, Delbert P. Warner and James B. Raden ABSTRACT An electromechanical resonator is provided incorporating a vibrating reed driven by electrostatic forces between itself and an output electrode. The output is represented by capacitance changes between the reed and an output electrode. Multiple resonators may be used to form band-pass filters and the like.

24 Claims, 25 Drawing Figures Patented Aug. 22, 1972 3,6855%3 8 Sheets-Sheet l Attorney Pate med Aug.22,1972 3,66,53

a Sheets-Sheet 2 Patented Aug. 22, 1972;

8 Sheets-Sheet 5 Patented Aug. 22, 1972. 3,686,53

8 Sheets-Sheet 5 Patented Aug. 22, 1972 3,686,53

8 Sheets-Sheet 7 L M P1 4205 ug; (B)

Patented Aug. 22, 1972 8 Sheets-Sheet 8 E l EN Q8 EQQJ l. 3% bqm mfi -$CW E5 EEE 8 ELECTROMECHANICAL RESONATOR The invention relates to electromechanical resonators.

The invention provides an electromechanical resonator including at least one vibrator element vibratable in bending oscillations by electrostatic forces between itself and first electrode means, the output of the resonator being represented by capacitance changes between a vibrator element and second electrode means.

The foregoing and other features according to the invention will be better understood from the following description with reference to the accompanying drawings, in which:

FIGS. 1A and 1B respectively illustrate one arrangement for the electromechanical resonator according to the invention and the equivalent circuit for this resonator,

FIGS. 2A and 28 respectively illustrate an electrostatic transducer which forms part of the resonator according to FIG. 1A and the equivalent circuit for the transducer.

FIGS. 3A and 3B respectively illustrate a modified arrangement of the resonator according to FIG. 1A and the equivalent circuit of this modified'arrangement,

FIGS. 4A and 4B respectively illustrate another arrangement for the electromechanical resonator according to the invention and the equivalent circuit for this resonator,

FIGS. 5A and 53 respectively illustrate a modified arrangement of the resonator according to FIG. 4A and the equivalent circuit of this modified arrangement,

FIG. 6 illustrates a further modified arrangement of the resonator according to FIG. 4A,

FIGS. 7A and 7B respectively illustrate another arrangement for the electromechanical resonator according to the invention and part of the equivalent circuit of this alternative arrangement,

FIGS. 8A and 83 respectively illustrate a modified arrangement of the resonator according to FIG. 7A and the equivalent circuit of this modified arrangement.

FIGS. 9, l0 and 11 illustrate multi-element electromechanical resonatorsaccording to the invention,

FIGS. 12A to 12C respectively illustrate a multi-element arrangement of the electromechanical resonator according to FIG. 7A and the equivalent circuits of this arrangement, FIG. 13 illustrates a multi-element electromechanical resonator according to the invention with mechanical and electrical coupling between the elements, I

FIGS. 14A and 14B respectively illustrate a further arrangement of the electromechanical resonator ac cording to the invention and the equivalent circuit of this further arrangement, and

FIG. 15 illustrate a frequency response curve for a modified arrangement of the electromechanical resonator according to FIG. 12A,

In its simplest form the electromechanical resonator according to the invention is a frequency dependent variable capacitance forming an electromechanical transducer. Referring to FIG. 1A of the drawings, the devices vibrator element 1 forms the mechanical part of the transducer as well as the moving plate of the variable capacitance. The vibrator element spans over a conducting plate 2 which forms the second plate of the variablecapacitance. The vibrator element is of length l and is fixed at one end to a ground plane 3. The ground plane which is connected to the input G1 via a capacitance C is biased above earth potential by means of a d.c. voltage source B1, the positive side of which is connected to the ground plane via a high impedance represented by the resistance R1. The resistance Rl and d.c. voltage source B1 are shunted by a decoupling-capacitance C The output of the resonator is taken across a load capacitance C connected between the plate 2and earth potential and shunted by a resistance R3. The incoming signal to the vibrator element causes a variable electrostatic force to act on the metallic element and excites it into flexural vibra tion corresponding to its natural frequency. The vibrating element causes small variations in the capacitance between itself and the plate 2 and this variation is detected across the capacitance C to provide the resonator output. The natural frequency of the resonator is dependent on the material and physical dimensions of the vibrator element as well as the boundary conditions between the element and its surroundings.

The electrostatic force between two conducting surfaces depends on the square of the voltage between them. Hence, to avoid frequency doubling a large d.c. bias voltage B1 is required as shown in FIG. 1A. This bias voltage is also necessary for the operation of the output transducer and to provide the required sensitivity which increases linearly with increases in bias voltage.

The equivalent circuit of the resonator according to FIG. 1A is illustrated in FIG. 1B of the drawings. Assuming that the movement of the vibrator element about an equilibrium position i.e. the position when the electrostatic force is balanced by the restraining elastic force in the material of the element, is small and that the voltage variation about a mean biasing voltage E is also small then linear action can be obtained.

Considering, the components of the equivalent circuit the equations outlined below give their relation ship with the electromechanical resonator.

When there is a voltage E between the vibrator element 1 and the plate 2 then this gives rise to an electrostatic force F given by lF|=VzE(de/d.x) (I) where C is the capacitance between the plate 2 and the resonator element 1, x is the displacement of the vibrator element 1 from itsequilibrium position and is equal to (1 -11,

d is the distance between the plate 2 and the vibrator element 1, and d is the value of d when E is zero.

E is the total voltage across the plate 2 and the vibrator element 1, I and (dc/dx) is the rate of change of C with respect to x. At the equilibrium position the values of E, C and d are respectively E C and d E=[E +esin mt] where e is the peak value of the ac. driving voltage.

If E e and x is small, then the operation is practically linear and the electrostatic force component due to the input signal is proportional to E e sin wt. Hence the sensitivity of the resonator is considerably increased.

For a rectangular vibrator element the capacitance The electrostatic transducer which forms part of the of the transducer C (a! We,,)/(d,,x) Farads 3 electromechanical resonator according to FIG. 1A and 2 its equivalent circuit are respectively illustrated in zelwfo FIGS. 2A and 28.

L. n .9... ,fi. The references used for the elements of the where equivalent circuit according to FIG. 2B are the same as lis the length of the vibrator element and have the same relationships as the references used a is the ratio of the length of the plate 2 to the length in the equivalent circuit according to FIG. 1A. This apof the vibrator element. plies also to the equivalent circuits which will be W is the width of the vibrator element and s is the discussed in subsequent paragraphs.

dielectric constant for air and thus for small move- The equivalent circuit according to FIG. 28 uses the ment the change in capacitance varies linearly impedance analogy between electrical and mechanical with x when x is small in value. networks and it was arrived at by setting up Lagranges The equivalent parameters of the vibrator element energy equations for generalized forces and co-orare L C and r dinates in the mechanical and electrical parts of the L [equivalent to mass (M)] 1.23plA Kgm (4) transducer according to FIG. 2A. These equations are where p is the density of the material of the vibrator as shown below: element, and A is the area of the vibrator element i.e. W

X t (thickness) d y) 5U a? (09:) a Compliance C [equivalent to Y stiffness (s) d (5y l 1 and Z Z 5 5 V (10) C 15 21, g metre] Newton where o a .s. .1 t A, "w (5) f ande are generalized forces 7 x and q are generalized co-ordinates where Y Youngs modulus of elasticity. Losses due )is the kinetic energy of the system to mping em m o uQ) Where Q is e q U is the potential energy of the system ty factor of the resonant circuits and (0 is the resonant f and x are respectively the force on and the displacequ n y 1/ u ument of the mechanical side of the transducer If an ideal transformer i.e. the transformer T1 in FIG. 8 d q are res ctively the voltage and the charge on IE, with an impedance ratio is used between the th le tri l side, electrical and the mechanical Parts of the transducer The kinetic energy of the transducer according to i then L C and r can be expressed in terms of Hen- FIG. 2A is given by the expression ries farads and ohms respectively provided that is x2 representing the value of the electromechanical transformation ratio of an electrostatic transducer. Thus Where M is the mass of the moving element 1 and qo o 40 it is the velocity of the mechanical side of the transwhere q is the charge across the element and the plate (meet Y 2 at the equilibrium position and equals n o- The potential energy of the'transduce'r is the sum of the The electromechanical coupling coefficient m is elastic energy of the mechanical parts, the electrostatic given y the ratio t/(Qa fl for a rectangular energy of the electrical part, and the electrical energy element and small displacements can be estimated due to the bias voltage. However, if the equilibrium from the fellowihg equation: position is used as a reference some of the potential k 1g E 6 l 1 energy terms will cancel out.

m T 15.2 Y td The ideal transformer ratio dz in FIG. 2B is equal to q ld Thus it can be seen that for an ideal electro- Equation (7) shows that K is dependent on the e static transducer using the impedance analogy, as is feetive length of the Plate 2, the a the the case with the equivalent circuit of FIG. 2B, the material of the vibrator element and its length and f ll i relations are ti fi d:

thickness, and the gap between the vibrator element and the plate. The e f (qo/ o) 8 ([2) Due to the negative capacitance factors of the plates, V and the current i q= q.,/d it A g (13) the compliance of the vibrator element is increased If two identical ideal transducers, as above, are con slightly and the effective compliance is given by the folnected together by their mechanical parts to form a sinlowing equation: gle unit and the electrical part of one of the transducers 1 C ff ti C 1 2 2 2 is made the input whilst the electrical part of the other I M e cc V6 g j k2 a (8 of the transducers is made the output, then it can be and the resonance frequenc then appears to be shifted Show on applymg the boundaIy commons that to l/(21r) w/l/(L C effective) from the natural p2= (l/z) pl (14) resonance frequency of the element i.e. l/( 211') The resistance R2 is the source resistance of the Where input G1 and the output of the resonator is taken p and p are the voltages associated with the electriacross the load resistance R3 and the capacitance cal parts of the said one of and the said other of the Cow. transducers respectively, and

r', and are the currents associated with the electrical parts of the said one of and the said other of the transducers respectively.

The above equations (14) and thus show an inversion between the input and output of the single unit which can be allowed for by taking say the output transformer ratio (1) to be of opposite sign to the input transformer ratio (b It should be noted that the bias voltage B1 could be connected to the plate 2 instead of the vibrator element 1 if this is found to be more convenient.

The electromechanical resonator can alsobe driven with the variable capacitance C shunting the rest of the circuit as shown in FIG. 3B which illustrates the equivalent circuit of the resonator illustrated in FIG. 3A. In this arrangement, the plate 2 provides the input and the output electrode for the resonator. The input and the output are connected to the plate2 respectively via appropriate capacitances C and C.

The electromechanical resonator according to the invention can have more than one conducting plate, for example as illustrated in FIG. 4A wherein two

plates

4 and 5 are utilized. In this case it can be driven as a two terminal or as a four terminal network. The equivalent circuit of a two plate four terminal resonator is illustrated in FIG. 4B wherein it can be seen that an additional transformation is introduced i.e. the transformer T2 and the variable capacitance C2 of the second plate is also included.

The bias voltage can be applied separately to the two

plates

4 and 5 as illustrated in FIG. 5A. The equivalent circuit of this arrangement as a two plate four'terminal resonator is illustrated in FIG. 5B wherein it can be seen that an additional transformation is introduced i.e. the transformer T2 and the variable capacitance C2 of the second plate is also included.

The plates 4'and 5 can be on opposite sides of the vibrator element as illustrated in FIG. 6.

The bias voltages on the plates can have the same polarity when the

plates

4 and 5 are on the same side of the vibrator element. In this case the ratios 41), and of the transducers will have opposite signs, and the overall transformer ratio will be negative. The

plates

4 and 5 can as previously stated be on opposite sides of the vibrator element and have the same polarity of bias; in this case the ratios d), and will have the same sign. They can also be on the same side of the vibrator element but with different polarities of bias, and in this case and (1) will again have the same sign and the overall ratio positive. The electromechanical resonator could have more than two plates and the overall transformer ratio between any two plates can be chosen to be positive or negative depending on the relative position of the plates and the polarities of the bias voltages.

By having electrostatic input and output transducers, the resonator has a direct capacitive coupling i.e. the capacitance C12 in FIGS. 4B and 5B, between the input and output plates. This direct coupling will introduce an attenuation peak higher or lower than the resonance frequency depending on whether the overall transformer ratio is positive or negative respectively. Controlling the value of the direct coupling will allow variation in the position of the attenuation peak and the frequency selectivity of the resonator. The direct coupling should however be kept as small as possible to improve the selectivity of the resonator. The input and I output plates can be isolated from each other by using a tromechanical resonator according to the invention can be extended to an N-port network to cover an N- plate device. For example, in FIGS. 8A and SE a resonator having five plates is illustrated. In this arrangement the plates 9 and 11 are connected to earth potential, the plate 12 and the element 1 form the input transducer and the plates 8 and 10 and the element 1 form the output transducers. Earthing a plate will effectively cause a slight shift in the resonance frequency due to the negative capacitance of the corresponding port. It will also slightly increase the effective capacitance C, of the element 1 to earth by the addition of the corresponding capacitance of the earthed plate to the element 1.

FIG. 88 illustrates the equivalent circuit of the five plate resonator according to FIG. 8A where 2F is the total force in the system and k is the system loop velocity. With a closed loop 2F will be equal to zero.

If a plate is situated on the opposite side of the vibrator element to the plate 12, or if an extra bias voltage is used on a plate then the (1) associated therewith should take the appropriate sign.

By using multiplate resonators, higher order networks can be obtained. The five plate resonator according to FIG. 8A could be modified such that it functions as a band pass filter with two attenuation peaks i.e. one in the upper stop band and one in the lower stop band by having the plates 9 and 11 connected to earth potential, the plate 10 and the element 1 forming the input transducer, the plates 8 and 12 and the element 1 fonning the output transducers and the plate 8 biased positive with respect to the element 1 potential. Alternatively, in order to obtain the same result an output transducer can be formed by the plate 10 and the element 1, input transducers can be formed by the plates 8 and 12 and the element 1, and in this instance the plates 9 and 11 would be connected to earth potential and the plate 8 would be biased positive with respect to the element 1 potential.

The electromechanical resonator can be made with more than one resonator element tuned to different frequencies, for example as illustrated in FIG. 9 and thereby provide a device which can be used in coding and decoding equipment or selective calling systems. Referring to FIG. 9 the vibrator elements 14 to 17 are of different lengths thereby giving rise to the different frequencies and are fixed at one end to a ground plane 18. The application of the input signal and the dc. bias voltage to the resonator is effected in a manner as previously outlined and the device can be operated as before with a single plate 19 or with an additional plate 20 which is illustrated by a chain dotted line.

The electromechanical resonator according to the invention can also have a multi-element system, with strips or rods supported at one or both ends or at the nodal points, with mechanical or electrical coupling,

between the elements, with electrical couplings between the plates, or both thereby providing a more complex multi-element filter examples of which are illustrated in FIGS. 10, ll, 12 and 13. i

The electromechanical resonator according to FIG. is provided with vibrator elements 21 to which are of the same length and fixed at one end to a ground plane 28. An input plate 26 is provided which forms the input transducer with the element 21 and an output transducer with the element 25.

The vibrator elements are coupled to each other by means of a coupling element 31 which is in the form of a strip, rod or the like. The coupling element 31 traverses each of the vibrator elements and is secured to each of them. The vibrator element 21 is also coupled to the vibrator element 24 by means of a coupling element 32 which is in the form of a strip, rod, or the like.

Vibrational energy generated at the vibrator element 21 by the input transducer is transmitted to the vibrator element 22 via the section of the coupling element 31 situated therebetween and to the vibrator element 24 via the coupling element 32 which causes these vibrator elements to vibrate in a mode of mechanical vibrations. The vibrational energy generated at the vibrator element 22 is then transmitted to the vibrator element 23 via the section of the coupling element 31 situated therebetween which cause the element 23 to vibrate in a mode of mechanical vibration. This process is repeated until the vibrational energy of each of the vibrator elements has been transmitted to the next adjacent vibrator element via the respective section of the coupling element 31. The mode of mechanical vibrations of the vibrator element 25 is detected by the output transducer which provides an output signal that has been filtered as desired. The coupling between non-adjacent vibrator elements produces attenuation peaks in the resonator output signal.

The electromechanical resonator according to FIG. 11 operates in exactly the same manner as but is constructed differently to the resonator according to FIG. 10. The only difference is that the

vibrator elements

22 and 24 are connected at one end to a ground plane instead of to the ground plane 28.

In order to increase the frequency of operation, the vibrator elements 21 to 25 could be connected at both ends between two ground planes and mechanically or electrically coupled to each other. This arrangement reduces the dimensional tolerances of the resonator and coupling elements and therefore makes them more economical to produce.

Referring to FIG. 12A, the electromechanical resonator illustrated therein comprises two

vibrator elements

29 and 32 which are respectively connected at one end to ground planes 33 and 34. The vibrator elements span over a conducting coupling plate 35 and a T-shaped conducting plate 36 which is connected to earth potential. The

vibrator elements

29 and 32 also respectively span over conducting

plates

37 and 38 which each form one plate of an electrostatic transplanes are each biased above earth potential by means of the dc. voltage sources B1, the positive side of which is connected to the ground plane via a high impedance represented by the resistance R1. The plate 36 is the screen plate and isolates the

plates

35, 37 and 38 from each other.

In operation, the vibrational energy generated at the element 29 by the input transducer gives rise to a variable electrostatic force on the plate 35 which acts on the element 32 and causes it to vibrate in a mode of mechanical vibrations. These vibrations are detected by the output transducer which provides an output signal that has been filtered as desired.

It can therefore be seen that the coupling between the

elements

29 and 32 is effected electrically.

The equivalent circuit of the resonator according to FIG. 12A is illustrated in FIG. 12B. The capacitance between the two elements is represented by the capacitance C15. The resistance R is normally zero if only one coupling plate is used. However, if the plate 35 is divided into two parts each part being associated with a separate one of the elements and the two parts are connected together via an amplifier unit then the resistance R can be replaced by an amplifier in the equivalent circuit.

FIG. 15 illustrates the frequency response of the electromechanical resonator according to FIG. 12A when modified by biasing the plate 37 above earth potential by means of a dc. voltage source, the positive side of which is connected to the plate 37 via a high impedance resistance, dividing the plate 35 into two parts, in a manner as outlined in the preceding paragraph, and connecting the two parts together by an amplifier unit. The value of the dc. voltage source used to bias the plate 37 is greater than the value of the source Bl thereby allowing a positive overall transformer ratio to be obtained for that part of the resonator associated with the vibrator element 29 and an attenuation peak higher than the resonance frequency of the element 29.

The vibrator element 32 in this arrangement therefore has only one bias voltage applied thereto and a negative overall transformer ratio will result thereby giving rise to an attenuation peak lower than the resonance frequency of the element 32.

By connecting the two

vibrator elements

29 and 32 in cascade in a manner as previously outlined gives rise to the frequency response illustrated in FIG. 15 which simulates the response of a band pass filter with attenuation peaks higher and lower than the mid-band frequency.

The resonance frequency fi, and the attenuation peak frequency f of each half of the resonator when tested individually were as follows:

Half including Half including vibrator element 29 vibrator element 32 f,, 4012 KHZ f, 3.980 KHz f.= 4.090 KH; 3,815 m (Bandwithk 36 KHz (bandwidtm 4 l KHz The

vibrator elements

29 and 32 which were of beryllium copper, had nominal dimensions of:

Length 2.0 mm Width W 0.2 mm Thickness r 0.025 mm As can be seen from FIG. 15, the response of the cascaded elements had a 3 db bandwidth of .55 Hz. Wider bandwidths can however be obtained by shifting the resonance frequencies of the two vibrator elements provided that the resulting increased attenuation ripple in the pass band is acceptable.

The capacitances C12, C34 and C14 are respectively representative of the direct coupling capacitance between the

plates

37 and 35, the

plates

35 and 38 and the

plates

37 and 38. These capacitances can in practice be made very small by suitable arrangement of the layout of the elements and the plates.

The capacitances C1, C2, C3 and C4 are respectively representative of the capacitance between the element 29 and the plate 27, the element 29 and the plate 35, the element 37, and the plate 35 and the element 32 and the plate 38.

The transformers associated with the input transducer and the transducer formed by the plate 35 and the element 29 have been combined to give a single transformer with the ratio 1 Similarly, the transformers associated with the output transducer and the transducer formed by the plate 35 and the element 32 have been combined to give a single transformer with the ratio 1 v If the

elements

29 and 32 are at the same potential and R'is zero then the equivalent circuit of FIG. 128 can be reduced to give the equivalent circuit illustrated in FIG. 12C. The capacitance C represents the coupling between the vibrator elements; if the coupling is mechanical C represents the compliance of the coupling element.

It will be appreciated that various other configurations of the electrical and mechanical coupling arrangements outlined in preceding paragraphs are possible for the electromechanical resonators according to the invention.

Electrical and mechanical coupling are utilized in the electromechanical resonator according to FIG. 13. This resonator comprises two of the resonators according to FIG. 1 and a third resonator without the plate 2. The vibrator element of the third resonator is mechanically coupled to the vibrator element of one of the other two resonators via the coupling element 39 and is electrically coupled to the vibrator element of the other of the two resonators via the conducting coupling plate 40. The plates 2 and the vibrator elements 1 form the input and output transducers for the device.

There is a wide choice in the shape and the material of the vibrator elements and in the arrangement of the conducting plates. The vibrator elements can be in the form of a disc or plate and the conducting plates can also be in the form of discs or concentric rings under or above the disc vibrator elements. This therefore allows obtained and a bandwidth wider than can be obtained with quartz crystals. Capacitance ratios of the order of 40 have been obtained on experimental devices. It will therefore be appreciated that this feature of the resonators according to the invention gives rise to a multitude of various network arrangements in ladder or lattice form; one of the simplest is illustrated in FIGS. 14A and 143.

As shown in FIG. 14A, the resonator illustrated therein utilizes two of the resonators according to FIG. 1 with the plates 2 coupled together'via an amplifier unit A. It should however be noted that the amplifier unit A could be omitted and the plates 2 would in this instance be directly coupled or formed by a single plate.

The equivalent circuit of the resonator according to FIG. 14A is illustrated in FIG. 14B. The input and output resonators are respectively represented by those parts of the circuit enclosed by the chain dotted lines 41 and 42. The coupling capacitance between the resonators is represented by the capacitance C12. If the elements are screened from each other or are at the same voltage then the capacitance C12 will be very small i.e. will tend towards zero and therefore the circuit tends toward a ladder network.

The electromechanical resonator can be biased with an ac. voltage instead of or superimposed on the dc. voltage and thus it would under these conditions act as a modulator circuit which can be useful in multiplex channel translating equipment.

The device also offers the interesting possibilities for making microminiature oscillators and transformers.

The electromechanical coupling coefficient of an electrostatic transducer is inversely related to the gap between the fixed and moving plates of the transducer. To obtain high electromechanical coupling the gap would have to be very small. This tends to increase the resistance of the still air in the gap, to the movement of the element, and thus reduces the overall quality factor Q of the resonant element. The air resistance can be reduced and the Q of the circuit increased considerably by enclosing the element and conducting plate or plates in an evacuated enclosure or by using a conducting plate with a mesh of micro holes in it to reduce the dash pot effect of the still air.

The electromechanical resonators according to the invention are completely passive and the activity comes only in the detector circuit. The passive tuned unit with its electrostatic transducers is a reversible device, by variation of the coupling it can be made symmetrical or unsymmetrical. Thefrequency range of the resonator is from a few hundred Hz to a few hundred KI-Iz. The limiting factor is mainly the physical dimensions, particularly the length. However, the most suitable range is l to 30 KHz. At low frequencies, below I KHZ, the effect of environmental vibrations, if proved to be a problem, can be reduced by using a balanced resonant element, or a free-free arrangement where the vibrator element is supported at its nodal points.

The electromechanical resonators outlined in the preceding paragraphs are compatible with use in hybrid integrated circuits either as a separate unit or as an integral part of a more complex network. Thus, the techniques could well provide the sharply tuned integrated circuit so often required.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.

I claim:

1. An electromechanical resonator comprising first electrode means including an input fixed electrode and an input electrically conductive vibrator element, second electrode means including an output fixed electrode and an output electrically conductive vibrator element, and bias means for establishing an electric field between the members of each of said first and second electrode means, the input vibrator element being operatively coupled to the output vibrator element and caused to vibrate at its natural frequency solely by electrostatic forces between itself and said input fixed electrode resulting from modulation of said electric field by an input signal applied thereto.

2. An electromechanical resonator as claimed in claim 1 wherein said input vibrator element is coupled to said output vibrator element so as to form a single vibrator unit functioning as both the input and the output vibrator elements.

3. An electromechanical resonator as claimed in claim 1 wherein said input vibrator element is elongated and anchored at one end to a ground plane and vibratable in bending oscillation about said anchored end by said electrostatic forces, the output of the resonator being represented by capacitance changes between said input vibrator element and said output vibrator element, the latter being of corresponding electrical shape with the former.

4. An electromechanical resonator as claimed in claim 1 wherein the second electrode means and a third electrode means are placed adjacent to, and spaced apart from the same side of the vibrator element.

5. An electromechanical resonator as claimed in claim 2 wherein the output conductive vibrator element. of said second means is coupled to said third electrode means so as to provide a single conductive member.

6. An electromechanical resonator as claimed in claim 4 wherein the second electrode means are provided by a first conductive member, wherein a third electrode means are provided by a second conductive member which is connected to an output terminal, wherein the first and second conductive members coupled together, wherein the vibrator element is biased above earth potential and wherein the vibrator element is connected to an input terminal.

7. An electromechanical resonator as claimed in claim 1 wherein the second electrode means and a third electrode means are adjacent to, and spaced apart from opposite sides of the vibrator element.

'8. An electromechanical resonator as claimed in claim 23 wherein the second electrode means includes at least one input conductive member, and wherein a third means includes at least one output conductive member.

9. An electromechanical resonator as claimed in claim 1, wherein the resonator includes a single vibrator element, and the resonators input and output terminals are each connected to the single conductive member.

10. An electromechanical resonantor as claimed in claim 1 wherein the resonators input terminal is connected to the vibrator element.

11. An electromechanical resonator as claimed in claim 1, wherein the resonator includes a plurality of vibrator elements which are each adapted to vibrate in bending oscillators at a different frequency to the other claim 5, which also includes, between adjacent conductive members,an additional conductive member which is connected to earth potential, and which is adjacent to, and spaced apart from, the vibrator element.

13. An electromechanical resonator as claimed in claim 1 which includes an electrical voltage source connected between the vibrator element and earth potential.

13. An electromechanical resonator as claimed in claim 1 which includes an electrical voltage source that is connected between earth potential and each conductive member, and wherein the vibrator element is connected to earth potential.

v15. An electromechanical resonator as claimed in claim 13 wherein the electrical voltage source is a dc voltage source, the positive side of which is connected, via a high impedance to the vibrator element, and the negative side of which is connected to earth potential, and wherein the high impedance and dc. voltage source are shunted by a decoupling capacitance.

16. An electromechanical resonator as claimed in claim 8 wherein a plurality of output conductive members are provided which are each connected to a separate output terminal.

17. An electromechanical resonator as claimed in claim 8 wherein a plurality of vibrator elements are provided, wherein an input conductive member is associated with one of the vibrator elements, wherein an output conductive member is associated with another one of the vibrator elements, and wherein the resonator includes coupling means situated between adjacent vibrator elements.

18. An electromechanical resonator as claimed in claim 17 which also includes coupling means situated between non-adjacent vibrator elements.

19. An electromechanical resonator as claimed in claim 17 wherein the coupling means are either electrical.

20. An electromechanical resonator as claimed in claim 19 wherein the electrical coupling means includes a conductive plate which is spaced apart from the vibrator elements, and which is arranged transverse to the longitudinal axes of the vibrator elements.

21. An electromechanical resonator as claimed in claim 19 wherein the electrical couplingmeans includes for each vibrator element, a conductive plate which is adjacent to, and spaced apart from the associated vibrator element, and wherein each conductive plate is connected to a separate one of the other conductive plates via an amplifier unit.

22. An electromechanical resonator as claimed in claim 6 wherein the first and second conductive members are coupled together via an amplifier unit.

23. An electromechanical resonator as claimed in claim 1 wherein the conductive members are each provided with a mesh of micro-holes therein which are arranged to reduce air resistance between the member and the associated vibrator element.

24. An electromechanical resonator as claimed in claim 1 which includes an evacuated chamber within which the electrode means are located.

Claims

5. An electromechanical resonator as claimed in claim 2 wherein the output conductive vibrator element of said second means is coupled to said third electrode means so as to provide a single conductive member.

8. An electromechanical resonator as claimed in claim 1 wherein the second electrode means includes at least one input conductive member, and wherein a third electrode means includes at least one output conductive member.

9. An electromechanical resonator as claimed in claim 1 wherein the resonator includes a single vibrator element, and the resonator''s input and output terminals are each connected to the single conductive member.

10. An electromechanical resonantor as claimed in claim 1 wherein the resonator''s input terminal is connected to the vibrator element.

11. An electromechanical resonator as claimed in claim 1, wherein the resonator includes a plurality of vibrator elements which are each adapted to vibrate in bending oscillators at a different frequency to the other vibrator elements.

12. An electromechanical resonator as claimed in claim 5, which also includes, between adjacent conductive members, an additional conductive member which is connected to earth potential, and which is adjacent to, and spaced apart from, the vibrator element.

14. An electromechanical resonator as claimed in claim 1 which includes an electrical voltage source that is connected between earth potential and each conductive member, and wherein the vibrator element is connected to earth potential.

15. An electromechanical resonator as claimed in claim 13 wherein the electrical voltage source is a DC voltage source, the positive side of which is connected, via a high impedance to the vibrator element, and the negative side of which is connected to earth potential, and wherein the high impedance and DC voltage source are shunted by a decoupling capacitance.

21. An electromechanical resonator as claimed in claim 19 wherein the electrical coupling means includes for each vibrator element, a conductive plate which is adjacent to, and spaced apart from the associated vibrator element, and wherein each conductive plate is connected to a separate one of the other conductive plates via an amplifier unit.