US2356248A - Oscillator arrangement - Google Patents

Description

Aug. 22,. 1944.

H. W. KOREN OSCILLATOR ARRANGEMENT Filed May 31, 1941 3 Sheets-Sheet 2 LOAD INVENTOR. Hez'man PM Karen 3 Mull QM ATTORNEY Aug. 22, 1944. H. w. KOREN I OSCILLATOR ARRANGEMENT 3 She tQs-Sheet 3 Filed May 31, 1941 Patented Aug. 22, 1944 OSCILLATOR ARRANGEMENT Hciman W. Koren, Bronx-N. Y., assignor to Sonotone Corporation, Elmsford, N. Y., a corporation of New York Application May 31, 1941, Serial No. 395,927

24 Claims.

This application is a continuation-in-part of my application Serial No. 349,045, filed August 1, 1940.

This invention relates to electric oscillation generators of the electronic type. Among the objects of the invention is a novel electric oscillation generator arrangement without resonant circuits in which an electron tube amplifier is combined with a regenerative feed-back circuit formed solely of resistor and condenser elements for producing sinusoidal oscillations continuously variable over a large part of the frequency range, without resort to resonant circuit inductances; and improved methods and arrangements for controlling and operation such oscillation generator.

The foregoing and other objects of the invention will be best understood from the following description of exemplifications thereof, reference being had to the'accompanying drawings wherein Fig. 1 illustrates diagrammatically an electric oscillation generator exemplifying theinvention;

Fig. 2 is a diagram illustrating certain modifications of an oscillation generator of Fig. 1;

Figs. 3 and 4 are schematic diagrammatic views illustrating oscillation generators, such as shown in Fig. 1, using modified forms of feed-back generators;

Fig. 5 is a diagram showing the circuit connections and thecooperative relationship of the elements of an audiometer utilizing an oscillation generator of the type shown in Fig. 1 and exemplifying certain other features of the invention;

Fig. 6 is a horizontal sectional View of a gear mechanism used in an oscillator control arrangement of the invention; and

Fig. '7 is a sectional View along line !-1 of Fig. 6.

All prior. audiometers used, as a source of audio-frequency oscillations, either oscillation generators operating with, a resonant circuit formed of magnetic core inductances and condensers Or beat frequency oscillators in which the desired audio-frequency was produced by heterodyning radio frequency oscillations of different frequencies produced by two radio frequency oscillation generators.

In general, electron tube amplifiers generate oscillations of a given frequency when the amplified output is fed back to the input side of the amplifier if the input voltage is approximately 180 out of phase with the voltage in the output circuit, and the amplifier gain is sufficient to cover the losses in the oscillator circuits.

One type of audio-frequency oscillator which was used in prior audiometers employed a resonant circuit formed of a magnetic core inductance and a condenser to fix the frequency at which the voltage fed back to the grid of the oscillator tube had the phase and the magnitude required to generate sustained oscillations of the given frequency. However, since it is difiicult to provide satisfactory magnetic core inductances required for such low frequency oscillation generators, such audio-frequency oscillators capable ofdelivering substantially pure tone oscillations continuously variable over a substantial part ofthe audio-frequency range, particularly in the low audio-frequency range, require elaborate switching arrangements and are cumbersome.

To overcome these difficulties, the prior audiometers designed for operation with a continuously variable frequency range used beat frequency oscillation generators. With such beat frequency oscillation generators, it is possible to continuously vary the generated oscillations through the entire audio-frequency range by keep- 1 ing the frequency generated by one of the radio frequency oscillators fixed and varying the frequency of the other radio frequency oscillator through the audio-frequency range, the latter being by a turn of a single dial associated with a variable condenser'forming part of the resonant circuit of such oscillator. However, the design of such beat frequency oscillators involves many problems and they have various disadvantages.

The frequency stability of the output of suchv Since audiometers should enable a person without technical skill to make accurate measurements of hearing, such beat frequency oscillators have obvious disadvantages for use in audiometers. As a result, audiometers using oscillators with fixed frequencysteps, although lacking in the advantage of a continuously variable testing frequency, have heretofore found wider use than variation of the oscillation frequency by variable condensers over much greater frequency range than heretofore considered possible.

In Nichols Patent 1,442,781, filed July '7, 1921, Van der Pol Patent 2,024,389, filed January 14, 1933, and in an article by Lattmann and Salinger in E. N. T. (Elektrische Nachrichten Technik), vol. 13, pages 133-142, published April, 1936, are described electron-amplifier tube oscillation generators in which the phase controlling feed back circuit is formed without resonant circuits and only of resistance and condenser elements which are so proportioned and arranged that for a given setting of its parameters, one and only one frequency is shifted in phase by the proper amount and fed back to the input grid of the amplifier in phase with its voltage at a gain sufficient to produce sustained oscillations, and that for all other frequencies the voltage fed back to the input grid is sufficiently out of phase and too small to sustain oscillations.

The E. N. T. article, in particular, gives the results of extensive work on such oscillators. shows than in oscillation generators operating with one tube amplifier, the output voltage of which is 180 out of phase relatively to its input Voltage, the two section feed back network formed of resistors and condensers must be designed so that it gives an overall phase shift only at the desired frequency to assure that the feed back voltage impressed on the output side of the amplifier is in phase with the input voltage of the tube, at the desired oscillation frequency, and that in oscillation generators using two amplifier tubes each of which gives a phase shift of 180 or a zero overall phase shift, the two section feed back network formed of resistors and condensers must be designed so that it gives an overall zero phase shift only at the desired frequency to assure that the feed back voltage impressed on the first tube is in phase with the input voltage of this tube at the desired oscillation frequency, the article giving the equations for the design of the feed back networks. The article also shows that the frequency at which such feed back network delivers an input voltage which is in phase with the input of the desired oscillations is given by an equation in which the resistor and condenser elements of the feed back network appear as positive parameters.

It has also long been realized that the most convenient way of varying the frequency of such oscillation generators is to leave the resistances of the feed back network fixed and to vary its oscillation frequency determining characteristics by using as feed back condensers commercially available gang-tuning condensers the capacity of which may be varied in theratio of about lto 20. In order to make possible the use of commercially practical sizes of such Variable condensers in oscillation generators designed for the low frequency range, such as the MMiO-il'? quency range, high resistances of the order of a megohm have to be used for the resistor-condenser feed back network. This in turn brings into effect the stray capacity of the wiring and the other elements associated with the feed back network, and increases the effective capacity which determines the frequency characteristics of such network thereby materially limiting the range over which the frequency maybe varied by such variable condensers to a fraction of the theoretically possible range, namely, to the ratio of about 1 to 10 in the audio-frequency range, for instance.

As a result, the continuously variable range of prior oscillator generators of such type was limited to relatively small steps, and different feed back networks and switches had to be switched .into the oscillation circuit for generating oscillations in the different steps of the desired frequency range.

, According to the invention, the foregoing difficulties heretofore encountered with such oscillation generators are eliminated by combining with the feed back network of such oscillation generator negatively acting capacitive means which are so proportioned and correlated to the other elements of the feed back network as to reduce or suppress the positive capacity effect of the elements of the feed back network in determining its frequency characteristics.

By using feed back networks, the frequency characteristics of which are determined in accordance with the invention by positively as well as negatively effective capacitive elements,

it is possible, not only to suppress the effect of the stray capacities which heretofore limited the range over which the frequency of such oscillators could be continuously varied, but also to increase the range over which the frequency ,2 can be continuously varied with a single set of. variable condenser or resistor elements beyond the range heretofore considered even theoretically possible.

In Fig. 1 is shown diagrammatically an oscillation generator arrangement exemplifying one practical form of the invention as developed for use in an audiometer. The oscillations are produced by electron amplifier tube units 2|, 22 and a feed back network 23 interconnected between theirinput and output sides, and the generated oscillations are impressed by the two push-pull connected amplifier tube units 22, 24 on the two push-pullconnected amplifier tube units 26, 21 forming the power amplifier stage which delivers the oscillations to an output transformer 29, the secondarywinding of which is connected to a load.

In the form shown, the amplifier is designed to be energized from a conventional power supply system comprising, for instance, a rectifier 3| and a filter network formed of choke coils resistors and condensers interconnected so as to deliver from an A. C'. or D. C. -volt domestic supply line, indicated by the plug 32, a filtered positive plate or anode voltage at the terminal +33 to the power amplified stage, and at the,

terminal +34 to the preceding amplifier stages. The cathodes of the several tube units are indirectly heated by heater filaments not shown, which are likewise supplied through plug 32.

'The first oscillation amplifier tube unit 2| operates as a high gain amplifier and is shown in the form of a pentode having its anode connected through a coupling resistance 41 and a current adjusting resistor 42 to the positive supply terminal +34, its screen grid being maintained at the required positive operating potential through a resistance 43, interconnected between the resistances 4|, 42 and its cathode being connected through a self-biasing resistor 44 to the low potential or grounded side of the amplifier indicated by a lead G, suitably connected bypass condensers completing the circuits.

The output of the first amplifier tube is impressed through a blocking condenser 46 on the input grid of tube unit 22 which is connected through a grid resistor 4'! to ground. The tube unit 22 operates as a phase invertor and is shown in the form of a triode which impresses on its output coupling resistor 48 a voltage which is in phase with the voltage impressed on the input grid of the first amplifier tube 2|.

The voltage developed across the coupling resistance 48 of the triode 22 is'impressed through a blocking condenser and a feed back lead 52 including a serially connected resistance-53 upon the entrance terminals of a feed back network, the outgoing terminals of which are connected between the input grid and the low potential or grounded side of the first amplifier tube 2|.

The feed back network is formed of a first network section consisting of a series resistance RI and a shunt condenser C4 and a second network section consisting of a series condenser C2 and a shunt resistance R2.

An adjustable-tap resistor 54 is connected across the entrance terminals of each feed back network to form a voltage divider, enabling accurate adjustment of the component of the output voltage impressed on the entrance terminals of the feed back network.

As pointed out above, it was long known that if two resistance coupled amplifier tubes of sufiicient gain are interconnected with such resistance condenser feed back network which applies to the input grid of the firstamplifier tube "a sufiicient' voltage of correct phase, sustained oscillations will be generated by such arrangement at the frequency F0 given by:

1 avenue, If the resistances and the condensers of the two network sections are made equal to R and C, respectively, this equation is reduced to:

1 21am .4 In such arrangement, with the first tube 2| first tube unit 2|, each section of the feed back network 23 will produce only at the frequency F0 given by Equation 1 or 2 an overall phase shift zero, and feed back to the input grid of the first tube 2| a voltage which is in phase with ltsgrid input voltage of tube 2|, and bring about the generation of oscillations. of the desired frequency if. the amplifier has an' overall gain sufficient to cover the losses in thefeed back network 23, and in the other circuit elements associated with the oscillating circuit. g Since the losses in a feed backnetwork defined by Equation 2 givea voltage attenuation oi -3 only, an amplifier operatingwith a relatively small gain will be sufiicient to generate the desired oscillations. By making the condensers Cl, C2 of the feed back network variable and controlling the overall gain of the two tubes 2|, 22 so that at each setting of the variable con densers the overall gain of the two tubes 2|, 22 is just sufllcient to cover the losses, it would thus be theoretically possible to vary the frequencies of the sustained oscillations over the range determined by the variable capacity range of the two condensers Cl, C2 of the feed back network 23. 1

However, as pointed out above, when using commercially available variable condensers of a practical size for such oscillation generators designed for the low frequency range, the feed back resistances of the feed back network must be made of the order of a megohm or more. Because of the large magnitude of the feed back resistances RI, R2, the stray capacity of the circuit wiring and the associated elements of the oscillator circuits modifies the operating characteristics of the feed back network.

Thus in the case shown in Fig. 1, the stray capacities associated with the feed back network 24 may be represented by a condenser C3 connected in shunt to the resistor R2 of the second feed back network section. If such stray capacity 03 is taken into consideration, the frequency at which the feed back network 24 will produce oscillations will be defined not by Equation 2 but by the following equation:

Equation 3 shows that the efiect of the stray capacities increases the effective capacity of the condensers forming part of the feed back network 24. Since the minimum capacity of commercially available variable condensers is about of its maximum capacity, the effect of the stray capacity C3 as indicated by Equation 3 raises the value of the minimum capacity that could be obtained with a given variable condenser. I

As a result, the range over which the frequency of such oscillation generator may be varied by commercially available condensers of a. practical size is limited to much smaller value. Thus, in the audio-frequency range from 20 to 20,000 cycles, for instance, the maximum range over which the frequency could be continuously varied with a practicalvariable condenser is practically limited to only about 1 to 10. Accordingly, switching means would have to be provided for switching in threev different sets of resistors into the feed back circuit 24 of the oscillator of Fig. 1 in order to cover the range from 20 to 20,000 cycles. a

The invention is based on the discovery that the operating characteristics of resistance condenser feed back networks used in such oscillation generators can be modified in away heretofore censidered impossible .and that-the operating range cver which the frequency of such oscillations could be continuously varied may be radically increased by combining with such feed back networks capacitive means which oppose the positive capacitive effect of the other condenser elements of the feed back circuit in determining its frequency and phaseshifting characteristics. Thus in Fig. l, the condensers ct'co'nnected in shunt to the resistance RI of the firstfeed back network's'elction was found to have an opposing or negative capacitive eifect in relation to the capacitive effects of the -condensers C1, C2 as well as the stray capac'ityCii in determining the frequency characteristics of this feed back net work 23. A mathematical analysis confirms this experimental discovery and shows that when a condenser C4 is connected in shunt to the resistor RI of the feed back network in the way illustrated in Fig. 1, the equations for the frequency at which the so modified feed back network 23 will give an overall phase shift zero are: r

in the absence of a stray capacity and if a stray capacity C3 is effective in shunt to resistor R2 of the feed back network.

It can be shown that the attenuation produced by a feed back network 23, in which a stray capacity C3 is effective in shunt to the feed back resistor R2, rises above 3 as the capacity of the condensers CI, C2 of the feed back network 23 is reduced to raise the frequency and that if the negatively acting capacity Cl is made as large as the positively acting capacities of the feed back condensers Cl, C2 and the stray capacity, the attenuation will rise to 5 as the capacity of the variable condensers is decreased to the minimum value.

The combination of a negatively acting capacity with such resistor condenser feed back network makes it thus possible not only to completely suppress the effect of the stray capacity C3 in limiting the frequency range of the feed back network, but also to reduce to zero the minimum capacity eifective in the feed back network, or to reduce to zero the effect of the capacityof the variable condensers of the network in their minimum capacity position. Theoretically, it would thus be possible to vary the frequency range obtainable with commercially available condenser up to an infinitely high value.

A practical oscillator actually constructed in the way indicated in Fig. 1 with a feed back network 23 using variable condensers C|, C2 having a maximum capacity of 730 X farad. and requiring three different sets of feed back resistors interchangeably switched into the feed back circuits for covering in three steps the ranges from 12 to 200 cycles, from 200 to 2,000 cycles, and from 2,000 to 20,000 cycles, on being modified in accordance with the invention by combining with it a negatively acting condenser C4 of 100 X 10* farad. connected in shunt to the feed back resistor RI, and using the same set of resistors RI, R2, enables an increase of the frequency range over which the frequency was continuously variable by 100 times with only a single set of such feed back resistors.

In the form of the oscillator arrangement shown in Fig. 1, the triode 22 which serves as a phase invertor element in the oscillation generating circuit is utilized also to cooperate with the associated triode 24 as a push-pull exciting stage for driving the push-pull amplifier stage with its power amplifier tubes 26, 28. The cathodes of the two triodes'22, 24 are connected in their push-pull circuit through balancing selfblasing resistors, and their two anodes are connected in the push-pull circuit across two coupling resistors 44, the high potential points of presses a corresponding rectified voltage..on a

which are. connected through similar blocking condensers 5| to the high potential points of the push-pull connected grid resistors 56 of the power amplifier tubes 26, 28, the cathodes of which are likewise connected in their push-pull circuit through self-biasing resistors.

A tap on the grid'input resistor of the power amplifier tube 26 supplies the exciting voltage for the push-pull inverter triode 24 and enables accurate adjustment of the exciting voltage so that it will be equal, and 180 out of phase relative to the voltage across the input grid of the triode 22.

To assure frequency stability and a pure wave form that is free from distortion, the oscillation generating amplifier is controlled in any known way by automatically controlling its gain so as to operate the oscillation amplifier as a class A amplifier, restricting the oscillations to the linear part of the tube characteristics and by applying a suitable amount of negative feed back.

In the arrangement of Fig. 1, the self-biasing resistor 44 has no shunting by-pass condenser and is connected in series with the cathode of the oscillation amplifier tube 2| in such relation to the resistor 4| of its anode circuit 'as to apply to its grid a negative feed back voltage sufficient to reduce distortion, but insufficient to stop oscillations. The cathode resistor 44 may beused to automatically control the gain of the amplifier tube by making it in the form of an incandescent lamp filament arranged to operate at a temperature at which its resistance rises with the current so as to automatically vary the grid bias of the amplifier tube 2| and maintain its operation in the linear part of its characteristic at a gain which is just sufiicient to supply the losses in the feed back network and the other parts of the oscillator circuit and produce the desired sustained oscillations.

As an aid in practicing the invention and without in any was intended to restrict thereby its scope, there are given below the design data of a feed back oscillation generator of the type shown in Fig. 1 which proved successful in actual operation:

"Tube 2| is a 6SK7 pentode, and the two triodes 22, 24 are a 6C8 tube. Feed back network resistors RI, R2 are of two megohms each. The feed back condensers CI, C2 are commercially available gang tuning condensers of 730 micromicrofarad. The negatively acting shunt condenser C4 has 100,000 micro-microfarads. When usin'gsuch feed back network without the negatively acting shunting condenser C4, the variable condensers Cl, C2 could vary the frequency sinusoidal oscillations only over the range between about cycles and 1000 cycles. Upon connecting the negatively acting condenser C4 across the resistor RI, the frequency range of sinusoidal oscillations produced by adjusting the same variable condensers Cl, C2 may be varied over a hundred times greater range and even more.

Various arrangements other than that shown in Fig. 1 may be utilized for automatically controlling the gain of the amplifier oscillation generator to assure that it operates as a class .A amplifier on the linear part of its characteristics over the entire operating range. Thus, as shown in Fig. 2, the oscillatory voltage developedacross the anodes of the power amplifier tubes 26, 28

is impressed through two leads including blocking condensers BI, and current limiting resistors 62 on a two phase rectifier 63 which in turn imrectifier-load resistance 64 connected between the low potential lead of theamplifier, indicated by a ground sign, and an adjustable midtap on a balancing resistor 65 connected betweenthe rectifier anodes. An adjustable tap on the rectifier load resistor 64 is connected through an additional voltage dropping resistor 68 to the low potential end of the feed back network resistor R2 so as to apply therethrough to the input grid of the amplifier tube 2| a bias voltage which keeps its operation in .the linear part of its characteristic and at a gain sufficient to generate sustained oscillations. The effectiveness of the feed back network resistor R2 in performing phase controlling functions is not affected by this ar rangement because its low potential end is connected to the low potential or ground lead G of the amplifier by a condenser 6! which is large enough so as to have a substantially negligible impedance at the frequencyof the generated oscillations and to hold the low potential end of resistor R2 at the low alternating potential of the.

lead G. The large capacity of the by-pass condenser 61 and of the by-pass condenser 68 connected across the rectifier load resistor 64 serve also in association with it and with resistor 66 as a filter which absorbs the oscillation ripples of the rectified oscillations and delivers a ripplefree biasing voltage to the grid of. the amplifie tube 2|. v

Various other prior-art oscillator generating arrangements using resistor-condenser feed back networks, without inductances, for producing sustained sinusoidal oscillations, may have their feed back circuits combined with negativelyacting capacitive elements for improving their operation characteristics and enlarging their frequency range.

Thus, as shown in the oscillation arrangement of Fig. 3 the amplifier A impresses a component of its output voltage on a feed back network H composed of a first network section formedof a series condenser Cl and a shunt resistor RI, and a second network section composed of a series resistor R2 and a shunt condenser C2, the outlet terminals of the second network section being connected to the input side of the amplifier A. If the resistances and condensers 0f the feed back network H are made equal to R and C, respectively, and the amplifier A is designed, to deliver an output voltage that is in phase with its input voltage and to operate at a gain suflicient to cover the losses in the feed back network and in the other elements of its operating circuits in? terconnected with the feed back network, it will produce sustained oscillations of a frequency given by Equation 2, as in the case of Fig. 1.

However, the frequency range of the oscillator arrangement shown in Fig. 3, when designedfor operation at low frequencies, will be limited by the effect of stray capacities which may be represented as a condenser C3 connected parallelto the resistor R1 of feed back network TI, and the frequency at which it will ive an overall phase shift required for-producing oscillations will be determined by Equation 3 instead of Equation 2, because the effect of the stray capacities increases the positive capacity effect of the condensers cl, C2, forming part of the feed back network 'H. By connecting a negatively acting condenser C4; in parallel to the feed back network resistor R2, and making it equal to the stray capacities C3, the effect of the stray capacities C3 may be suppressed. By further increasing the magnitude of the negatively acting condenser C4, the effect of the positively acting condensers Cl, C2 in determining the frequency characteristics and the range over which the frequency of, the oscillation generator may be varied, may be greatly reduced, and the frequency range greatly increased in a way analogous with that described in connection'with Fig. 1.

If, as shown in Fig. 4 under similar conditions an amplifier A impresses a component of its output voltage on a feed back network 72 composed of a first network section'having a series resistor RI and a series condenser Cl, and a second network section having a shunt condenser C2 and a shunt resistor R2, the outlet terminals of the second network section being connected to the input side of the amplifier A, amplifier A will produce sustained oscillations ofa given frequency accordingto Equation 2 as in the case of Figs. 1 and 3. Y Similarly, the effect of the stray capacities may be represented by a condenser C3 connected parallel to the resistor R2 of feed backnetwork l2, and the frequency at which it will give an overall phase shift required for producing oscillations will be determined not by Equation 2, but by Equation 3, because the eiiect of the stray capacities increases the positive capacity effect of the condensers forming part of the feed back network 12. t 7

By connecting a negatively acting condenser C4 in shunt to the feed back network resistor RI, and making it equal to the stray capacities C3, the equation of the frequency at which the so modified feed back network 12 will give an overall phase shift Zero and produce sustained oscillations becomes:

being similar to Equation 5 and showing that in a way similar to Figs. 1, 2 and 3 such negatively acting condenser element may be used not only tosuppress the effect of the stray capacities, but also to reduce the effect of the positively acting condenser. elements Cl, C2 in determining the frequency characteristics of the network and the range over which the frequency of an oscillation generator using such feed back network may be varied by the variable. condenser elements CI, C2 of; the feed back network.

Although the principles of the invention underlying the oscillation arrangements described above have many other applications, they have particular value for work in the low frequency field. One application of such oscillator, in the form of a novel audiometer arrangement in connection with which it was developed, will now be described.

Fig. 5 is a diagrammatic illustration of an audiometer utilizing the'oscillator arrangement of the invention described in connection with Figs. 1 to 4 and embodying various other inventive features pointed out hereinafter. Although an oscillator arrangement of the invention of the type described in connection'with Figs. 1 to 4 using only a single feed back network with resistors and variable condensers is sufficient for producing oscillations. continuously variable'over the entire audio-frequency range important for practical auqiometers, there are various applications in which it is desirable to extend the range of the frequency obtainable with a single feed back network of such character and accordingly, the audiometer arrangement is shown provided with two feedback networks which are. automatically switched intothe oscillation amplifier circuit as the frequency is varied from one part of the frequency range to the other.

In the audiometer of Fig. 5, the amplifier tube units 2|, 22, 24, 26, 28 are connected in the way shown in Fig. 2 to produce oscillations when eitherthe feed back network LF or the feed back network HF is connected by the two relays 8|, 82

between the output side of the amplifier triode 22 and the input side of the amplifier tube 2|.

The feed back'networks LF and HF are composed of resistor-and condenser elements RI, Cl, R2, C2 as in the oscillator arrangement of Fig. 2. The'p'arameters of the feed back network LF are chosen to produce oscillations variable over a lower part of the frequency range and the parameters of the feed back network HF are chosen to produce oscillations variable over a higher part-of the frequency range. The component of the feed back voltage applied to the input side of each feed backnetwork is adjusted by a variable tap resistor 54 and linear operation of the oscillation generating amplifier over the frequency range is assured by the automatic control of the bias of the grid of the amplifier tube 2| through a rectifier arrangement including-a rectifier 63, such as shown in Fig. 2'.

The relays 8|, 82 are of the direct current type and have their coils connected in series between ground and a positive terminal 35 which is connested through a filter network including an inductance, a current limiting resistor, and a condenser-to the rectifier-3| of the power supply sys-' tem. When the relay 8| is energized, as in the position shown in Fig. 5, its two sets of down-- wardly flexed contacts connect the input grid of the amplifier tube 2| and the lead from the adjustable tap of the rectifier resistor-65 to the terminals of the'resist'or R2 forming the outgoing terminals of the feed back network LF, the corresponding terminals of the other feed back network HF being connected to the tube circuits in the released normally biased position of the contacts of relay 8|.

Whenthe other relay 82 is deenergized, as in the condition shown in Fig. 5, its upper releasedcontactscomplete the feedback connection between resistor 53. and theentrance terminal of thefeed back resistor RI of the feed back network LF, the corresponding terminal of the other feed back network HF being. similarly intercom nected when the relay 82 is energized. Intheposition shown in Fig. 5, the relay 82 is held deenergized .by completing at the closed contacts of an auxiliary switch 82 a short circuit around its actuating coil, the switch 83 being so arranged that when it opens and restores the energization of the relay 82, the latter completes at its lower flexed contacts a short circuit for the coil of the other relay 8|, thereby releasing its two sets of contacts to their normally biased unfiexed position.

In other, words, when the auxiliary switch 83 is closed and completes its circuit, the energizing relay 8| and therdeenergizing relay 82 connect thefeed back network LP in the oscillator circuit, and when the switch 83 is opened, thereby energizing relay 82 and deenergizing relay 8 the two relays disconnect the feed back network LF and connect instead the feed back network HF in the oscillator circuit.

The frequency of the generated oscillations is continuously varied over a large continuously variable frequency range made possible by the use of several of such phase-shifting or controlling networks LF, H F by a frequency se lector croon- In the form shown diagrammatically in Fig. 5,

the frequency selector 9| has a driving gear 92 which, in the position shown, drives a condenser gear 93 mounted on a common shaft 94 with the rotors of the variable condensers C|, C2 of the feed back network LF, so that movement of the frequency selector 9| to the left actuates the condensers C C2 to continuously increase their capacity thereby raising the frequency of the generated oscillations.

The length of the selector gear 92 is 30 proportioned that when, in the form shown, its leftward motion brings the condensers CI, C2 of the feed back network L) to their position of minimum capacitive coupling, the left end of the selector gear 92-is brought into engagement with a similar condenser driving gear 95 which rotates through a common shaft 94 the variable condensers Cl, C2 of the feed back network HF so as to vary them in a similar way in the course of the further motion of the frequency selector 9| while the right end of its gear 92 leaves it driving engagement with the condenser driving gear 93 the feed back network LF.

The frequency selector 9| is also provided with two condenser locking cam sections 96, 91 adjoining the ends of its driving gear 92 and arranged so that locking cam section 96 maintains 1 locking engagement with a locking projection 98 of condenser gear 95 of the network HP in order to keep its variable condensers Cl, C2 locked in the same operating position as long'as the selector gear 92 rotates the variable condensers C|, C2 of the other network L) and that the frequency selector gear 92 in the course of its leftward motion reaches the position in which it disengages from the condenser gear 93 of the network LF and engages the condenser gear 95 of the other network HF the locking projection 98 of its condenser gear 95 is released by locking cam section 96 of the frequency selector 9|, and its otherlocking cam section 9'! establishes a similar looking engagement with a similar locking projection 98 of the condenser gear 93 of the network LF in order to keep its variable condensers CI, C2 locked in the same operating positions as long as selector gear 92 rotates the variable condensers Cl, C2 of the feed back network HF.

In other words, the frequency selector 9| is so arranged that during a short transition portion of its motion when it breaks the driving connection to the variable condensers of one feed back network and establishes its driving connection with the variable condensers of the adjacent feed back network, it remains in driving engagement with the variable condensers of both feed back networks, thus providing a period of transition during which the frequency controllingelementsated to disconnect one feed back network from the oscillator circuit at the moment during when the two feed back networks, are in-a condition at which they generate oscillations of the same frequency. I

In the form shown, suchautomatic action, is obtained by utilizing the condenser gear 93 to keep thetcontacts of the control switch 83 of the relays SI, 82 closed as long as the condensergear 93 is being rotated, and to open the contacts of the switch 83 only at the point when, during the transition period, the contacts of the switch 83 are brought into engagement with an interrupter member 99 operated by condenser gear 93. I

In the practical construction of such control arrangement, the condenser gear% is made in the form of a metal disc, of brass, for instancv and the contacts of the. switch 83 are biased to press on the opposite sides of the gear so that its circuit is kept closed as long as the contact of switch 83 rides on the metallic surfaces of the condenser gear 93. The interrupter member 99 of the gear 93 is made in the form of an insulating insert extending through the wall of thegea-r so that when the front end of the insulatinginterrupter insert 99, in the course of its anticlockwise rotation starts to enter between the contacts of the control switch 83 and opens the switch circuit thereby actuating the relays 8|, 32 to disconnect the feed back network LF and connect the feed back network HF into the oscillator cir' cuit.

This switching occurs at the moment of the transition period While the frequency selector gear 92 is in driving engagement with condenser gears 93, 95 of both feed back networks LF, HF, the driving engagementof the frequency selector 9| with the condenser gear 93 being broken when the points of the switch contacts 93 reach an intermediate portion of the insulating interrupter insert 99, this beingthe position when the'ccndenser locking section 9'! establishes its locking engagement with the locking projection 98 of condenser gear 93, asindicated by their dotted line positions 98',99,respectively. I

In' other words, in the frequency selector arrangement described above, the frequency se-' lector 9| with its various elements are so .arranged that as the frequency selector 9| is moved fromthe position shown toward the left, it will continue to rotate the condenser gear 93' of the frequency selector LF until, at the'point when its condensers Cl, C2 are 'aboutto reach their position of minimum capacity, the driving gear 92 of the frequency selector also establishes 'drivi-ng'engagement with the condenser gear 95 of the feed back network HE; and that during the transition period of the further movement of the frequencyselector 9| toward the left, while both-condenser gears 93, 95 are rotated, a point is reached at which the relay con trol switch 83 is opened'at the moment when both feed back networks arei'n a condition in which they will generate the same frequency,

I the switching operation of the relays connecting atthat moment the feed backnetwork HF in lieu of feed back network LF into the oscillator circuit; and that when, atthe endof the transitionperiod of the motion of thejfrequency' selector 9| its gear releases the condenser gear 93 of the network LF and looks it by its locking cam 91, the insulating interrupter insert'99 of thegear 98 will hold the relaycontrolswitch 83 open and keep" the feed back network HF connected-in the oscillator circuit,""'as long"a'sI In order to assure that the frequency gen-v erated by the two feed back networks LF, HF at the moment of transition is identical, small trimming condensers or trimming resistors may combine with the feed back networks. The moment at which the relay control switch 83 is actuated from the closed to the open position for cutting out one feed back network and interconnecting the other into the oscillator circuit may be adjusted by an adjustable mounting of the switch 83, so that it may be shifted in the direction indicated by the arrow 83 and locked in the adjusted position, corresponding to the position when the frequency generated by the two feed back networks are identical. The adjustable resistors 54 connected to the entrance terminals of the two feed back networks make it possible to adjust their operation to assure that at the moment of transition from one feed back network to the other, not only the frequency of the generated oscillations shall be identical, but that the oscillations impressed on the output transformer shall be ofthe same intensity. r 1

By an arrangement of the type described above, a simple motion imparted to the frequency selector 9|, from the low frequency end of its scale to the high frequency end of its scale as indicated on its index will actuate the oscillator circuits to generate the oscillations varying over the large frequency range made possible by the use of two or more feed back networks covering successive parts of the desired frequency spectrum which are automatically switched into the oscillating circuit as the frequency is varied.

One structural form of an oscillator control mechanism of the type described in connection with Fig. 5 is shown in Figs. 6 and '7. As shown in Figs. 6 and 7, a frequency selector or control, shaft I36, carrying at its inner end the frequency selector plate 9|, is rotatably mounted in a mounting wall I31, located behind a front panel I38 of the oscillator arrangement, and has secured to its front end'a frequency control operating knob FC. A dial with the frequency scale FS is amxed to the frequency selector shaft I36 behind a window FSW of the front panel|38 through which a portion of the frequency scale is exposed, and a strip I39 of transparent material bearing the frequency index PI being placed immediately behind the window for indicating on the scale PS the different frequency settings.

The rotary condensers CI, C2 of the feed back networks LF, HF are mounted together with the other elements of the feed back networks in a shielded casing I40 of metal supported within a.

suitable housing, in the space extending behind the mounting wall |3l, the shaft 94 of the rotary condensers of the low frequency network LF car. rying on its outer end the gear 93 and shaft 94 of the rotary condensers of the feed back network HF 95.

carrying on its outer end the condenser gear.

in a way analogous to that described in connec-' tion with Fig. 5.

In the form shown, the driving gear disc 9| and the two condenser gears 93, 95 are made of standard gear blanks having the same wall thickness. The teeth of the condenser gears 93, 95 are slightly offset relatively to the teeth of the driving gear segment 92 of the frequency selector disc 9|, and the portion of its periphery which has no teeth has secured thereto a locking plate segment Ml arranged so that its peripheral edge is aligned in overlapping relation to the teeth of the two condenser gears 93, 95 and serves as the locking cam sections 96, 91 for interlockingly engaging one of the two condenser gears 93, 95 while the other is rotated by the driving gear 92, except for the duration of the short transition period during which the driving gear segment 92 engages and drives both condenser gears 93, 95 in a way analogous to that explained in connection with Fig. 5.

The side wall portions of the condenser gears 93, 95 which are aligned in the plane of the looking cam sections 98, 91 of the locking segment |4| are provided with shallow depressions I42 arranged so that a tooth portion bordering one peripheral edge of the depression serves as the locking projection 98 which becomes engaged by the periphery of the locking segment I4! whenone of the condenser gears 93, 95 is locked.

In the form shown in Figs. 6 and '7, the clockwise rotation of the frequency selector disc 9| imparts through its driving gear segment 92 a counterclockwise motion to the condenser gear 93 of the condenser rotor plates of the feedback network LF rotating them counterclockwise fromtheir inward to their outward position while the peripheral edge of the locking plate |4i engages the overlapping tooth projection 98 of the gear 95 and holds it locked so that the condenser rotor plates of the high frequency feed back network HF are kept locked in their inward position in which the condensers have maximum capacity coupling.

Since standard commercially available rotary condensers are usually designed so that when their rotary plates are in the inward position, they can be rotated only in outward direction, the rotary condenser plates of the feed back network HF will remain locked in their inward position a slong as the locking projection 990i the condenser gear 95 is engaged by the locking segment |4| of the frequency selector disc 9i and is prevented from turning in outward counterclockwise direction.

Shortly before the counterclockwise rotation of condenser gear 93 brings its rotary condenser plates to the end of their outward path, the looking projection 98 of the gear 95 is released from its locking engagement with the peripheral locking segment portion 97 of the frequency selector 9| and the adjoining teeth of the driving gear segment 92 come into engagement with the teeth of the gear segment 95 so that during the short transition period, until the condenser rotor plates driven by gear 93 reach the end of their outward path, the rotary condenser plates of both feed back networks will be rotated by the driving gear 92 ofthe frequency selector disc 9|.

At the end of the transition period, in the course of the rotation of the frequency selector disc 9|, its peripheral locking segment portion '96 comes into engagement with the locking teeth projection 98 of the condensergear 93, and looks it and the rotary condenser plates of the low frequency network LF against counterclockwise rotation, a stop projection I43 mounted in the path of a stop pin I44 extending, for instance, from the hub of the condenser gear 93 preventing counter-clockwise rotation thereof. As a result, during the further clockwise rotation of the frequency selector disc 9 the condenser rotor plates of the low frequency feed back network LF remain locked in their outward position, while the frequency selector gear 92 rotates through condenser gear 95 the rotary condenser plates of the high frequency network from their inward toward their outward position.

In an analogous way, counterclockwise rotation of the frequency selector 9| will return first the condenser rotor plates of the high frequency network HF that are connected to the condenser gear 95 from their outward position to their inward position and after looking them in their inward position, at the end of the transition period, the further counterclockwise rotation of the frequency selector disc 9| returns the rotary condenser plates of the low frequency network from their outward to their inward position.

As shown in Fig. 7, the condenser gear 93 of the low frequency feed back network is provided with the insulating insert 99 for controlling the operation of a spring contact switch 83 which operates the relays in the way described in con-' nection with Fig. 5.

The rotational movement of the driving gear 92 is limited by a stop pin I36 extending from its body and engaging in its end positions either the right or the left side of a stop I41 suitably mounted in the path of the stop pin M6.

The principles of the invention explained in connection with specific exemplifications thereof will suggest to those skilled in the art many other applications and modifications of the same. It is accordingly desired that the appended claims be construed broadly, and that they shall not be limited to the specific details shown and described in connection with exemplifications thereof.

Iclaim:

1. In a control arrangement for a frequency discriminating circuit, including at least two frequency discriminating variable impedance units each of which has a movable impedance control element and is designed for selective operation as part of said circuit: control means for increasing the range over which the characteristic frequency of said frequency discriminating circuit is continuously variable, comprising a control member movable over a predetermined range to establish a driving connection with one control element for moving it over its range when the control member is moved over one part of its range, and to establish a driving connection with the other control element for moving it over its range when the control member is moved overanother part of its range so as to vary the impedance of one variable impedance unit while the control 'member is moved over one part of its range and to vary the impedance of the other variable impedance unit while the control memher is moved over the other part of its range;

said variable impedance units and the cooperating parts of said control member and said control elements being so designed and correlated that both of said control elements are actuated to conjointly move while said control member is moved over a limited intermediate transition part of its range and that in one position of said control member along the transition part of its range the impedance of said two variable impedance units has values which give said frequency discriminating circuit substantially the same frequency characteristic; said control means including locking means operative to maintain the control element of one of saidimpedance units locked and prevent the variation of its impedance when said control member moves over a part of its range for varying the impedance of the other variable impedance unit.

2. In a control arangement for a frequency discriminating circuit, including at least two frequency discriminating variable impedance units each of which has a movable impedance control element and is designed for selective operation as part of said circuit: control means for increasing the range over which the characteristic frequency of said frequency discriminating circuit is continuously variable, comprising a control member movable over a predetermined range to establish a driving connection with one control element for moving it over its range when the control member is moved over one part of its range, and to establish a driving connection with the other control element for moving it over its range when the control member is moved over another part of its range so as to vary the impedance of one variable impedance unit while the control member is moved over one part of its range and to vary the impedance of the other variable impedance unit while the control member is moved over the other part of its range; said variable impedance units and the cooperating parts of said control member and said control elements being so designed and correlated that both of said control elements are actuated to conjointly move while said control member is moved over a limited intermediate transition part of its range and that in one position of said control member along the transition part of its range the impedance of said two variable impedance units has values which give said frequency discriminating circuit substantially the same frequency characteristic; said control means including locking means operative to maintain the control element of one of said impedance units locked and prevent the variation of its impedance when said control member moves over a part of its range for varying the impedance of the other variable impedance unit; said locking means being operated to permit actuation of both of said control elements when said control member is moved over a transition part of its range.

3. In a control arrangement for a frequency discriminating circuit, including at least two frequency discriminating variable impedance units each of which has a movable impedance control element and is designed for selective operation as part of said circuit: control means for increasing the range over which the characteristic frequency of said frequency discriminating circuit is continuously variable, comprising a control member movable over a predetermined range to establish a driving connection with one control element for moving it over its range when the control member is moved over one part of its range, and to establish a driving connection with the other control element for moving it over its range when the control member is moved over another part of its range so as to vary the impedance of one variable impedance unit while the control member is moved over one part of its range and to vary the impedance of the other variable impedance unit while the control member is moved over the other part of its range; said variable impedance units and the cooperating parts of said control member and said control elements being so designed and correlated that both of said control elements are actuated to conjointly move while said control member is moved over a limited intermediate transition part of its range and that in one position of said control member along the transition part of its range the impedance of said two variable impedance units has values which give said frequency discriminating circuit substantially the same frequency characteristic; said control means includin switch means actuated to selectively interconnect one of said variable impede ance units in said circuit and to disconnect the other of said variable impedance units from said circuit in response to themotion of the control member over a portion of said transition part of its range.

4. In a control arrangement for a frequency discriminating circuit, including at least two frequency discriminating variableimpedance units each of which has a movable impedance control element and is designed for selective operation as part of said circuit: control means forincreasing the range over which the characteristic frequency of said frequency discriminating circuit is continuously variable, comprising a control member movable over a predetermined range to establish a driving connection with one control element for moving it over its range when the control member is moved over one part of its range, and to establish a driving connection with the other control element for moving it over its range when the control member is moved over another part of its range so as to vary the impedance of one variable impedance unit while the control member is moved over one part of its range and to vary the impedance of-the other variable impedance unit while the control member is moved over the other part of its range; said variable impedance units and the cooperat ing partsof said control member and said control elements being so designed and correlated that both of said control elements are actuated to conjointly move while said control member is moved over a limited intermediate transition part of its range and that in one position of said control member along the transition part of its range the impedance of said two variable impedance units has values which give said frequency discriminating circuit substantially the same frequency characteristic; said control means including locking means operative to maintain the control element of one of said impedance units locked and prevent the variation of its impedance when said control member movesover a part of its range for varying the impedance of the other variable impedance unit; said locking means being operated to permit actuation of both of said control elements when said control member is moved over a transition part of its range; said control means including switch means actuated to selectively intercon nect one of said variable impedance units in said circuit and to disconnect the other of said variable impedance units from said circuit in response to the motion of the control member over a portion of said transition part of its range.

5. In a control arrangement for a frequency discriminating circuit, including at least two frequency discriminating variable impedance units each of which has a movable impedance control element and is designed for selective operation as part of said circuit: control means for increasing the range over which the characteristic frequency of said frequency discriminating circuit is continuously variable, comprising a control member movable over a predetermined range to establish a driving connection with one control element for moving it over its range when the control member is moved over one part of its range, and to establish a driving connection with the other control element for moving it over its range when the control member is moved over another part of its range so as tovary the impedance of one variable impedance unit while the control member is moved over one part of its range and to vary the impedance of the other variable impedance unit while the control member is moved over the other part of its range; said variabl impedance units and the cooperating parts of said control member and said control elements being so designed and correlated that both of said control elements are actuated to conjointly move while said control member is moved over a limited intermediate transition part of its range and that in one position of said control memberalong the transition part of its range theimpedance of said two variable impedance units has values which give said frequency discriminating circuit substantially the same frequency characteristic.

6. A control arrangement as defined by claim 5, characterized by the. fact that the control means include a locking element movable to one position in which one of said impedance control elements is locked and'to another position in which the other of said impedance control elements is locked; said control member and said locking element being so interconnectediand so designed and correlated that when said control member is moved over a part of its range for varying the impedance of one variable impedance unit said locking element is actuated to maintain the control element of the other variable impedance unit locked and prevent variation'of its impedance. a

'7. A control arrangement as defined by claim 5, characterized by the fact that the control means include a locking element movable to one posi- 1 tion in which one of said impedance control elements is locked and to another position in which the other of said impedance control elements is locked; said control member and said locking element being so interconnected and so designed and correlated that when said control member is moved over the transition part of its range said locking element is maintained in a position in which both control elements will be actuated to move and that when said control member is moved over a part of itsrange beyond its transition part for varying the impedance of one variable impedance unit said locking element is actuated to maintain the control element of the other variable impedance unit locked and prevent 'variation of its impedance.

8. A control arrangement as defined by claim 5, characterized by the fact that the control means include switch means actuated in response to the movement of said control memb r Ov theportion of said transition part of its range for selectively interconnecting one and disconnecting the other of said variable impedance units in said circuit.

9. A cont ol arrangement as defined by claim 5, characterized by the fact that the control means include a locking element movable to one position in which one of said impedance control elements is locked and to another position in which the other of said impedance control elements is locked; said control member and said locking element being so interconnected and so designed and correlated that when said control member is moved over a part of its range for varying the impedance of one variable impedance unit said locking element is actuated to maintain the control element of the other variable impedance unit locked and prevent variation of its impedance; and switch means actuated in response to the movement of said control member over the portion of said transition part of its range for interconnecting one of said variable impedance units in said circuit.

10. A control arrangement as defined by claim 5, characterized by the fact that the control means include a locking element movable to one position in which one of said impedance control elements is locked and to another positio'n in which the other of said impedance control elements is locked; said control member and said locking element being sointerconnected and so designed and correlated that when said control member is moved over the transition part of its range said locking element is maintained in a position in which both control elements will be actuated to move and that when said control member is moved over a part of its range beyond its transition part for varying the impedance of one variable impedance unit said locking element is actuated to maintain the control element of the other variable impedance unit locked and prevent variation of its impedance; and switch means actuated in response to the movement of said control member over the portion of said transition part of its range for interconnecting one of. said variable impedance units in said circuit.

11. In an electric oscillation generator: an electric amplifier having an input side, an output side and a frequency discriminating feed back circuit interconnected between said output side and said input side for applying to said input side a voltage which is in phase with one phase of the voltage at said input side at a predetermined 0 frequency so as to generate oscillations of said frequency; said frequencydiscriminating circuit including only resistance and capacity impedance units for determining the frequency characteristics of said circuit; each of at least two ofsaid impedance units being of the same kind and being a variable impedance unit having an impedance control element movable overa predetermined range for varying its impedance over a variable range thereof; each of said variable impedance units being designed for selective operation as a part of said circuit; control means for varying the frequency of the generated oscillations including means for increasing the range over which the characteristic frequency of said frequency discriminating circuit is continuously variable, comprising a control member movable over a predetermined range to establish a driving connection with one control element for moving it over its range when the control member is moved over said control elements being so designed, and correlated that both of said control elements are actuated to com'ointly move'while said control member is moved over a limited intermediate transition part of its range and that in one position of said control member along the transition part of its range the impedance of said two variable impedance units has values which give said frequency discriminating circuit substantially th same frequency characteristic.

12. An electric oscillation generator as defined ,by claim 11, characterized by the fact that the control means include a locking element movable to one position in which one of said impedance control elements is locked and to another position in which the other of said impedance 'control elements is locked; said control member and said locking element being so interconnected and so designed and correlated that when said control member is moved over a part of its range for varying the impedance of one variable impedance unit said locking element is actuated to maintain the control element of the other variable and prevent variation of impedance unit locked its impedance.

13. An electric oscillation generator as defined by claim 11, characterized by the fact that the control means include a locking element movable to one position in which one of said impedance control elements is locked and to another position in which the other of said impedancecontrol elements is locked; said control member and said locking element being so interconnected and, so designed and correlated that when said control member is moved over the transition part of its range said locking element is maintained in a position in which both control elements will be actuated to moveand that when said control member is moved over a part of its range beyond its transition part for varying the impedance of one variable impedance unit said locking element is actuated to maintain the control element of the other variable impedance unit locked and prevent variation of its impedance.

14. An electric oscillation generator as defined by claim 11, characterized by the fact; that the control means include switch means actuated in response to the movement of said control member over the portion of said transition part of its range for selectively interconnecting one and disconnecting the other of said variable impedance units in said circuit.

15. An electric oscillation generator as defined by claim 11, characterized by the fact that the control means include a locking element movable to one position in which one of said impedance control elements islocked and to another position in which the other of said impedance control elements is locked; said control member and said locking element being so interconnected and so designed and correlated that when said control member is moved over a part of its range for varying the impedance of one variable impedance unit said locking element is actuated to maintain the control'elemem; of the other variable impedance unit locked and prevent variation of its impedance; and switch means actuated in response to the movement of said control member over the portion of said transition part of its range for interconnecting one of said variable'impedance units in said circuit.

' 16. An electric oscillation generator as defined by claim 11, characterized by the fact that the control means include a locking element movable toione position in which one of said impedance control elements is locked and to another position in which the other of said impedance control elements is locked; said control member and said locking element being so interconnected and so designed and correlated that when said control member is moved over the transition part of its range said locking element is maintained in a position in which both control elements will be actuated to move and that when said control member is moved over a part of its range beyond its transition part for varying the impedance of one variable impedance unit said locking element is actuated to maintain the control element of the other variable impedance unit locked and prevent variation of its impedance; and switch means actuated in response to the movement of said control member over the portion of said transition part of its range for interconnecting one of said variable impedance units in sai circuit. v 17.-In combination, an electric amplifier having an input side and an output side and a feed .back circuit interconnected between said outputside-and said input side for applying to said input side a voltage which is in phase with the phase of the voltage at said input side at a predetermined frequency, saidfeed back circuit including only resistance and capacity elements for determining the frequency characteristics of said-feed back circuit and at least one capacity element so connected parallel to a resistance element of said ,feed back circuit as to have an effect opposite to said first mentioned capacity elements in determining the frequency characteristics of said feed back circuit.

18. In combination, an electric amplifier having an input side and an output side and a feed back circuit interconnected between said output side and said input side for applying to said input side a voltage which is in phase with the phase of the voltage at said input side at a predetermined frequency, said feed back circuit including resistance elements and at least two capacity elements for determining the frequency characteristics of said feed back circuit, at least one of said capacitive elements being connected parallel to a resistance element of said feed back circuit so as to have an effect opposite to that of at least one other of said capacitive elements in determining the frequency characteristics of said feed back circuit.

19. In a generator of electric oscillations, an

electric amplifier having an input side and an output side and a feed back circuit interconnected between said output side and said input side for applying to said input side a voltage which is in phase with one phase of the voltage at said input side at a predetermined frequency so as to generate oscillations of said frequency, said feed back circuit including only resistance and capacity elements for determining the frequency characteristics of said feed back circuit and at least one capacity element so connected parallel to a resistance element of said feed back circuit as to-have an effect opposite to said first mentioned capacity elements in determining the frequency characteristics of said feed back circuit.

20. In a generator of electric oscillations, an electric amplifier having an input side and an output side and a feed back circuit interconnected between said output side and said input side' for applying to said input side a voltage which is in phase with the phase of the voltage at said input side at a predetermined'frequency so as to generate oscillations of said frequency, said feed back circuit including resist- "ance elements and at least two capacity elements for determining the frequency characteristics of said feed back circuit, at least one of said capacitive elements being connected parallel to a resistance element of said feed back circuit so as to have an effect opposite to that of at least one other of said capacitive elements in determining the frequency characteristics of said feed back circuit.

21. In combination; an electric amplifier having an input side and an output side and a feedback circuit interconnected between said output side and said input side for applying to said input side a voltage which is in phase with the phase of the voltage at saidinput side at a predetermined frequency; said amplifier including at least two electron-tube amplifier stages; said feed back circuit including only resistance and capacity elements for determining the frequency characteristics of said feed back circuit and at least one capacity element so connected parallel to a resistance element of said feed back circuit as to have an effect opposite to said first mentioned capacity elements in determining the frequency characteristics of said feed back circuit.

cuit including resistance elements and at least two capacity elements for determining the frequency characteristics of said feed back circuit; at least one of said capacitive elements being connected parallel to a resistance element of said feed back circuit so as to have an effect opposite to that of at least one other of said capacitive elements in determining the frequency characteristics of said feed back circuit.

23. 'In a generator of electric oscillations: an electric amplifier having an'input side and output'side and a feed back circuit interconnect between said output side and said input side i r applying-to said input side a voltage which is phase with one phase 'of the voltage at said input side at a predetermined frequency so as to generate oscillations of said frequency; said amplifier including at least two electron-tube amplifier stages; said feed back circuit includingonly resistance and capacity elements for determining the frequency characteristics of said feed back circuit and at least one capacity element so connected parallel to a resistance element of said feed back circuit as to have an effect opposite to said first mentioned capacity elements in determining thefrequency characteristics of said feed back circuit.

24. -In a generator of electric oscillations: an electric amplifier having an input side and an output side and a feed back circuit interconnected between said output side and said input side for applying to said input side a voltage which is in phase with the phase of the voltage at said input side at a predetermined frequency so as to generate oscillations of said frequency; said amplifier including at least two electron tube amplifier stages; said feed back circuit including resistance elements and at leasttwo capacity elements for determining the frequency characteristics of said feed back circuit; at least one of said capacitive elements being connected parallel to a resistance element of said feed back circuit so as to have an effect opposite to that of at least one other of said capacitive elements in determining the frequency characteristics of said feed back circuit.

HEIMAN W. KOREN.

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