US2027521A - Oscillation generator - Google Patents

Description

Jan. 14, 1936.

F. H. BRAKE OSCILLATION GENERATOR Filed June 24, 1953 2 Sheets-Sheet l Z ero emperafane -expams/'on fem Jan. 14, 1936. F H BRAKE OSCILLATION GENERATOR 2 Sheets-Sheet 2 Filed June 24, 1933 gli- Patented Jan. l, i36

UNITED STATES PATENT OFFICE OSCILLATION GENERATOR Application June 24, 1933, Serial No. 677,512

10 Claims.

This invention relates to electrical oscillators, and in particular to oscillators in which the frequency of oscillation is continuously variable over a substantial band of operating frequencies.

The problem of providing vacuum tube oscillators which exhibit a negligibly small drift or variation in oscillation frequency is an old one. It is well known that when a vacuum tube is connected to a periodic or tuned load circuit, with feedback coupling suitable for the generation of sustained oscillations, the fundamental frequency of these oscillations is approximately the same as the free oscillation frequency of the tuned load circuit. But it is also well known that in a practical apparatus the oscillation frequency will drift or vary, for any given setting of the physical coils and condensers employed in the system, with the ambient temperature, the local temperature of the vacuum tube, and with the applied voltages. Such frequency drifts, while usually a small percentage of the absolute frequency, may seriously limit the utility of the oscillation generator for communication and measurement purposes, particularly at the so-called high communication frequencies of the order of 3000 to 10,000 kilocycles per second. In recent years, it has become fairly common practice to use as the main frequency-determining element of the oscillation generator a low-decrement electro-mechanical oscillator of one of the following types: piezoelectric crystal, magneto-striction vibrator; magnetic vibrator (tuning fork). 'I'he use of such devices possesses three important disadvantages; rst, the fundamental frequency is not continuously variable as an operating adjustment, and except for corrective changes made as an intallation adjustment in the vicinity of the operating frequency, only discrete frequencies may be employed, each of which requires a different electromechanical oscillator; second, such frequency determining elements are expensive and inherenily diillcult to maintain in reliable operation at the higher communication frequencies; third, although when used according to the best modern practice the variation in frequency due to changes in external circuit constants and supply voltage may be made negligibly small by careful design, it is still necessary in precision work, to control the temperature of oscillators of any of the known types with a high degree of precision; in certain (ci. 25o-36) useful applications for such a generator, two are particularly important, viz: radio transmitters for military communications where the most useful communication frequency may be determined by the exigencies of the moment, and laboratory oscillation generators which must be continuously variable for measurement purposes but highly stable for any given setting of the tuning control. I have made a thorough study of the characteristics of oscillation generators of the general class in which a vacuum tube is the power-converting element, but the main frequency-determining element is a tunable electric circuit containing inductance and capacity and connected to either the plate or grid circuit of the tube, with a feedback coupling, by known means, to the other electrode. The principal causes of drift in the oscillation frequency are as follows (in this specification I shall use the term "drift" to denote a change in frequency originating in the generator itself and hence usually undesirable, reserving the term variation for a change in frequency produced deliberately by a change in one or more of the operating controls of the system) 1. Temperature changes.

2. Voltage changes.

With regard to changes in the supply voltages impressed upon the generator, it is possible by the use of a main frequency-determining circuit having a low decrement, and by the use of known methods and degrees of feedback coupling, to reods form no part of the present invention and are excluded from further consideration. With regard to temperature effects, I desire for a better understanding of my invention to sub-classify such effects according to their origin and detailed results, as follows:

'Iype (a) changes in the interelectrode capacity of the vacuum tube;

Type (b) changes in the reactance of either the inductance coils or the condensers of the main frequency-determining circuit;

'Iype (c) changes in the stray or parasitic capacities of the circuit between leads, terminals, etc.

In accordance with the present invention, the net eiects of temperature changes on the oscillator frequency are reduced or eliminated by balancing certain of the temperature eifects against each other, and by introducing additional elements to neutralize other effects. It is also desirable to reduce the absolute magnitude of the several temperature effects but the same methods of compensation may be applied to suppress frequency drift whether or not the component partsv of the receiver are so constructed as to reduce the tendency towards a frequency drift.

An object of the invention is to provide a generator of electric oscillationsin which the oscillation frequency is readily adjustable to a predetermined value regardless of the length of time during which this generator has been in operation prior to the adjustment. A further object is to provide an oscillation generator in which the frequency of oscillations is continuously variable throughout a substantial band of frequencies but which has a negligible overall temperature coeffcient of frequency. A further object is to provide an adjustable oscillation generator in which both the instantaneous frequency and the rate of change of frequency with temperature are adjustable. These and other objects of the invention will be apparent from the following description and appended drawings, in which:

Fig. l is a schematic circuit diagram of a typical oscillation generator employing my invention;

Figs. 2 and 3 are, respectively, a plan view of one section of a tuning condenser, and a side elevation of a coil assembly suitable for use in the circuit of Fig. 1;

Fig. 4 is a line drawing showing diagrammatically the construction and circuit connections of a frequency-control element employed in my invention; and

Fig. 5 is a collection of curves showing the variation in frequency, with time, of a typical system constructed in accordance with my invention.

In the schematic circuit diagram, Fig. l, the reference numeral I identifies a triode vacuum tube of an oscillation generator which includes a tuned grid circuit comprising the inductance 2 and a tuning condenser 3, and a feedback coil 4 in the plate circuit andcoupled to grid coil 2. The grid circuit includes a conventional grid 'leak and condenser 5 and the load circuit (not shown) into which the oscillator works may be coupled to the 4network by coupling condenser 6 and lead 1. The cathode K is heated in the usual manner, and the plate P is energized from a suitable high voltage source, indicated by +B. 'I'he elements mentioned constitute one known type of oscillation generator and, as will be apparent from the following description of the novel features, the invention is equally applicable to other typesv of oscillators. l

In accordance with the invention, the inductance 2 and tuning condenser 3 are of such physical construction as to render the resonant frequency of the tuned circuit substantially independent of the temperature of these elements. A further compensation for temperature changes 5 is provided by the thermostatic condenser which is shunted across coil 2, the condenser being shown schmatically as a vane or electrode 8 carried by a thermostatic strip S and thereby moved toward and away from a xed electrode I0 10 in accordance with the temperature of the strip.

In transmitters and other apparatus employing oscillators, it is the usual practice to house the parts in a container which, while it usually does not provide thermal insulation, affords protec- 15 tion from direct air currents. Good design practice calls for the location of the tube and the main frequency determining elements 2, 3 in separate adjacent compartments and the invention will be described with reference to this type of construction but it will be understood that the invention is applicable to other physical structures in which the rates of temperature change of the various elements are substantially different from those which are observed with the conventional transmitter constructions.

The significance of the novel structure will be best understood by first considering the causes and the nature of the temperature changes to which the several parts of an oscillation genera- 30 tor are subject. Since the observed eect of temperature upon drift varies with time after the transmitter is placed in operation, it is apparent that the several temperature eiects noted above should be analyzed on a time basis. Changes in the interelectrode capacities of the tube take place rapidly when the oscillator is placed in operation. For practical purposes the temperature of all parts of the tube may be considered as determined, during operation of the oscillator, 40 by the temperature, and hence the rate of liberation of heat from the lament or cathode of the tube. When the tube is turned on and the cathode heated to incandescence, the small heat capacity of all elements of the tube produces a rapid 45 rise in temperature which is large in comparison with any changes which may be due to variations of the temperature of the ambient air, and which quickly establishes stable temperature conditions that are but slightly,. if at all, affected by the 50 ambient temperaturaf The coil 2 and condenser 3, and the various stray capacities external to tube I which go to make up the main frequency-determining circuit, are subjected during operation to varia- 55 tions in temperature which are relatively slow compared with the self-generated temperature changes of the vacuum tube. This arises from the fact that these elements are all made of, or thermally connected with, bodies of relatively 60 large heat capacity; their temperature changes are produced by a small amount of convection and conduction from the tube and by changes i. e., those which are closely associated with the tube and subject during operation toy a rapid rise to a temperature considerably above the ambient temperature; and all others, which undergo relatively slow-changes in temperature which may be no greater than the variations in ambient temperature.

Of the second class of elements, those aected I by type (c) effects or changes in parasitic capacities and capacities between terminals are stabilized, as far as practicable for thermal changes, by the use of rigid conductors clamped in position; by the use of dielectrics of the preferred classes which are described hereinafter, and by slight adjustments of the additional means hereinafter described.

Of the elements, affected by the type (b) effects, changes in the reactance of coils and condensers `(such as the elements 2, 3) are first minimized in absolute value by the use of one or more of the preferred insulators at all points where mechanical supports are required. The residual frequency drift originating in the coil and condenser is then reduced or eliminated by methods of construction which produce a decrease in capacity fora change in temperature of that sense which causes an increase in inductance and vice versa. In other words, the tuning condenser and inductance of the main frequency-determining circuit, which are both large and are heated and cooled slowly and equally, are made to have temperature coeilicients of capacity and inductance, respectively, of opposite sign and substantially equal value. Since the most important quantity determining the oscillation frequency isy the product of this inductance by this capacity, the result is the reduction or suppression of thermal variation in this product. l

It is apparent that the design of apparatus to compensate for the usual temperature effects may be simplified by employing materials which reduce the tendency towards a frequency drift. I have discovered that the molded condensation products (of the types sold under the name Bakelite which are commonly employed as insulating materials have a high temperature coefficient of dielectric constant, and that certain other materials are characterized by a low temperature coefficient of dielectric constant. The synthetic resins commonly employed for tube bases, tube sockets and condenser insulation have dielectric constants of the order of iive to eight times that of air, and have a temperature coefficient of dielectric constant as high as 2% per degree centigrade.

Furthermore, I have discovered that, with a commonly used insulating material of the molded synthetic resin type (employed only at points in an oscillation generator where good mechanical design demands a solid insulating material), the dominating portion of the thermal frequency drift may be that due to changes in the dielectric constant of the said insulating material. It is therefore preferable to employ, where solid insulating material 'is required, such materials as have a low temperature coefficient of dielectric constant. I have found that appropriate materials are hard rubber compositons of the type sold commercially as 11 x 2B ceramics of the vitried steatite or aluminum silicate type such as sold under the trade name Isolantite and boro-silicate glass of the low-expansion, heat-resistant type such as sold under the trade mark Pyrex. When measured on a 1000 cycle capacity bridge, these preferred types of insulating materials exhibit the following properties:

Tem rature coe cient of dielectric constant (ZP-60 C.)

Material As noted above, the corresponding values for the molded synthetic resins commonly employed as insulating material in radio equipment run as high as 2% per degree centigrade. The problem of compensating for temperature effects is slmpliiied by the use of insulating materials having a low temperature coeiilcient of dielectric constant, which term includes any suitable materials that have temperature coeiiicients not substantially larger than those of the above tabulation.

An appropriate construction for the tuning condenser is illustrated in Fig. 2. The steel frame Il constitutes a pivotal support for the rotor plates l2, in the usual manner, and is threaded to receive the pairs of screws I3 which support the stator assembly. The ends of the screws and of the rods Il are similarly recessed to receive the insulating spheres l5 which are of material having a low temperature coeflicient of dielectric constant, being preferably borosilicate glass balls. The mounting rods I4 are formed of a metal having a high temperatureexpansion coefiicient, such as brass, and the statorV plates I6 as well as the rotor plates I2 are of metal having a low or zero temperatureexpansion coeilicient, such as the nickel-iron alloys sold under the trade-mark Invar. With this construction, the temperature coefficient of capacity of the condenser is negative, i. e., the capacity decreases with an increase in temperature. The temperature coeflicient is constant within wide temperature limits, and predeterminable with satisfactory precision for a given design of condenser.

A coil assembly for use with such a condenser is shown in Fig. 3, and comprises copper wire or wires vII on a threaded tube I8 of a material, such as the ceramic Isolantite, which has a low temperature coefficient of dielectric constant. The winding form may be mounted on any appropriate support I9, such as the base of a shield can. The copper conductor expands with increasing temperature and, by suitable choice of the conductor size and spacing, the percentage increase in inductance for rising temperatures may be made approximately equal to the percentage decrease in capacity of the tuning condenser for the same temperature rise. Thus when the two are combined to form the main frequency-determining element of the oscillation generator, they will produce, over a relatively slow change in temperature of the type described, no contribution to the frequency drift.

There remains to be stabilized the class of variations which are designatedabove as type (a) i. e., changes in the interelectrode capacities of the vacuum tube. As has been explained, these variations are characterized by a relatively rapid rise in temperature during operation, to

values substantially above the ambient temperature. 'I'he thermostatic condenser which compensates for these changes should therefore be of extremely low heat capacity in order that it may respond quickly to changes in temperature. As shown in Fig. 4, the thermostatic condenser is mounted on the partition wall 20 which divides a'portion of the metal housing into compartments for receiving the tube I and ior housing the coil 2 and condenserf 3. The tube base 2| and the' socket 22 are preferably formed4 of one of the insulating materials noted above, thereby reducing the rates of change ci the interelectrode capacities with temperature.

The movable plate 8 of the condenser is an aluminum disk of light gauge and small heat capacity, and the mounting oi the thermostatic strip' 9 on the partition wall 20 Vpermits use of that wall as the xed electrodeof the condenser. The upper end of strip 9 is secured to and spaced from an insulating plate 23, such as a disk of mica, by a bolt 24 and spacing collar 25. The plate 23 extends across an aperture in the wall 20 and the bolt 24 is connected to the high potential terminal of the tuned circuit 2, 3 by a conductor 26, the low potential circuit terminal being grounded on wall 20 by a lead 21. The conductor 26 is of low heat conductivity and may be a i'lne wire, such as #36, B & S gauge copper; or, if a fine conductor' is undesirable for mechanical reasons, a material of low heat conductivity, such as manganin alloy, may be used. By thus reducing the leakage of heat from the moving plate of the condenser, its ,rate of response to heat generated in tube l is accelerated.

The sections @8, 9b of the thermostatic strips are thin metal strips of Invar and of nickelsilver, respectively, and the strip therefore fiexes away from the partition wall 2@ with rising temperatures, thus reducing the capacity of the condenser. T'ne surfaces or strip es and disk t adjacent the tube are blackened to facilitate the absorption of heat. This type of thermostatic condenser is appropriate for use with a genera-l class of low power tubes in which'the tube ele- Vments* are-.all mounted-on supports sealed at one end of the tube. have found experimentally that in anoscillation generator having the circuit shown in Fig. l., that portion -oi the frequency drift -which'is contributed by the inter- *electifode -capacides the' direction of decreasing irreouency vfor' increasing temperature, and vice versa; and that when. using tubes ci the type described, the frequency drift is consistent, r'evernbleand' reproducible. Consequently, with -suchtubes and the circuit arrangement of Fig. i, the capacity oi the thermostatic condenser shculd decrease with rising temperature to compensate for the thermal capacity changes in the tube. It the tube is so constructed that the resultant drift is in the opposite sense, the mounting of thebimetallic strip 9 is reversed to obtain a capacity increase with rising temperature.

It is obvious that with suitable care in design 'the thermostatic condenser could be incorporated within the tube I, in which case a most active response could be obtained. VSuch a construction would be within the scope of this invention. One or more such small thermostatic condensers could be included within the tube to hold the various interelectrode capacities invariant. l

The curves of Fig. 5 compare the perfomance of an oscillation `generator embodying the invention with a noncompensated generator employing the usual synthetic resin insulation at all high potential points in the Fig. 1 circuit, ex-

cept in the main timing condenser but including the base oi' the vacuum tube. The tube was oi the base terminal type, as shown in Fig. 4, and

operated at a plate power input of approximately `4 watts with the circuit adjusted to give a power conversion eiliciency oLabout 50%. Time in minutes from the instant the filament circuit was closed is plotted along the horizontal axis and the decrease in the frequency oi oscillation, expressed as a percentage of the nominalor desired frequency, is plotted along the vertical axis. Curve A represents the performance oi the noncompensated oscillation generator employing, except as noted, insulating material having a relatively high temperature coeiilcient of dielectric constant. It is to be observed that the frequency decreases rapidly for the rst ve minutes; this portion of the frequency drift is due mainly to the relatively rapid heating of the various parts of the vacuum tube, particularly the base, after the filament is turned on.

These curves were taken for a constant ambient temperature, and consequently the eiects on the remainder of the system are a gradual heating Cmainly by conduction and convection from the hot tube), which causes a further steady drift in the same sense but at a reduced rate. The curve is plotted for only the rst 20 minutes after the generator was turned on, but it is a noteworthy fact that under these conditions the steady change in frequency indicated by the course of curve A maypersist for hours, producing a total drift, even at constant supply voltage, to a frequency diering from the initial frequency by as much as 9.5% or more.

Curve B was plotted.V from data obtained with the same initial circuit constants as for the generator 'of curve A, but with insulation of the preferred type used-throughout, including a ceramc base in the tube. The inductance and tuning condenser 3 were designed to have opposite and approximately equal temperature codenser wasnot employed.

The noteworthy nrnditions stated in the` preceding Vsentence masa stabilization -oi the frequency after therst ewminutes at a .frequency appneciablyy less than the :nominal requency :bm practicediylseonstant.

The initial rapdi'reaeency drift, due mainlyto the heating ci the tube, is reduced in magnitude but still persists.

'static condenser received suilicient -heat to operate it fully. Then the heat from the tube produced a decrease in the thermostatic condenser capacity which reduced the frequency drift practically to zero. The capacity of the thermostatic condenser quickly became stabilized at a value less than its initial valuewhich practically restored the oscillation frequency to its initial value, where it remained thereafter. Y

.eiiicients of reactance, but the thermostatic conf For simplicity, I have omitted a graphical showing of experimental results with my invention for changes in the ambient temperature. I

have found in practical systems that an oscillator which is properly adjusted to suppress the drift during warming-up periods at constant ambient temperature is usually properly adjusted for a similar degree of suppression of the drift which would normally accompany wide changes in ambient temperature after warming-up. This arises from the fact that the thermostatic condenser is usually varied, by changes in the temperature of the oscillator housing, at least as rapidly and widely as the base of the tube, and by the use o1' mechanical designs which will be obvious to one skilled in the art, the main frequency-determining circuit may be so arranged that the condenser and coil thereof are equally heated and cooled by thermal conduction through the housing. Furthermore, the enclosure of the thermostatic condenser in a compartment immediately adjacent the vacuum tube tends to maintain this condenser at a temperature determined largely by the heated tube, even though the ambient temperature falls to very low values. Experience with the system soon indicates the adjustments which should be made to take care of extreme operating conditions. For example, I have found experimentally with this invention embodied in a radio transmitter designed for use on aircraft. that by adjusting the thermostatic condenser for under-compensation of warming-up drift at constant room temperature a satisfactorily low frequency driftmay be obtained under the extreme operating condition where the apparatus is cooled to below 0 F. before the tube is turned on. With this particular type of correction, I have adjusted the thermostatic condenser to a predetermined initial value, transported the equipment to a region of low temperature and kept it there until all parts thereof were cooled to a temperature of 5 F., then started the oscillator and increased the ambient temperature to a value of 70 F., while constantly measuring the frequency, and obtained a maximum variation from the normal frequency throughout the test of less than .05%. This is a degree of frequency stability, for the given temperature range, whichcompares favorably with that obtained with piezoelectric oscillators, and possesses the great advantage of being obtained in a system whose operating frequency is continuously variable at the will of the operator.

While I have described by invention in connection with one particular circuit, the novel principles involved are obviously applicable to any one of a number of circuits used in oscillation generators, and also to a wide variety of organizations of the physical apparatus.

I claim: l

1. In an oscillation generator, the combination with a vacuum tube having a base of insulating material in which connections to the tube electrodes are mounted, of a tunable frequency-determining circuit including an inductance and tuning condenser, said inductance and condenser including means to prevent drift of the oscillation frequency with changes in temperature of said inductance and condenser, and means compensating for those changes in interelectrode capacities which result from changes in the temperature of said tube base.

2. An oscillation generator as claimed in claim l, wherein said tube base comprises material having a low temperature coelcient of dielectric constant.

3. In an oscillation generator of the type including an inductance coil-condenser circuit continuously tunable over a range of frequencies, the combination with a vacuum tube having a base of insulating material in which electrode connections are mounted, and an oscillatory network, of means included in said network to compensate for the frequency drift normally resulting from changes in the temperature of a coil and tuning condenser, and means comprising a temperature-responsive reactance in said network in heat-transfer relationship to said tube, to compensate for frequency drift normally resulting from changes in the temperature of the tube.

4. An oscillation generator comprising a vacuum tube, and a frequency-determining network associated with said tube, the main frequencydetermining elements of said network comprising an inductance and a tuning condenser, said elements having temperature coeiiicients of opposite sign and of substantially the same magnitude, said inductance and tuning condenser each including solid insulating members, and said insulating members having a low temperature coeflicient of dielectric constant.

5. A vacuum tube oscillator system comprising a frequency-determining network whose effective value is substantially independent of rapid changes in temperature, said network including an inductance and a tuning condenser having temperature coeflicients of opposite sign and of substantially the same magnitude, a vacuum tube subjected during operation of the oscillator to relatively rapid changes in temperature which tend to alter the oscillation frequency of the system at a relatively rapid rate, and a supplemental frequency-determining element having an effective value which varies with the temperature of the tube and in a sense to oppose that alteration of the oscillation frequency which arises from the changing temperature of the said tube.

6. An oscillator system as claimed in claim 5, wherein said supplemental element comprises a thermostatic condenser, one electrode of said condenser comprising a bimetallic strip.

7. In an oscillator, a metallic housing having a plurality of compartments, a tube socket in one compartment, a bimetallic strip in said one compartment mounted on and insulated from one wall of the said compartment, a fixed electrode cooperating with said strip to form a thermostatic condenser, an oscillatory network in said housing and exterior to said compartment, and circuit elements connecting said condenser to said network, said fixed electrode comprising a portion of the compartment wall on which said strip is mounted.

8. The method of control of the frequency of oscillations of a generator including a tube, and a tuned circuit of the xed inductance-variable condenser type, which comprises balancing temperature-produced variations in the reactance of said inductance by temperature-produced variations in the reactance of said tuning condenser, and compensating for temperature-produced variations in the inter-electrode capacities of the tube.

9. The method of obtaining frequency stability with an oscillator of the type including a tube and a pair of primary frequency determining elements, one of the said elements being adjustable to determine the oscillatory frequency, which method comprises subjecting said primary fretemperature-produced variations in the react-` ance of one element by temperature-produced variations in the reactance of the other e1ement,'

and independently compensating for frequency variations arising from the heating of the tube.

10. Apparatus for producing electrical oscillations of a predetermined frequency which is substantially independent of the ambient temperature, said apparatus comprising primary Irequency determining elements positioned adjacent to each other and subjected to substantially the same temperature variations, and a space current device and a thermostatic capacitive element adjacent each other and subject to similar temperature iluctuations, said primary elements having substantially equal temperature coeftlcients of opposite sign, the said capacitive element and space discharge device comprising secondary frequency determining elements having substantially equal temperature coemcients of 10 opposite sign.

FREDERICK DRAKE.

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