Title:
INTEGRATED CIRCUIT FOR ELECTRONIC TIMEPIECES
United States Patent 3727151
Abstract:
An integrated electronic circuit operating in conjunction with an electromagnetic transducer for sustaining a tuning fork in vibration, the fork serving as a frequency standard for a timepiece. The electronic circuit includes a super-gain transistor whose output is connected through an external power source to the drive coil of the transducer associated with the tuning fork to produce current pulses for actuating the fork. The voltage induced in the phase-sensing coil of the transducer is applied to the input of the transistor through a coupling capacitor, a resistor providing base current for the transistor, whereby regeneration occurs to sustain oscillation at a rate determined by the resonance frequency of the fork. The capacitor has a value in the picofarad range, this small value being offset by a large resistor value in excess of 100 megohms, the resultant decrease in base current being compensated for by an increase in current gain effected by the super-gain transistor. The capacitor, the resistor and the transistor are fabricated as a monolithic device, making possible a miniature timepiece construction.
Application Number:
05/244644
Publication Date:
04/10/1973
Filing Date:
04/17/1972
View Patent Images:
Export Citation:
Assignee:
Bulova Watch Company, Inc. (New York, NY)
Primary Class:
Other Classes:
331/108D, 368/167, 968/487
International Classes:
G04C3/10; H01L49/02; G04C3/00; H03B5/30
Field of Search:
331/156,116M,18D 58/23AO,23TF
Other References:
Electronic Design 8, pg. 112, Apr. 11, 1968..
Primary Examiner:
Kominski, John
Claims:
I claim
1. An integrated circuit chip adapted for association with the electromagnetic transducer of an electro-mechanical resonator to sustain the resonator in vibration at its natural frequency, said transducer having a drive and a phase-sensing coil, said chip being constituted by a super-gain transistor, a capacitor having a value in the picofarad range and a resistor in the megohm range, the input of said transistor being connectable through said capacitor to said phase-sensing coil and the output thereof being connectable through an external power source to said drive coil, said resistor being connected between said input and output of said transistor; said transistor, said capacitor and said resistor being fabricated to define monolithic structure.
2. An integrated circuit chip adapted for association with the electromagnetic transducer of an electro-mechanical resonator to sustain said resonator in vibration at its natural frequency, said transducer having a drive coil and a phase sensing coil, said chip comprising:
3. An arrangement as set forth in claim 2, wherein said resonator is a tuning fork.
4. An arrangement as set forth in claim 3, wherein said tuning fork is provided with a permanent magnet on each tine, one cooperating with said drive coil and the other with said phase-sensing coil.
5. An arrangement as set forth in claim 2, wherein the value of said capacitor is less than 400 picofarads.
6. An arrangement as set forth in claim 2, wherein said resistor has a value in excess of 100 megohms.
7. An arrangement as set forth in claim 2, wherein said integrated circuit is formed on a chip having three terminals, one being connected to the emitter, the second to the collector and one end of said resistor, and the third to one end of the capacitor, the other end of said resistor and the other end of said capacitor being connected on said chip to said base.
1. An integrated circuit chip adapted for association with the electromagnetic transducer of an electro-mechanical resonator to sustain the resonator in vibration at its natural frequency, said transducer having a drive and a phase-sensing coil, said chip being constituted by a super-gain transistor, a capacitor having a value in the picofarad range and a resistor in the megohm range, the input of said transistor being connectable through said capacitor to said phase-sensing coil and the output thereof being connectable through an external power source to said drive coil, said resistor being connected between said input and output of said transistor; said transistor, said capacitor and said resistor being fabricated to define monolithic structure.
2. An integrated circuit chip adapted for association with the electromagnetic transducer of an electro-mechanical resonator to sustain said resonator in vibration at its natural frequency, said transducer having a drive coil and a phase sensing coil, said chip comprising:
3. An arrangement as set forth in claim 2, wherein said resonator is a tuning fork.
4. An arrangement as set forth in claim 3, wherein said tuning fork is provided with a permanent magnet on each tine, one cooperating with said drive coil and the other with said phase-sensing coil.
5. An arrangement as set forth in claim 2, wherein the value of said capacitor is less than 400 picofarads.
6. An arrangement as set forth in claim 2, wherein said resistor has a value in excess of 100 megohms.
7. An arrangement as set forth in claim 2, wherein said integrated circuit is formed on a chip having three terminals, one being connected to the emitter, the second to the collector and one end of said resistor, and the third to one end of the capacitor, the other end of said resistor and the other end of said capacitor being connected on said chip to said base.
Description:
BACKGROUND OF THE INVENTION
This invention relates generally to electronically controlled timepieces provided with an electro-mechanical resonator as a frequency standard, and more particularly to an integrated circuit for such timepieces.
In the patents to Hetzel U.S. Pat. Nos. 2,900,786 and 2,971,323, there are disclosed electronic time pieces including a self-sufficient timekeeping standard constituted by a mechanical resonator in the form of a tuning fork whose vibrations are sustained by two electromagnetic transducers operating in conjunction with a battery-energized transistor circuit. The vibratory action of the fork is converted into rotary motion to operate the time-indicating hands of the timepiece.
In timepieces of the type disclosed in the Hetzel patents, each electromagnetic transducer is associated with a respective tine of the fork and includes a magnetic element attached to the end of the tine and vibrating therewith. The magnetic element on one tine reciprocates with respect to a stationary main drive coil, and that on the other tine moves back and forth with respect to a stationary phase-sensing coil. The drive coil is connected to the output circuit of the transistor, while the sensing coil is connected to the input thereof, whereby alternating voltage induced in the sensing coil renders the transistor conductive to produce current pulses in the drive coil for magnetically actuating the tines. Thus the electronic circuit functions essentially as a feedback oscillator, the rate of feedback being governed by the frequency of the mechanical resonator.
Fundamental to all solid state oscillators is a transistor amplifier, for the process of oscillation simply involves a connection of output to input so that certain conditions are fulfilled. The electronic circuit in a tuning-fork timepiece of the above-described type is that of a resonant feedback oscillator which differs from a conventional electronic oscillator in that in place of an inductance-capacitance combination, an equivalent electro-mechanical resonator is used.
While the invention will be described in connection with tuning fork oscillators, it is to be understood that the same principles are applicable to oscillators incorporating other forms of electromechanical resonators, such as those electronic timepieces which employ oscillating balance wheels as the timekeeping standard, the balance wheel being sustained in oscillation by the electronic circuit. And while mechanical resonators are disclosed herein in connection with timepieces, these frequency standards have many other practical applications, such as in resonant filters and optical choppers.
In commercially available electronic timepieces of the tuning-fork or balance wheel type, the associated electronic drive circuit is presently composed of discrete elements which at least include a transistor, a capacitor, and a resistor. When a battery-operated electronic timepiece is designed to be confined within a watch casing or in a miniature housing of similar dimensions where space is at a premium, the use of microelectronics technology to reduce the size and cost of the electronic circuit would appear to be in order. But for the reasons to be now explained, it has not heretofore been possible to fully exploit integrated circuits for the purpose of miniaturizing the electronic circuit of electronic timepieces employing mechanical resonators.
As is well known, a conventional circuit is made up by interconnecting separate components, whereas in an integrated circuit, the several components are fabricated on a single piece or chip, which performs the function of the circuit. The chief advantages of integrated circuits are extremely small size, low cost and high reliability.
The fabrication of a single crystal monolithic integrated circuit involves the formation of transistors, resistors, and capacitors in a single silicon substrate, with sufficient insulation between the components to minimize parasitic interaction therebetween.
In fabricating single-crystal monolithic structures, one begins with a p-type wafer or silicon as a substrate. An epitaxial layer of n material is grown on the surface of the wafer, and integrated circuit components are made by using a masking technique to permit successive diffusions of alternating p and n materials into the epitaxial layer. Transistors are made by using three of the layers in an n-p-n or a p-n-p sandwich; diodes are made by using two of the layers or by properly connecting transistors to form diodes. After all diffusions have been completed, a layer of silicon dioxide exists over the surface of the wafer. This layer serves to protect or "passivate" the integrated circuit so that it will not be contaminated by its environment.
Resistors are generally obtained by using a diffused layer in which current is forced to flow in a direction parallel to the p-n junction thus formed, or by depositing thin films of resistive material on the top of the silicon dioxide layer. Capacitors are obtained by using p-n junction capacitances or by depositing conducting material above the silicon dioxide layer. Connections between components are made by depositing a conducting material over the oxide and using a masking technique to select desired areas.
The junction capacitor is the easiest type of capacitor to fabricate in integrated circuits. Junction capacitors can be made in the picofarad range to be used primarily for decoupling and by-pass capacitors. The metal-oxide-silicon (MOS) capacitor is formed by an n + /region (emitter diffusion) and a metal film usually of aluminum separated by a silicon dioxide dielectric.
In some cases the terminal performance requirements of integrated circuits are such that capacitors made in a single crystal monolithic circuit are unacceptable, because the proper values cannot be obtained, because variations with temperature are too large or because of parasitics which cannot be tolerated. In such cases, one can use a thin film technique for forming the capacitor, in which event the bottom plate is made by depositing an aluminum layer on the final S 1 O 2 layer of the monolithic structure. Next one deposits a layer of dielectric material over the aluminum layer, above which goes another layer of aluminum to produce the top plate of the capacitor. But here too, the largest values obtainable on chips are in the picofarad range.
A detailed description of integrated technology for fabricating resistors, capacitors and transistors, may be found in the test "Analysis and Design of Integrated Circuits," by Lynn, Meyer & Hamilton -- 1967 -- McGraw Hill Book Co.
Inasmuch as electronic circuits of the type heretofore employed in conjunction with tuning forks and other forms of mechanical resonators make use of a capacitor whose value lies in the microfarad range, this requirement precludes full integration of the circuit. One cannot, using diffusion or thin film techniques, fabricate on a chip a capacitor having, say, a 0.1 or 0.03 mfd value or any other value in the microfarad range.
All that one could do, prior to the present invention, was to make up a hybrid circuit in which an integrated circuit chip containing the resistor and transistor, was combined with a discrete capacitor of the desired high value. But such hybrids are more expensive to manufacture, and are not as compact and reliable as a straightforward integrated circuit.
SUMMARY OF THE INVENTION
In view of the foregoing, it is the primary object of this invention to provide a solid state electronic circuit operable in conjunction with an electromagnetically-actuated tuning fork or other mechanical resonator for sustaining the resonator in vibration at its natural frequency, the circuit being constituted by components all of which lend themselves to fabrication in a monolithic device. Because the capacitor has a value in the picofarad range, it may be fabricated by thin film, MOS or diffusion techniques, and the need for a hybrid device is obviated.
More specifically, it is an object of this invention to provide a highly-compact, low cost integrated circuit composed of a transistor, a capacitor, and a resistor, which circuit, when combined with drive and phase sensing coils of electromagnetic transducers associated with a tuning fork constitutes an efficient Hartley oscillator for sustaining the fork in vibration.
A significant feature of the invention resides in the fact that when the tuning fork functions as a frequency standard or time base for a timepiece, the highly compact integrated circuit makes possible greater miniaturization of the timepiece than has heretofore been feasible. Essentially the same arrangement may be used to produce tuning-fork tone generators, filters and other low-frequency devices employing a mechanical resonator as a frequency-determining element.
Briefly stated, in one preferred embodiment of the invention, these objects are attained in an integrated circuit operating in conjunction with electromagnetic transducers for sustaining a tuning fork in vibration, the fork serving as a frequency standard for a timepiece. The electronic circuit includes a super-gain transistor whose output is connected through an external power source to the drive coil of an electromagnetic transducer associated with the fork to produce current pulses for actuating the fork, the voltage induced by the vibrating fork in the phase-sensing coil of an electromagnetic transducer associated therewith being applied to the input of the amplifier through a coupling capacitor whose value lies in the picofarad range. Also provided is a resistor whose value exceeds 100 megohms to provide base current for the transistor, whereby regeneration occurs in the amplifier to sustain oscillation at a rate determined by the resonance frequency of the fork.
The small value of the capacitor is offset by a large resistor value (exceeding 100 megohms), as a consequence of which the resultant base current is extremely low. But compensation for the low level of base current is effected by the super-gain transistor which produces output current at a level sufficient to actuate the fork. Because the picofard capacitor, the megohm resistor and the super-gain transistor all lend themselves to integrated circuit fabrication, the electronic circuit may be produced in the form of a tiny chip.
OUTLINE OF THE DRAWING
For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawing, wherein:
FIG. 1 is a schematic diagram of a tuning-fork frequency standard, including an integrated circuit in accordance with the invention;
FIG. 2 is the equivalent electrical circuit of the tuning fork and its associated electronic circuit in simplified form;
FIG. 3 is a schematic circuit showing in another form, the simplified electronic circuit associated with the equivalent circuit of the fork.
DESCRIPTION OF THE INVENTION
Referring now to FIG. 1, there is shown a frequency standard employing a tuning fork 10 having a pair of flexible tines 10A and 10B, and a mounting stem 11. When used as a time base for a watch or other miniature timekeeping device, the vibratory action of the fork is converted into rotary motion for operating the hands by means of a suitable motion transformer. This may be in the form of an indexing mechanism or by means of a magnetic escapement. Or the tuning fork oscillations may be used to provide periodic electrical pulses to activate an electronic time display.
The sole concern of the present invention is with respect to the electronic circuit associated with the fork or other mechanical resonator for sustaining it in operation, the system functioning as a frequency standard for timekeeping or any other known purpose. Hence the use to which the fork or resonator is put and the manner in which this is effected, form no part of the present invention.
Tuning fork 10 is provided with electromagnetic drive and phase-sensing transducers, the first being constituted by a permanent magnet 12 mounted on one tine of the fork and cooperating with a stationary coil 13, the second by a permanent magnet 14 associated with a stationary coil 15. While two separate drive and phase-sensing transducers are shown, in practice, these may be combined into a single electro-magnetic transducer having phase-sensing and drive coil sections, as disclosed for example, in the Bennett-Koehler U.S. Pat. No. 3,517,288.
Associated with the electromagnetic transducers is an integrated circuit chip, generally designated by numeral 16, the circuit thereof being formed by a transistor 17 having base B, collector C and emitter E electrodes, a capacitor 18 and a resistor 19.
Capacitor 18 is connected in series with resistor 19, the junction thereof being connected to base B of the transistor. The other end of capacitor 18 is connected to chip terminal T 1 . The other end of resistor 19 is connected to collector C and to chip terminal T 2 . Emitter E is connected to chip terminal T 3 . Thus, to install the chip in the tuning fork watch, only three connections thereto are required.
Terminal T 1 is connected to one end of phase-sensing coil 15 which is connected in series with drive coil 13. The junction of the two coils is connected to terminal T 3 . Terminal T 2 is connected in series with a DC power supply or battery 20 to the other end of drive coil 13.
This transistor, which is an amplifier, has its output connected through battery 20 to drive coil 13. In operation, when transistor 17 is rendered momentarily conductive, a current pulse from the battery is delivered to the drive coil 13, the resultant magnetic field producing a thrust on tine magnet 12, thereby actuating the fork. The voltage induced by tine magnet 14 in phase-sensing coil 15 is applied through capacitor 18 to base B of the transistor to overcome the bias thereon resulting from base current flow through resistor 19, to control the instant or phase position, in the course of each vibratory cycle, when the drive pulse is to be delivered to the drive coil.
The tuning fork and its associated electronic circuit fall into the resonant-feedback class of useful transistor oscillators, this class being analyzed in detail in the "Transistor Manual" Seventh Edition (1964), published by the General Electric Company. The resonant feedback oscillator uses either inductance-capacitance resonators or their electromechanical equivalent.
In the standard form of resonant-feedback transistor oscillator, the output of the transistor acting as an amplifier is coupled to the tuned primary of a transistor whose secondary is connected to the input to provide regeneration or positive feedback, causing the amplifier to oscillate. In the present arrangement shown in FIG. 1, the resonant feedback oscillator is a Hartley circuit in which the series-connected phase and drive coil windings form an autotransformer in place of the two winding transformers of the standard circuit.
We shall now consider the qualitative aspect of the frequency standard shown in FIG. 1. This is best done in the context of the equivalent circuit of the tuning fork resonator, for we are primarily concerned with the electrical effects of the oscillator circuit on the resonator.
In FIG. 2, the electrical equivalent to the tuning fork 10 is represented by inductance L f , capacitance C f , and resistance R f , all being in parallel relation to provide a parallel-resonant circuit whose frequency is determined by reactive values of inductance and capacitance, and whose "Q" is determined by the value of resistance. Since the tuning fork is a medium Q device, as compared, say, to a high Q resonator such as a piezoelectric crystal, the value of resistance is significant.
In FIG. 2, one can neglect the switching transistor and concentrate on the series-connected resistor 9 and capacitor 18, connected in shunt relation with the equivalent tuning fork resonator circuit. Resistor 19 has a value R o which appears between collector C and base B, and capacitor 18 has a value C o presented between base B and emitter E. These values may be replaced by their parallel equivalent, as shown in FIG. 3, wherein:
C o 1 = C o (1+ (ω C o R o ) 2 ) - 1
In the above equations, the symbol ω represents the angular frequency of the fork, that is 2 π multiplied by the frequency of the fork.
From an examination of equation II, it will be evident that to minimize the value of capacitance C o 1 , the values R o and C o should be large. It is desirable to minimize the value of C o 1 in comparison with C f in order to minimize the frequency shift of the system away from the natural frequency of the fork. If the value of C o 1 is not small relative to C f , then the fork is forced to operate at a frequency displaced from its natural frequency, in which event the amount of power required to drive the fork off-frequency is increased. Moreover, the system is then more sensitive to variations in capacitance C o 1 , which is undesirable.
But in order to provide an integrated circuit, it is essential to make the value of C o small (less than 400 picofarads) otherwise it cannot be fabricated using integrated circuit technology. One is then faced with the necessity of offsetting the effect of using a small capacitor in the picofarad range by making the value R o very large (greater than 100 megohms).
However, resistance R o is the main determinant of the base current of the transistor, and since the transistor is a current amplifying device, the decrease in base current resulting from an increase in the value of resistance R o will, in the case of a standard transistor of the type heretofore used in electronic timepieces, not provide sufficient current in the collector output circuit to drive the tuning fork. This collector current supplies the energy sustaining the fork in vibration, and for a given amplitude of fork vibration, one requires a definite amount of energy as determined by the Q of the fork.
I have found that the reduction in base current brought about by the use of a base current resistor whose value exceeds 100 megohms, can be offset by using a super-gain transistor of the type disclosed for example in Electronic Products Magazine (Apr. 19, 1971) in the article entitled "Super-Gain Transistors for IC's."
As pointed out in this article, super-gain transistors (also known commercially as super-beta transistors) are standard bipolar transistors which have emitters that have been diffused for extremely high currents. In the normal diffused transistor, the D-C current gain (Beta) is inversely related to the base width. But as the emitter is diffused deeper into the base region, the beta increases to several thousands. Typical current gains of 5,000 are obtainable at one microampere collector currents.
In the above qualitative analysis of the electronic circuit for driving the tuning fork, I have disregarded the effect of the transistor. In reality, when the transistor is turned on once each cycle to generate the drive pulse, the equivalent circuit of the class-C operated Hartley oscillator is more complicated. However, the main qualitative aspects of the circuit are still essentially the same as those described previously, hence there is no need to consider the more complicated aspects of the circuit behavior.
In summary, in order to make possible total integration of all elements forming the electronic circuit associated with an electromagnetically-actuated fork or other mechanical resonator, it becomes necessary to make use of a capacitor in the picofarad range, (i.e., 200 picofarads), for this value can readily be fabricated by existing microelectronics technology. This small capacitance value then dictates a much higher value for the base current resistor (>100 megohms), thereby giving rise to a markedly reduced base current. This in turn necessitates a super-gain transistor to provide adequate drive current for actuating the associated tuning fork to sustain it in vibration.
While there have been shown and described preferred embodiments of an integrated circuit for electronic timepieces, it will be appreciated that many changes and modifications may be made therein without, however, departing from the essential spirit of the invention.
This invention relates generally to electronically controlled timepieces provided with an electro-mechanical resonator as a frequency standard, and more particularly to an integrated circuit for such timepieces.
In the patents to Hetzel U.S. Pat. Nos. 2,900,786 and 2,971,323, there are disclosed electronic time pieces including a self-sufficient timekeeping standard constituted by a mechanical resonator in the form of a tuning fork whose vibrations are sustained by two electromagnetic transducers operating in conjunction with a battery-energized transistor circuit. The vibratory action of the fork is converted into rotary motion to operate the time-indicating hands of the timepiece.
In timepieces of the type disclosed in the Hetzel patents, each electromagnetic transducer is associated with a respective tine of the fork and includes a magnetic element attached to the end of the tine and vibrating therewith. The magnetic element on one tine reciprocates with respect to a stationary main drive coil, and that on the other tine moves back and forth with respect to a stationary phase-sensing coil. The drive coil is connected to the output circuit of the transistor, while the sensing coil is connected to the input thereof, whereby alternating voltage induced in the sensing coil renders the transistor conductive to produce current pulses in the drive coil for magnetically actuating the tines. Thus the electronic circuit functions essentially as a feedback oscillator, the rate of feedback being governed by the frequency of the mechanical resonator.
Fundamental to all solid state oscillators is a transistor amplifier, for the process of oscillation simply involves a connection of output to input so that certain conditions are fulfilled. The electronic circuit in a tuning-fork timepiece of the above-described type is that of a resonant feedback oscillator which differs from a conventional electronic oscillator in that in place of an inductance-capacitance combination, an equivalent electro-mechanical resonator is used.
While the invention will be described in connection with tuning fork oscillators, it is to be understood that the same principles are applicable to oscillators incorporating other forms of electromechanical resonators, such as those electronic timepieces which employ oscillating balance wheels as the timekeeping standard, the balance wheel being sustained in oscillation by the electronic circuit. And while mechanical resonators are disclosed herein in connection with timepieces, these frequency standards have many other practical applications, such as in resonant filters and optical choppers.
In commercially available electronic timepieces of the tuning-fork or balance wheel type, the associated electronic drive circuit is presently composed of discrete elements which at least include a transistor, a capacitor, and a resistor. When a battery-operated electronic timepiece is designed to be confined within a watch casing or in a miniature housing of similar dimensions where space is at a premium, the use of microelectronics technology to reduce the size and cost of the electronic circuit would appear to be in order. But for the reasons to be now explained, it has not heretofore been possible to fully exploit integrated circuits for the purpose of miniaturizing the electronic circuit of electronic timepieces employing mechanical resonators.
As is well known, a conventional circuit is made up by interconnecting separate components, whereas in an integrated circuit, the several components are fabricated on a single piece or chip, which performs the function of the circuit. The chief advantages of integrated circuits are extremely small size, low cost and high reliability.
The fabrication of a single crystal monolithic integrated circuit involves the formation of transistors, resistors, and capacitors in a single silicon substrate, with sufficient insulation between the components to minimize parasitic interaction therebetween.
In fabricating single-crystal monolithic structures, one begins with a p-type wafer or silicon as a substrate. An epitaxial layer of n material is grown on the surface of the wafer, and integrated circuit components are made by using a masking technique to permit successive diffusions of alternating p and n materials into the epitaxial layer. Transistors are made by using three of the layers in an n-p-n or a p-n-p sandwich; diodes are made by using two of the layers or by properly connecting transistors to form diodes. After all diffusions have been completed, a layer of silicon dioxide exists over the surface of the wafer. This layer serves to protect or "passivate" the integrated circuit so that it will not be contaminated by its environment.
Resistors are generally obtained by using a diffused layer in which current is forced to flow in a direction parallel to the p-n junction thus formed, or by depositing thin films of resistive material on the top of the silicon dioxide layer. Capacitors are obtained by using p-n junction capacitances or by depositing conducting material above the silicon dioxide layer. Connections between components are made by depositing a conducting material over the oxide and using a masking technique to select desired areas.
The junction capacitor is the easiest type of capacitor to fabricate in integrated circuits. Junction capacitors can be made in the picofarad range to be used primarily for decoupling and by-pass capacitors. The metal-oxide-silicon (MOS) capacitor is formed by an n + /region (emitter diffusion) and a metal film usually of aluminum separated by a silicon dioxide dielectric.
In some cases the terminal performance requirements of integrated circuits are such that capacitors made in a single crystal monolithic circuit are unacceptable, because the proper values cannot be obtained, because variations with temperature are too large or because of parasitics which cannot be tolerated. In such cases, one can use a thin film technique for forming the capacitor, in which event the bottom plate is made by depositing an aluminum layer on the final S 1 O 2 layer of the monolithic structure. Next one deposits a layer of dielectric material over the aluminum layer, above which goes another layer of aluminum to produce the top plate of the capacitor. But here too, the largest values obtainable on chips are in the picofarad range.
A detailed description of integrated technology for fabricating resistors, capacitors and transistors, may be found in the test "Analysis and Design of Integrated Circuits," by Lynn, Meyer & Hamilton -- 1967 -- McGraw Hill Book Co.
Inasmuch as electronic circuits of the type heretofore employed in conjunction with tuning forks and other forms of mechanical resonators make use of a capacitor whose value lies in the microfarad range, this requirement precludes full integration of the circuit. One cannot, using diffusion or thin film techniques, fabricate on a chip a capacitor having, say, a 0.1 or 0.03 mfd value or any other value in the microfarad range.
All that one could do, prior to the present invention, was to make up a hybrid circuit in which an integrated circuit chip containing the resistor and transistor, was combined with a discrete capacitor of the desired high value. But such hybrids are more expensive to manufacture, and are not as compact and reliable as a straightforward integrated circuit.
SUMMARY OF THE INVENTION
In view of the foregoing, it is the primary object of this invention to provide a solid state electronic circuit operable in conjunction with an electromagnetically-actuated tuning fork or other mechanical resonator for sustaining the resonator in vibration at its natural frequency, the circuit being constituted by components all of which lend themselves to fabrication in a monolithic device. Because the capacitor has a value in the picofarad range, it may be fabricated by thin film, MOS or diffusion techniques, and the need for a hybrid device is obviated.
More specifically, it is an object of this invention to provide a highly-compact, low cost integrated circuit composed of a transistor, a capacitor, and a resistor, which circuit, when combined with drive and phase sensing coils of electromagnetic transducers associated with a tuning fork constitutes an efficient Hartley oscillator for sustaining the fork in vibration.
A significant feature of the invention resides in the fact that when the tuning fork functions as a frequency standard or time base for a timepiece, the highly compact integrated circuit makes possible greater miniaturization of the timepiece than has heretofore been feasible. Essentially the same arrangement may be used to produce tuning-fork tone generators, filters and other low-frequency devices employing a mechanical resonator as a frequency-determining element.
Briefly stated, in one preferred embodiment of the invention, these objects are attained in an integrated circuit operating in conjunction with electromagnetic transducers for sustaining a tuning fork in vibration, the fork serving as a frequency standard for a timepiece. The electronic circuit includes a super-gain transistor whose output is connected through an external power source to the drive coil of an electromagnetic transducer associated with the fork to produce current pulses for actuating the fork, the voltage induced by the vibrating fork in the phase-sensing coil of an electromagnetic transducer associated therewith being applied to the input of the amplifier through a coupling capacitor whose value lies in the picofarad range. Also provided is a resistor whose value exceeds 100 megohms to provide base current for the transistor, whereby regeneration occurs in the amplifier to sustain oscillation at a rate determined by the resonance frequency of the fork.
The small value of the capacitor is offset by a large resistor value (exceeding 100 megohms), as a consequence of which the resultant base current is extremely low. But compensation for the low level of base current is effected by the super-gain transistor which produces output current at a level sufficient to actuate the fork. Because the picofard capacitor, the megohm resistor and the super-gain transistor all lend themselves to integrated circuit fabrication, the electronic circuit may be produced in the form of a tiny chip.
OUTLINE OF THE DRAWING
For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawing, wherein:
FIG. 1 is a schematic diagram of a tuning-fork frequency standard, including an integrated circuit in accordance with the invention;
FIG. 2 is the equivalent electrical circuit of the tuning fork and its associated electronic circuit in simplified form;
FIG. 3 is a schematic circuit showing in another form, the simplified electronic circuit associated with the equivalent circuit of the fork.
DESCRIPTION OF THE INVENTION
Referring now to FIG. 1, there is shown a frequency standard employing a tuning fork 10 having a pair of flexible tines 10A and 10B, and a mounting stem 11. When used as a time base for a watch or other miniature timekeeping device, the vibratory action of the fork is converted into rotary motion for operating the hands by means of a suitable motion transformer. This may be in the form of an indexing mechanism or by means of a magnetic escapement. Or the tuning fork oscillations may be used to provide periodic electrical pulses to activate an electronic time display.
The sole concern of the present invention is with respect to the electronic circuit associated with the fork or other mechanical resonator for sustaining it in operation, the system functioning as a frequency standard for timekeeping or any other known purpose. Hence the use to which the fork or resonator is put and the manner in which this is effected, form no part of the present invention.
Tuning fork 10 is provided with electromagnetic drive and phase-sensing transducers, the first being constituted by a permanent magnet 12 mounted on one tine of the fork and cooperating with a stationary coil 13, the second by a permanent magnet 14 associated with a stationary coil 15. While two separate drive and phase-sensing transducers are shown, in practice, these may be combined into a single electro-magnetic transducer having phase-sensing and drive coil sections, as disclosed for example, in the Bennett-Koehler U.S. Pat. No. 3,517,288.
Associated with the electromagnetic transducers is an integrated circuit chip, generally designated by numeral 16, the circuit thereof being formed by a transistor 17 having base B, collector C and emitter E electrodes, a capacitor 18 and a resistor 19.
Capacitor 18 is connected in series with resistor 19, the junction thereof being connected to base B of the transistor. The other end of capacitor 18 is connected to chip terminal T 1 . The other end of resistor 19 is connected to collector C and to chip terminal T 2 . Emitter E is connected to chip terminal T 3 . Thus, to install the chip in the tuning fork watch, only three connections thereto are required.
Terminal T 1 is connected to one end of phase-sensing coil 15 which is connected in series with drive coil 13. The junction of the two coils is connected to terminal T 3 . Terminal T 2 is connected in series with a DC power supply or battery 20 to the other end of drive coil 13.
This transistor, which is an amplifier, has its output connected through battery 20 to drive coil 13. In operation, when transistor 17 is rendered momentarily conductive, a current pulse from the battery is delivered to the drive coil 13, the resultant magnetic field producing a thrust on tine magnet 12, thereby actuating the fork. The voltage induced by tine magnet 14 in phase-sensing coil 15 is applied through capacitor 18 to base B of the transistor to overcome the bias thereon resulting from base current flow through resistor 19, to control the instant or phase position, in the course of each vibratory cycle, when the drive pulse is to be delivered to the drive coil.
The tuning fork and its associated electronic circuit fall into the resonant-feedback class of useful transistor oscillators, this class being analyzed in detail in the "Transistor Manual" Seventh Edition (1964), published by the General Electric Company. The resonant feedback oscillator uses either inductance-capacitance resonators or their electromechanical equivalent.
In the standard form of resonant-feedback transistor oscillator, the output of the transistor acting as an amplifier is coupled to the tuned primary of a transistor whose secondary is connected to the input to provide regeneration or positive feedback, causing the amplifier to oscillate. In the present arrangement shown in FIG. 1, the resonant feedback oscillator is a Hartley circuit in which the series-connected phase and drive coil windings form an autotransformer in place of the two winding transformers of the standard circuit.
We shall now consider the qualitative aspect of the frequency standard shown in FIG. 1. This is best done in the context of the equivalent circuit of the tuning fork resonator, for we are primarily concerned with the electrical effects of the oscillator circuit on the resonator.
In FIG. 2, the electrical equivalent to the tuning fork 10 is represented by inductance L f , capacitance C f , and resistance R f , all being in parallel relation to provide a parallel-resonant circuit whose frequency is determined by reactive values of inductance and capacitance, and whose "Q" is determined by the value of resistance. Since the tuning fork is a medium Q device, as compared, say, to a high Q resonator such as a piezoelectric crystal, the value of resistance is significant.
In FIG. 2, one can neglect the switching transistor and concentrate on the series-connected resistor 9 and capacitor 18, connected in shunt relation with the equivalent tuning fork resonator circuit. Resistor 19 has a value R o which appears between collector C and base B, and capacitor 18 has a value C o presented between base B and emitter E. These values may be replaced by their parallel equivalent, as shown in FIG. 3, wherein:
C o 1 = C o (1+ (ω C o R o ) 2 ) - 1
In the above equations, the symbol ω represents the angular frequency of the fork, that is 2 π multiplied by the frequency of the fork.
From an examination of equation II, it will be evident that to minimize the value of capacitance C o 1 , the values R o and C o should be large. It is desirable to minimize the value of C o 1 in comparison with C f in order to minimize the frequency shift of the system away from the natural frequency of the fork. If the value of C o 1 is not small relative to C f , then the fork is forced to operate at a frequency displaced from its natural frequency, in which event the amount of power required to drive the fork off-frequency is increased. Moreover, the system is then more sensitive to variations in capacitance C o 1 , which is undesirable.
But in order to provide an integrated circuit, it is essential to make the value of C o small (less than 400 picofarads) otherwise it cannot be fabricated using integrated circuit technology. One is then faced with the necessity of offsetting the effect of using a small capacitor in the picofarad range by making the value R o very large (greater than 100 megohms).
However, resistance R o is the main determinant of the base current of the transistor, and since the transistor is a current amplifying device, the decrease in base current resulting from an increase in the value of resistance R o will, in the case of a standard transistor of the type heretofore used in electronic timepieces, not provide sufficient current in the collector output circuit to drive the tuning fork. This collector current supplies the energy sustaining the fork in vibration, and for a given amplitude of fork vibration, one requires a definite amount of energy as determined by the Q of the fork.
I have found that the reduction in base current brought about by the use of a base current resistor whose value exceeds 100 megohms, can be offset by using a super-gain transistor of the type disclosed for example in Electronic Products Magazine (Apr. 19, 1971) in the article entitled "Super-Gain Transistors for IC's."
As pointed out in this article, super-gain transistors (also known commercially as super-beta transistors) are standard bipolar transistors which have emitters that have been diffused for extremely high currents. In the normal diffused transistor, the D-C current gain (Beta) is inversely related to the base width. But as the emitter is diffused deeper into the base region, the beta increases to several thousands. Typical current gains of 5,000 are obtainable at one microampere collector currents.
In the above qualitative analysis of the electronic circuit for driving the tuning fork, I have disregarded the effect of the transistor. In reality, when the transistor is turned on once each cycle to generate the drive pulse, the equivalent circuit of the class-C operated Hartley oscillator is more complicated. However, the main qualitative aspects of the circuit are still essentially the same as those described previously, hence there is no need to consider the more complicated aspects of the circuit behavior.
In summary, in order to make possible total integration of all elements forming the electronic circuit associated with an electromagnetically-actuated fork or other mechanical resonator, it becomes necessary to make use of a capacitor in the picofarad range, (i.e., 200 picofarads), for this value can readily be fabricated by existing microelectronics technology. This small capacitance value then dictates a much higher value for the base current resistor (>100 megohms), thereby giving rise to a markedly reduced base current. This in turn necessitates a super-gain transistor to provide adequate drive current for actuating the associated tuning fork to sustain it in vibration.
While there have been shown and described preferred embodiments of an integrated circuit for electronic timepieces, it will be appreciated that many changes and modifications may be made therein without, however, departing from the essential spirit of the invention.