US2707232A - Impedance translating device - Google Patents

Description

April 26, R ADLER IMPEDANCE TRANSLATING DEVICE Filed Nov. 3, 1949 Wa/XVENTOR. V

BY 4 JMW2% W United States Patent 0 IMPEDANCE TRAN SLATING DEVICE Robert Adler, Chicago, Ill., assignor to Consolidated Electric Company, Chicago, 111., a corporation of Illinois Application November 3, 1949, Serial No. 125,249 8 Claims. (Cl. 250-36) This invention relates to an audio and lower frequency impedance translating device and in particular as applied to a variable frequency oscillator, as applied to a variable frequency resonant circuit, and simply as an impedance device having an apparent impedance of adjustable value, the apparent impedance for any given adjustment remaining constant over a wide range of voltages or frequencies of an external voltage applied to said device. It is an object of this invention to provide improved apparatus of that character.

It is well known to obtain variable capacitance through the use of rotatable condenser plates or through the use of multiple condensers having selectable taps. Where it is necessary that space requirements be kept at a minimum, the first of these possibilities is suitable only for higher frequencies, i. e.,small capacities, because, at audio and lower frequencies generally, the condensers become large due to the need for large capacities and the large spacing between plates. The second of these arrangements is not completely satisfactory because the capacity is variable only in finite steps and because of variable contact resistance. Furthermore, either of these arrangements requires a substantial operating force because of unavoidable, attendant friction and thus is useless where relatively small mechanical forces are available.

Fixed condensers can be made sufficiently small because metal foil types may be used and these can be made with properties substantially independent of variables such as temperature, voltage, etc.

It is well known to obtain variable inductance through the use of slide wire devices, multiple tap devices, coils in which a core is slidable, and variometers, the latter being a transformer having an air gap with a coil rotatably mounted therein.

Here, at high frequencies, air core coils can be used because the values of inductance needed are small, and since the permeability of air is constant variable inductances can be made stable, that is, the inductance does not change substantially with different voltages under varying temperature conditions, etc. Where the needed inductance values are higher, powdered metal cores have been used in variable inductances at high frequencies with good stability.

At audio frequencies and lower frequencies generally, it is necessary to utilize iron core devices of some sort to obtain the required values of inductance. Whether such audio frequency inductance is of the variometer type or any other one of the types indicated, inductance is proportional to the permeability of the iron, and since the permeability of iron is a function of the applied voltage, known variable inductance devices for audio frequencies are not stable.

Slide wire devices are unsuitable because of relatively high required operating force in applications where only small forces are available, and where continuous variations in inductance are needed multiple contact devices obviously are not satisfactory.

Fixed inductance devices can be made sufiiciently stable through the use of good iron and a well designed air gap.

Tuned circuits or oscillators of variable frequency in which such known variable reactance devices are employed are limited in their application or usefulness because of the limitations discussed above in the adjustability and stability of the impedance devices.

Conventional variable resistances such as carbon piles, slide wire resistors and tapped resistors also have various disadvantages such as instability, and variable contact resistance.

It is an object of the invention to provide an improved variable impedance which is stable over the desired range of voltage, frequency, temperature and other common variables.

While the invention is particularly applicable to alternating currents of relatively low frequencies, such as those within and below the range termed audio frequencies, it may be applied to circuits utilizing higher frequencies.

According to the invention, an impedance device of adjustable apparent or effective value is obtained by placing in series with a fixed impedance element having a stable impedance value an internal voltage which may buck or boost an external voltage applied to the device. This internal voltage is of the same character as the external or applied voltages, that is, if the external voltage is alternating, the internal voltage must be an alternating voltage of the same frequency and in proper phase relation therewith. Under these circumstances, as will subsequently be explained in greater detail, the current flow resulting from the application of an external voltage will be such as to indicate an impedance of greater or lesser value than the actual impedance of the fixed impedance element, depending upon the relative polarity or phase relationship of the internal voltage with respect to the applied voltage. The apparent impedance of the device can be varied by selectably adjusting the magnitude of the internal voltage in either bucking or boosting relationship with respect to the external or applied voltage.

Where the applied voltage is of variable magnitude, the internal voltage, in addition to being of the same character as the applied voltage, must be varied in magnitude in direct proportion to the external voltage in order to maintain a constant apparent impedance for a given setting or adjustment of the device. This may be accomplished by having the character and magnitude of the internal voltage derived from the applied voltage. The circuit is preferably arranged to isolate the applied voltage, as through the use of an electron amplifier tube circuit, since any current drawn from the source of applied voltage will cause a change in the apparent impedance of the device.

According to one embodiment of the invention, a selectable internal voltage is obtained from a rotary transformer which is continuously excited by the applied voltage through an electron tube circuit. This arrangement results in the character and magnitude of the internal voltage being derived from the applied voltage while calling for a negligible fiow of current from the source of applied voltage.

For any given applied voltage the internal voltage may be varied over a iven range by adjustment of the rotary transformer. The range of apparent impedance thereby obtained centers about the constant impedance of the fixed impedance element. This range may be increased or decreased by varying the gain of the isolating amplifier which excites the rotary transformer, the increased or decreased range of selectable impedance values still being centered about the same central value. This feature is of great practical value in bringing the device into proper adjustment.

The resonant frequency of a tuned circuit may be varied by utilizing such a variable reactance device therein, that is, a voltage is placed in series with either the capacitive or the inductive reactance of the resonant circuit, the value of this voltage being automatically maintained at some desired fraction of the voltage applied to the tuned circuit. The resonant frequency of such a circuit may be varied smoothly over a Wide range, and with any selected setting will remain constant even though the voltage applied thereto varies in magnitude.

The controlling or frequency determining portion of one type of oscillator is a resonant circuit comprising at least one condenser and one inductance coil. The output frequency of such an oscillator may be made selectable by utilizing a resonant circuit such as that described immediately above, in which case the output frequency may be smoothly adjusted and, for any selected setting, will remain constant even though the voltage or current output of the oscillator varies over a wide range.

finite steps of variability,

Accordingly, it is another object of the invention to provide an improved oscillator having a smoothly variable and selectable output frequency.

It is another object of the invention to provide an improved oscillator having an output frequency which is smoothly and selectably variable and which remains substantially constant for a given setting in spite of substantial variation of the voltage or current output thereof.

It is another object of the invention to provide an improved resonant circuit of smoothly variable and selectable resonant frequency.

It is another object of the invention to provide an improved resonant circuit having a resonant frequency which is smoothly and selectably variable and which remains substantially constant at any given setting in spite of substantial variation of the voltage applied thereto.

It is another object of the invention to provide an improved impedance device of smoothly variable and selectable apparent impedance.

It is another object of the invention to provide an improved impedance device of smoothly variable and selectable apparent impedance, such apparent impedance remaining substantially constant for any given setting of said device in spite of substantial variation of the magnitude of the voltage applied thereto.

It is another object of the invention to provide an improved impedance device having an apparent impedance which is smoothly variable and selectable over a given range, that range being smoothly and selectably variable.

It is another object of the invention to provide an improved impedance device having an apparent impedance which is smoothly variable and selectable over a given range, that range being smoothly and selectably variable about the same central value of impedance.

It is another object of the invention to provide an improved reactance device having a smoothly variable and selectable apparent reactance.

It is another object of the invention to provide an improved reactance device having a smoothly variable and selectable reactance, such apparent reactance remaining constant for a given setting of said device in spite of substantial variation of the magnitude or frequency of an alternating voltage applied thereto.

The invention, together with further objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, in which like parts are indicated by like reference numerals:

Figure 1 is a diagram illustrating one embodiment of the invention as applied to a variable impedance device for lse with an alternating voltage;

Fig. 2 is a diagram illustrating an embodiment of the invention as applied to a resonant circuit, and

Fig. 3 is a diagram illustrating an embodiment of the invention as applied to an oscillator.

With particular reference to Fig. l, the invention is shown embodied in a circuit including a pair of input terminals 11 and 12, a series arrangement therebetween of a fixed and stable capacitance condenser 13 and a rotor coil 14 mounted on the rotor 14' of a rotary transformer 15, and an amplifier tube 17. The transformer is excited by voltage applied to a stationary coil 16, the immediate source of this voltage being the tube 17 connected as a cathode follower. A cathode follower type circuit is preferably employed in the embodiment shown in Fig. 1 because it has certain desirable characteristics which are particularly advantageous in this application, as will subsequently be described in detail.

The circuit includes the tube 17, the plate 18 of which is connected by a conductor 19 to the positive side of a B battery 20. The negative side of this battery is connected to a biasing resistor 21 which has a capacitor 22 arranged in parallel therewith, this condenser permitting the flow of alternating current therethrough without appreciably altering the bias which is obtained by the flow of direct current through the resistor 21, all as well under stood in the art.

The other end of the resistor 21 is connected by a conductor 23 to one lead of the transformer exciting coil 16, the other lead of this coil being connected to the cathode 24 of the electron tube 17. A circuit is thereby completed through the exciting coil 16 through the tube 17 and the battery 20 and through the resistor 21 and capacitor 22 back to the exciting coil. A capacitor 25 may be connected in parallel with the exciting coil 16 for the purpose of creating a circuit in conjunction with the exciting coil 16 approximately resonant over a substantial range of frequency.

The voltage across coil 16 is controlled by the grid 26 of the electron tube 17, this grid being connected by a conductor 27 to the terminal 11. The other terminal 12 is connected by a conductor 28 to a point 29 at the lower point of the biasing resistor 21. The exciting coil 16 being connected in the cathode circuit of tube 17 makes this a cathode follower circuit, such a circuit inherently having linear amplification and an input im pedance which is very high with respect to the output impedance. A high input impedance is desirable since it is necessary to prevent drawing any current from the source of external voltage, otherwise there results a change in the apparent impedance of the device. Linear amplification is needed to excite the rotary transformer with a voltage whose instantaneous magnitude is a linear function of the magnitude of the external voltage, the purpose of which will subsequently become clear. The cathode follower circuit is also very stable, that is, the linear amplification is not materially affected by reasonable variations in the characteristics of the tube.

It will now be apparent that an external alternating voltage applied to the terminals 11 and 12 will determine the character and magnitude of the voltage applied to the transformer exciting coil 16. Stated otherwise, the character and magnitude of the exciting coil voltage is derived from the applied voltage while putting a negligible load on the source thereof, the amplifier tube 17 effectively isolating the transformer 15 from the source of the voltage applied to terminals 11 and 12. Some form of isolating means is, of course, needed to prevent the drawing of any appreciable current from the source of applied voltage by the control means.

The rotary transformer 15 is designed to operate over an unsaturated range, and, accordingly, for any given position of the rotor coil 14, the voltage output thereof is a linear function of the excitation voltage of coil 16. The voltage of cathode 24 following the voltage of the grid 26, it follows then that for any given position of the rotor coil 14, the output voltage thereof will be a fixed proportion of the voltage applied to terminals 11 and 12.

The angular position of the rotor 12 may be controlled by a control knob 20 which is mechanically interconnected with the rotor 14 as schematically indicated by the dash line 31. Rotation of the rotor 14' will alter the flux linkage between the exciting coil 16 and the rotor coil 14 and will result in a change in the magnitude of the rotor coil voltage with respect to the voltage applied to terminals 11 and 12. When the plane of the rotor coil is aligned with the magnetic field of the transformer the voltage output of the coil is zero, with the result that the apparent impedance of the device will be equal to the capacitive reactance of the condenser 13 plus the reactance of the rotor coil 14 which is preferably made very small (negligible) with respect to the reactance of the fixed capacitance condenser 13.

As the rotor coil 14 is turned from its neutral position, a voltage is induced therein of the same frequency as the frequency of the applied voltage, and this voltage can be made to buck or boost the applied voltage depending upon the direction in which the coil is turned. If the rotor coil output is made to boost the applied voltage, a greater current will flow in the condenser circuit with the result that the apparent capacitance of the circuit is increased. Similarly, if the coil is turned in the opposite direction, its output will buck the applied voltage with the result that the current in this circuit will be decreased and the apparent capacitance will be less.

The rotary transformer 15 is preferably so designed that the voltage output of the rotor coil 14 is a linear function of the position of the rotor 14' and hence of the control knob 30. This may be accomplished in a manner well known in the art by providing a uniform radial air gap between the pole faces and the rotor, with the magnetic field uniformly distributed thereover, and

by arranging the small number of turns of the rotor coil in closely spaced relationship. In place of the rotary type, other transformer designs may be used in which a continuously variable voltage ratio is obtained by means of movable coils or movable flux-carrying members. The

transform-er 15 serves as a variable voltage ratio device whose output voltage is a linear function of its excitation voltage and, preferably, a linear function of some mechanical displacement. Any device having such characteristics may be used in place of the rotary transformer.

If desired, the positioning of the rotor coil 14 can be made an automatic or semi-automatic function of some external apparatus. However, since the embodiment of the invention presently being described concerns only the impedance device itself, the details of such a control circuit will not be described herein.

The impedance of the exciting coil 16 is made very high relative to the impedance of the tube circuit and consequently there is good voltage regulation in the tube circuit. The impedance of the rotor coil is preferably made small with respect to that of fixed capacitor 13 so that good voltage regulation is obtained throughout the internal voltage generating system, all as well understood in the art.

The voltage amplification ratio of the cahode follower 7 tube circuit is inherently less than one. This is not objectionable in this particular application since the exciting coil 16 may be adapted for operation with this somewhat smaller voltage.

The range in magnitude of the internal voltage, and, hence, the range in magnitude of the apparent impedance are limited, for any given voltage gain in the amplifier, by the maximum usable voltage ratio of the rotary transformer. The impedance range is centered about the impedance value of the fixed impedance 13 since a given maximum internal voltage is arranged in either bucking or boosting relationship, and when the internal voltage is zero, i. e., neither buck nor boost, the apparent impedance is that of element 13. However, this impedance range can be varied by changing the amplifier gain.

Variation in voltage amplification in a cathode follower amplifier is limited and the amplification is less than one in all cases and, accordingly, better control of this nature can be obtained by using a negative feedback amplifier such as that described below and shown in If desired, an amplifier may be inserted in the circuit to amplify the output of the rotary transformer before it reaches the load circuit of the applied voltage. Such an amplifier may be made adjustable so that the range of the internal voltage may be varied.

The increased or decreased impedance range obtained by varying amplifier gain will still be centered about the value of the fixed impedance 13. This characteristic of the impedance device has considerable practical advantage in the adjustment thereof. The central or neutral value of impedance is determined by selection of a fixed impedance element of the desired value. The range of apparent impedance for any given amplifier gain can then be determined by rotating the rotor of the transformer through its maximum desired angular displacement. If the variation in apparent impedance is then found to be too great or too small, the amplifier gain can be adjusted by any suitable means with the knowledge that any adjustment will result in a range of apparent impeilance centered about the original central or neutral va ue.

This variable impedance device may have incorporated therein, in place of the condenser 13, an inductance coil 13a of constant and stable value such as one obtainable by an iron core with an air gap, or a resistor 13b of constant and stable value, both of which are shown in phantom lines in Fig. 1. In each of these cases, the operation of the device remains the same, the frequency of the rotor coil output always being the same as that of the voltage applied to terminals 11 and 12 and in the proper phase relationship therewith, and the voltage magnitude always being a linear function of the magnitude of the applied voltage and of the position of the control knob 38.

Thus it is seen that the variable impedance device comprises simply a fixed impedance element and a voltage source arranged in series therewith, the output voltage of that source deriving its character and magnitude from the voltage applied to the device without burdening the latter, and the magnitude of the internal voltage being proportional to, and a linear function of, the magnitude of the applied voltage. The latter proportion is made adjustable, as by turning the rotor coil 14, in order that the apparent impedance be selectably or automatically adjustable.

The rotary transformer 15 employed in the embodiment of the invention described above is well known as a variable inductance device of itself, i. e., as a variometer. However, when used in that manner the inductance is not stable, i. e., the variable permeability of the magnetic circuit causes an undesired change in inductance when the voltage applied thereto is altered in frequency or magnitude. When used in this fashion, the coil 16 may be connected in series with coil 14 and the two windings connected to the voltage source. The inductance then presented to the voltage source is determined by the position of coil 14 whose flux adds to, or subtracts from, that due to coil 16. So long as there is no change in the fiux of the core, that is, so long as the applied voltage does not change, and the air gap does not change, etc., the value of inductance may be sensibly constant. But when the voltage changes, or the air gap changes, the flux through the core changes and hence the inductance changes. In other words, the permeability of the iron core is a function of the voltage, air gap, etc., inductance being a function of permeability, when permeability changes the inductance changes.

When the rotary transformer is used as in the circuit shown in Fig. l, the device is employed as a transformer or a ratio device rather than an inductance device and its variable perm ability has a negligible effect. That is, the voltage induced into coil 14, for any one position, is always equal to the voltage applied to coil 16 multiplied by a constant which is the turn ratio of coils 14 and 16 and a function of the position of coil 14.

if the voltage applied to coil 16 changes and the fiux through the core (and thus the permeability) changes, the voltage of coil 14, for the same position, is still the same proportion of the voltage of coil 16. The linearity of output to input voltage has been maintained. If the air gap should expand due to temperature, etc., so that the total flux drops a certain percentage, for example, ten percent, the voltage of coil 14 would not change so long as the voltage of coil 16 has not changed because the air gap fiux pattern has not changed. Again the linearity of input to output voltage has been maintained.

in other words, according to the invention, the transformer ratio of the rotary transformer is used which is a function of the transformer configuration and which is independent of permeability to obtain a linear and stable element, whereas the same rotary transformer used as an inductance device has inductance proportional to permeability and thus is non-linear and unstable.

Moreover, at audio and lower frequencies, the rotary transformer represents the most satisfactory instrument to produce smooth reproducible electrical characteristics. Since the rotor may be mounted on low-friction bearings, the moving force required is small compared to slide wire reactors which have high moving friction. The rotary transformer may be made quite small relative to a variable condenser of corresponding reactance.

Attention is now directed to Fig. 2 in which another embodiment of the invention is shown applied to a resonant circuit, its function being to vary the resonant frequency of such a circuit and to maintain such resonant frequency constant for any given settting of the device even though the external voltage applied to the circuit varies in magnitude. The terminals 11 and 12 are connected, as shown, to a circuit 41 consisting of an inductance coil 42, a condenser 4-3, and the rotor coil 14 arranged in series with the condenser 43. When the voltage output of the rotor coil is zero, the resonant frequency of the circuit 41 will be determined by the fixed reactances of the inductance coil 42 and the condenser since the reactance of the rotor coil 14 is of such small magnitude that it may be disregarded. Coil 42 and condenser 43 are stable elements, as already indicated.

If, however, the rotor coil 14 is made to produce a voltage of the same frequency as that applied to the terminals 11 and 12, the charging current in the condenser circuit will be made greater or lesser than normal depending upon whether the rotor coil voltage is in boosting or bucking relationship with the applied voltage. 1f the charging current is made greater, as by arranging the rotor coil voltage in boosting relationship, the apparent capacitance is increased and the resonant frequency of the resonant circiut 41 is lowered, as is readily understood by reference to the Well known formula:

in which fr is the resonant frequency, 11' is 3.1416, L is inductance in henries, and C is capacitance in farads. Similarly, if the voltage output of the rotor coil 14 is made to buck the applied voltage, the apparent capacitance is decreased, and the resonant frequency will increase.

The rotary transformer 15 may be identical to that described in Fig. l, a control knob 30 again being provided to control the position of the rotor coil 14. Excitation voltage is supplied to the stationary coil 16 through a negative feedback electron tube amplifier circuit. The tube 44 has a plate 45 which is connected to the transformer exciting coil 16 through a conductor 46. The other terminal of the exciting coil is connected through a conductor 47 to the positive terminal of a B battery 48. The negative terminal of this battery is connected through a resistor 49 to the cathode i) of the electron tube 44, a condenser 51 being arranged in parallel with the resistor 49. A condenser 25 may again be placed in parallel with the transformer exciting coil 16 to establish a substantially resonant load circuit. The plate current of the tube 44, and, hence, the excitation current, is controlled by the grid 51 which is connected through a resistor 52 to the terminal 11, the other terminal 12 being connected by a conductor 53 to a point 54 in the tube circuit.

In order that the voltage amplification factor of the amplifier be constant, which is necessary to preserve a linear relationship between the signal input and the voltage output, the amplifier is connected as a negative feedback amplifier. That is, plate 45 is connected to grid 51 through the condenser 55 and the resistor 56. With linear amplification thus obtained in the tube circuit, the voltage output of the rotor coil 14 will be a linear function of the applied voltage for the reasons stated above in describing the apparatus of Fig. 1. In this circuit also the input impedance is high so the transformer is effectively isolated from the resonant circuit.

Accordingly, when an alternating voltage of a certain frequency and magniude is applied to the circuit 41 through the terminals 11 and 12, the rotor coil 14 will produce a voltage whose character and magnitude are derived from the applied voltage. That is, the frequencies will be equal and the voltages will either be in phase or 180 de grees out of phase depending upon the position of the rotor coil 14. Also, the magnitude of the internal voltage will be a linear function of the magnitude of the applied voltage, the proportions being selectably adjustable by means of the control knob 30.

This internal voltage being in series with the condenser 43 will add to or subtract from the external voltage as applied to the condenser. This changes the effective capacitance of the condenser side of the tuned circuit, and, consequently, changes the resonant frequency of the circuit 41. As the magnitude of the applied voltage is increased or decreased, the internal voltage varies in direct proportion thereby maintaining a fixed apparent capacitance and a fixed resonant frequency. The ratio between the magnitudes of the internal and external voltages can be selectively adjusted by control of the position of the rotor coil 14 whereby the effective or apparent capacity and the resonant frequency are similarly adjusted.

The internal voltage may be placed in series with the inductance coil 42 instead of the condenser 43. The internal voltage will then vary the effective inductance of the coil and, consequently, the resonant frequency of the circuit.

In Fig. l, a cathode follower is employed and certain advantages thereof have geen discussed. In the circuit of Fig. 2, a negative feedback amplifier is employed to obtain linear amplification. Either of these amplifiers for energizing the primary or exciting coil 16 of the rotary transformer may be used in either type of circuit. Other forms of amplifiers may be used so long as the amplification factor is constant and the input impedance is high so as to effectively isolate the voltage coil 14 from the source of the voltage at terminals 11 and 12.

While inductance coil 42, condenser 43, and coil 14 are shown connected as a parallel resonant circuit, With voltage applied across the circuit, it will be understood that the applied voltage in coil 42 and condenser 43 may be arranged in a series circuit. Moreover, it is pointed out that while inductance 42 and condenser 43 of Fig. 2 are disclosed as forming a circuit whose resonant frequency may be varied by means of changing the magnitude of an impedance, it is equally applicable to producing a variation in the phase angle of an impedance.

The resonant circuit shown in Fig. 2 is readily adapted to use as the frequency determining portion of a variable frequency oscillator and is so shown in Fig. 3 where, however, a third form of isolating amplifier is shown which will be described briefly.

The isolating amplifier 75 shown in Fig. 3 is well known in the art and is connected in What is commonly referred to as a cathode degeneration circuit. The resistance 76 is of a substantial value and serves to reduce the variation in potential of the grid 77 relative to the cathode 81. The degeneration thus obtained eliminates the necessity of the negative feedback circuit shown in Fig. 2, while still maintaining linear amplification.

The resonant circuit 41 is connected as shown with the internal voltage generated by rotor coil 14 being in series with the inductance coil 42, this having been suggested as an alternative arrangement in the description above of the apparatus shown in Fig. 2. The terminal 12 is connected to ground as shown while the terminal 11 is connected to grid 77 and through a resistor 61 and a coupling condenser 62 to the plate 63 of an amplifier tube 64. The plate 63 of tube 64 is connected through a resistor 65 to a source of positive potential, such as a B battery while the cathode 66 of the tube is connected through a resistor 67 to ground.

The plate 73 of the tube 75, in addition to being connected to the exciting coil 16 of the rotary transformer 15, as in Fig. 2, is also connected through a conductor 79, a condenser 68 and a resistor 69 t0 the grid 70 of the tube 64, a grid leak resistor 71 being connected as shown. The entire circuit oscillates at a frequency governed by the resonant circuit 41, the in-phase feedback voltage, neces sary to produce sustained oscillations, passing from plate 63 of amplifier 64 to the grid 77 of the tube 75 through the condenser 62 and the resistor 61.

In oscillators, it is necessary to have an amplifier with an input circuit to which a portion of the energy from the amplified output necessary to sustain the oscillations is fed in proper phase. Since the amplifier now reamplifies the fed back portion of energy and feeds back a portion thereof, there is a progressive build-up of the amplitude of the oscillations until, in a properly designed amplifier, some element of the circuit becomes non-linear of otherwise changes its manner of operation. At this point the percentage of energy feedback is decreased and further amplification ceases. In such an oscillator the amplitude of the oscillations is maintained at a desired value. If no reduction in the percent of feedback occurs, the circuit would generate sufficient current to destroy itself. In general, it is not of prime importance as to what element in the oscillator circuit becomes non-linear and it may be the amplifier tube which comes to operate on the nonlinear portion of its characteristic curve.

In the apparatus of Fig. 3, the isolating amplifier 75 used in developing the internal voltage must provide linear amplification for the reasons discussed above in connection with amplifiers 17 and 44. Accordingly, of the amplifiers the circuit of tube 64 must be the nonlinear element.

The circuit of osciilator 64 is designed, then, by proper selection of circuit constants, that is, the cathode, grid and plate resistors and the plate voltage, such that the feedback amplifier 64 operates over a range of amplification in which it becomes non-linear when oscillations of the desired amplitude are generated. At the same time, the voltage fed back by amplifier 64 to the resonant circuit 41 and the isolating amplifier 75 is limited to a value which permits linear amplification by the isolating amplifier. Hence, a fundamental characteristic of the oscillator of Fig. 3 is the combination of the linear amplifier 75 for developing the internal voltage with the nonlinear, feedback amplifier 64. Stated otherwise, the ranges of linear operation of the two amplifiers are so related that the feedback amplifier may operate over a non-linear range of amplification, thereby sustaining oscillation at a predetermined level, whereby the voltage fed to the isolating amplifier is of such value that the latter amplifier operates over a linear range.

The resonant frequency of the circuit 41 may be altered by changing the angular position of the rotor coil 14 through operation of the control knob 30. This changes the internal voltage fed to the inductance side of the resonant circuit which in turn changes the apparent inductance and the resonant frequency of the resonant circuit 41.

The output voltage of the oscillator may be tapped off at any one of several points, but is preferably taken from a fixed secondary coil 72 of the rotary transformer 15. This particular point is selected because it is a relatively low impedance, high power source, and, consequently, can supply a substantial amount of power to its terminals 73 and 74 with good voltage regulation.

The oscillator circuit, shown in Fig. 3, is made to have a variable frequency output by including therein a voltage source in series with one of the reactance elements in the resonant circuit, this voltage source producing a voltage which is derived from the voltage applied to the resonant circuit. In order that the frequency of the oscillator may remain constant, the internal voltage place in series with one of the reactance elements should vary as a linear function of the applied or feedback voltage, this being accomplished in the circuit shown in Fig. 3 through the use of the linear amplifier and a generator, i. e. the rotary transformer 15, whose output voltage is a linear function of its excitation.

In the embodiment of Fig. 1, the voltage amplification of the cathode follower amplifier normally is less than one. Accordingly, the range of impedance variation is limited. This may be visualized by noting that for any given deflection of rotor 14a and for any given rotor coil 14, the voltage which may be supplied in series with condenser 13 is limited by the amplification of the amplifier.

In the embodiment shown in Fig. 2, the voltage amplification of the amplifier may be made greater than one by a reasonable amount through varying the values of resistors 56 and 52 relative to each other, while the gain of the amplifier shown in Fig. 3 may be varied by altering the value of the resistor 76. Such gain variation permits an increase in range of impedance or resonant frequency variation. The amplifiers of Figs. 2 and 3 may be used with the circuit shown in Fig. 1. Thus with an increase in amplification of a two to one ratio, for example, the voltage available for connection in series with condenser 13, is also increased by a factor of two. Consequently, with the same deflection of coil 14, the increase or decrease in apparent impedance has been magnified by a factor of two.

In the circuit of Fig. 2, an increase in the gain of the amplifier results in an increase in the range of resonant frequency variation with given apparatus. The normal resonant frequency of circuit 41 is determined only by the inductance coil 42 and condenser 43, and positioning of coil 14 to one side or the other of its neutral position produces bucking or boosting voltages which are in series with condenser 43. With increased amplification the coil 14, in any given position, produces increased voltages and hence greater deviation from the normal resonant frequency. This is of advantage in the circuit of Fig. 3 since the frequency range of the oscillations produced is increased.

A secondary amplifier for amplifying the output of the rotor coil 14, suggested in connection with the circuit shown in Fig. 1, can also be added, if desired, to the circuit shown in Figs. 2 and 3.

The invention has been described above as applied to a simple variable impedance, as applied to a resonant circuit, and as applied to an oscillator, and its use has been suggested as a means of changing the phase angle of an impedance. Many other applications of the invention may be found which fall in the scope of the invention, the particular applications described above being ones in which the invention performs a function peculiar to its nature and produces results previously unobtainable except through the use of more complex and otherwise less satisfactory apparatus.

While particular embodiments of the invention have been shown, it will be understood, of course, that the invention is not limited thereto since many modifications may be made, and it is, therefore, contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of the invention.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

1. In an oscillator having a resonant circuit for determining the output frequency thereof, said resonant circuit including a capacitive reactance element and an inductive reactance element in parallel relationship; a

nonlinear amplifier having its output connected across said resonant circuit for sustaining oscillation therein at a predetermined level, and a linear amplifier whose output is connected in series with one of said reactance elements, the input of said linear amplifier being connected across said resonant circuit whereby the magnitude of the output voltage thereof is a linear function of the magnitude of the voltage across said resonant circuit, and the input of said nonlinear amplifier being connected to said liner amplifier whereby the output of said nonlinear amplifier is controlled by said linear amplifier.

2. In an oscillator having a resonant circuit for determining the output frequency thereof, said resonant circuit including a capacitive reactance element and an inductive reactance element in parallel relationship; a nonlinear vacuum tube amplifier having its output connected across said resonant circuit for sustaining oscillation therein at a predetermined level, and a linear vacuum tube amplifier whose output is connected in series with one of said reactance elements, the input of said linear amplifier being connected across said resonant circuit whereby the magnitude of the output voltage thereof is a linear function of the magnitude of the voltage across said resonant circuit, and the input of said nonlinear amplifier being connected to said linear amplifier whereby the output of said nonlinear amplifier is controlled by said linear amplifier.

3. In an oscillator having a resonant circuit for determining the output frequency thereof, said resonant circuit including a capacitive reactance element and an inductive reactance element in parallel relationship; a nonlinear amplifier having its output connected across said resonant circuit for sustaining oscillation in said resonant circuit at a predetermined level, a voltage ratio device having its output connected in series with one of said reactance elements, said voltage ratio device having a linear relationship of input voltage to output voltage and being adjustable to vary said ratio, and a linear amplifier, the output of said linear amplifier being connected to the input of said voltage ratio device and to the input of said nonlinear amplifier, the input of said linear amplifier being connected across said resonant circuit.

4. In an oscillator having a resonant circuit for determining the output frequency thereof, said resonant circuit including a capacitive reactance element and an inductive reactance element in parallel relationship; a nonlinear amplifier having its output connected across said resonant circuit for sustaining oscillation in said resonant circuit at a predetermined level, a rotary transformer having its output coil connected in series with one of said reactance elements, said rotary transformer having a linear relationship of input voltage to output voltage and being adjustable to vary said ratio, and a linear amplifier, the output of said linear amplifier being connected to the input coil of said rotary transformer and to the input of said nonlinear amplifier, the input of said linear amplifier being connected across said resonant circuit.

5. An impedance device comprising a fixed impedance element, a voltage ratio device having its output arranged in series with said fixed impedance element, said voltage ratio device having a linear relationship of output voltage to input voltage, and an amplifier circuit including an electron tube, the input of said voltage ratio device being arranged in the cathode circuit of said amplifier whereby the magnitude of the output voltage of said amplifier is substantially a linear function of the magnitude of the input voltage, the input of said amplifier being arranged across said series arrangement of said fixed impedance element and the output of said voltage ratio device.

6. An impedance device comprising a fixed impedance element, a voltage ratio device having its output arranged in series with said fixed impedance element, said voltage ratio device having a linear relationship of output voltage to input voltage, and an amplifier circuit including an electron tube, the input of said voltage ratio device being arranged in the plate circuit of said amplifier, said amplifier including negative feedback means of such character as to maintain a linear relationship between the magnitude of the voltage output and the voltage input of said amplifier as the magnitude of the voltage input varies over a substantial range, the input of said amplifier being connected across said series arrangement of said fixed impedance element and the output of said voltage ratio device.

7. An impedance device comprising a fixed impedance element, a variable voltage ratio device having its output arranged in series with said fixed impedance element and having a negligible impedance relative to said fixed impedance, said voltage ratio device having a substantially linear relationship of output voltage to input voltage, an amplifier circuit, the input of said voltage ratio device comprising the output of said amplifier circuit, and means in said amplifier circuit to maintain the magnitude of the output voltage of said amplifier substantially as a linear function of the magnitude of the input voltage, the input of said amplifier being arranged across said series arrangement of said fixed impedance element and the output of said voltage ratio device.

8. Apparatus for presenting a resonant circuit of adjustable resonant frequency to an external alternating voltage, said apparatus comprising an inductive reactance element, a capacitive reactance element, a variable voltage ratio device having its output arranged in series with one of said reactance elements, said output having a negligible impedance relative to said reactance elements, said output and said last-named reactance element being arranged in parallel with the other of said reactance elements, an amplifier circuit, and means in said amplifier References Cited in the file of this patent UNITED STATES PATENTS 1,113,149 Armstrong Oct. 6, 1914 1,783,557 Bethenod Dec. 2, 1930 1,779,382 Mathes Oct. 21, 1930 1,946,047 Van der Pol et al Dec. 6, 1934 1,997,407 Hofer Apr. 9, 1935 2,071,564 McNicolson Feb. 23, 1937 2,140,339 Travis Dec. 13, 1938 2,155,404 Craft Apr. 25, 1939 2,220,770 Mayer Nov. 5, 1940 2,379,689 Crosby July 3, 1945 2,406,125 Ziegler Aug. 20, 1946 2,459,842 Royden Jan. 25, 1949

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