US2469803A - Compensated, controlled-feedback, amplitude modulated oscillator - Google Patents

Description

P. WEATHERS COMPENSATED CONTROLLED-FEEDBACK AMPLITUDE MODULATED OSCILLATOR May 10, 1949.

Filed Aug. 22, 1947 w w W M a w w W W 0 w my m m w 4 mm fl m 2 y W am 2 M u #7 T a 1 l4 .1 M Q l I m. a m w w 4 F 1, m J a .4. a .w m

i .W mw mm .0 I 11 M w M Y I. Mr M .Ufl ZlfiM a m a. 1, WE", J mI J! M (I .1 a I d, mm W M J a J m g n 2 m M m m v a a v F a WWW A fi w k 0. WWW WR km 4 IN VEN TOR. PAUL .Wsxraans Patented May 10, 1949 UNITE aeca'sos rric MPENSA'EED, CONTRULLED-FEEDACK,

v.ilflhll'EE MODULA'EED @SCELLATOR llaui Weatherafiaddon Heights, N. 3., assignor to Herbert K. Neuber, Philadelphia, Pa.

Application August 22, 1947, Serial No. 710,069 1 (Elaine. (Cl. 332-48) The present invention relates to modulated oscillators of the electronic tube type as shown. described and claimed, for example, in my copending applications Serial No. 636,702, filed December 22, 1945, for Oscillators, now Patent No. 2,436,129, granted Feb. 17, 1948 and Serial No. 715,377, filed December 11, 19%, for Oscillator, now Patent No. 2,436,129, granted Feb. 17, 15348.

In the first oscillator system referred to above, a self-excited electronic tube oscillator is provided with means for effecting a variable control of oscillation strength or modulation efiect thereon, and a resultant amplified modulation signal output. Modulation is applied to the oscillator in response to extremely small changes sired modulation efi'ects nd without appreciably in capacity, inductance or resistance in a control variation.

In the second of the above applications a selfcxcited, electronic-tube oscillator system is prosystems referred to.

vided with a modulation control circuit for varystrength of the oscillation without appreciably varying the frequency of the oscillations produced, whereby a greatly increased high fidelity modulation signal output is attained.

In each of these oscillator systems differentially variable capacity, inductance or resistance de vices provide the modulation source. In preferred embodiments of these oscillator systems a phonograph pickup device as the modulation source and in the form of difierentially variable capacity actuated by movement of the pickup stylus, serves to modulate the oscillator by the difierential control of feed-back energy from the anode circuit to the grid circuit.

In any case, the modulation source may com prise a small, light-weight push-pull variable capacitor or any similarly variable impedance, and the oscillator and associated circuits may be coupled thereto through a transmission line of appreciable length, without introducing unde limiting the frequency range of response of the impedance element, or the amplitude of the con trolling effect upon the oscillator and the fidelity of the resulting signal output therefrom.

In the reproduction of records by means of a pickup device of the character referred to, tonearm resonance or other undesirable frequency characteristic may be introduced into the modulation efiect upon the oscillator system, thereby causing distortion in the reproduction or a limitation of the efiective overall gain of the oscillator in the reproduction of signals from the record at the stylus.

It is, therefore, a primary object of the present invention to provide an improved modulated oscillator circuit which effectively operates to correct for any undesired frequency characteristic which may be applied to the oscillator as a result of modulation.

The invention is not limited to the oscillator However it is particularly effective in and adapted to a system as shown, described and claimed in the second of the applications hereinbefore referred to and will be shown and described in connection with a modulated oscillator system of that type.

It is, therefore, an object of the invention to provide an improved modulated oscillator of the self-excited electronlc-tube type having difierential modulation iced-back control for varying the magnitude or strength of the oscillations at the modulation frequencyby varying the input capacity of the oscillator, and controlling the voltage gain of the oscillator in .counter-phase to the modulation by controlled negative or inverse modulation frequency feed-back, whereby a desired modulation frequency correction may be efiected to eliminate an undesired modulation characteristic.

The invention, furthermore, is not limited to the reproduction of phonograph records in connection with the modulation of an oscillator but may be applied to any modulated oscillator system, the modulation characteristic of which requires correction in one or more portions of its frequency range. I

It is, therefore, a further object of the invention to provide a modulated oscillator systemwhich may be efiective in any portion of the modulation frequency range and which maybe embodied in or form part oi. the oscillator circuit, and also which may take advantage of the gain of the oscillator tube to amplify the corrective effect upon the modulation characteristic.

It is a still further object of the invention to provide a self-excited, modulated oscillator system having an improved modulation control circuit which will permit the oscillator signal output to be adjusted to optimum value in the presence of any undesired modulation characteristic without introducing cross-modulation, through correction of the percentage of modulation in the response range in which the undesired characteristic occurs.

For example, in the reproduction of phonograph records tone arm resonance may introduce a rapid rise in the percentage of modulation at low frequencies and this may in turn produce such resultant high modulation percentage that cross-modulation of the high frequencies may occur.

It is, therefore, an object of the invention also to provide an improved modulation control system for an electronic-tube modulated oscillator which introduces degeneration into the oscillator system at a predetermined frequency or frequency range whereby the percentage modulation may be corrected in that range and in which the degree of degeneration may be established at any desired predetermined value and the resultant frequency compensation may be adjusted so that the modulation percentage in the range referred to is controlled to a degree whereby it becomes possible to increase the overall gain or sensitivity of the oscillator without appreciable cross-modulation efiect resulting from undesired modulation effects from the modulation source or the associated circuits.

The invention will, however, be better understood from the following description, when con-' sidered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing:

Figure 1 is a schematic circuit diagram of an electronic-tube modulated oscillator system embodying the invention in a present preferred form;

Figure 2 is a schematic representation of the mechanical elements of a phonograph record reproducing system with which the circuit of Figure 1 is adapted for use;

Figure 3 is a graph showing curves illustrating certain operating characteristics of the circuit of Figure 1 in accordance with the invention, and

Figure 4 is a second schematic circuit diagram showing a modification oi the circuit of Figure l as a further embodiment of the invention.

Referring to Figure 1, an electronic-tube oscillator 5 is arranged for generating self-oscillations and for receiving modulation from a source 6 which itdelivers at a highly amplified value to output terminals I and 8, for example as described in my aforesaid application.

In accordance with the invention, the oscillator grid 9 is connected through a coupling or grid capacitor ill with the high potential terminal I l of a grid circuit l2 in which is connected a variable tuning inductance I3, between the terminal and a ground connection lead it for the oscillator system. The cathode l5 of the oscillator tube 5 is also connected to the ground lead [4 for the oscillator system. The cathode l5 of the oscillator tube 5 is also connected to the ground lead i4 through a feed-back modulation frequency correction net-work indicated at I6 and comprising in the present example a feed-back impedance H in the form of a resistor, which may be variable as indicated, and a shunt capacitor l8 therefor:

A grid resistor I9 is connected directly between the grid and the cathode as shown, forming a D.-C. biasing path. The resistor l9 may have a value of several megohms, such as ten megohms for example.

The output anode of the oscillator tube 5 is connected through a variable tuning inductance 2| in series with an output coupling impedance or resistor 22 to a suitable source of positive anode potential indicated by the lead 23. The lead 23 is provided with a by-pass capacitor 24, and the anode circuit lead 25 between the inductance 2| and the resistor 22 is by-passed to ground through a by-pass capacitor 26 for the oscillator frequency.

It will be noted that the output terminal 8 is also connected to the ground lead It and that the output terminal I is connected through a coupling capacitor 21 and filter resistor 28 in series, to the anode circuit lead 25.

A variable tuning core 29 is provided for the anode circuit inductance 2| as well as a shunt tuning capacity therefor indicated at 30. The latter comprises the inter-electrodal capacities of the tube and the reflected reactance through the inductance 2| from a modulator circuit comprising a feedback inductance 3| and a transmission line having leads 32 connected at one end of the outer terminals 33 and 34 of the inductance 3| and having the opposite end connected with a pair of spaced capacitor plates 35 and 36 in the modulation source 6. The leads 32 may be extended over a considerable distance to the modulation unit 6, and are suitably shielded as indicated at 31, the shielding element being connected to ground through a lead 38. The stray capacity between the leads 32 and the shield 31 are indicated by the capacities 40 and M while the stray capacity between the leads is indicated by the capacity 42.

The feed-back inductance 3| is provided with a center tap 43 which is connected conductively through a lead Mi with the hi h potential terminal ll of the grid circuit. The latter circuit is variably tuned by means of a movable tuning core 45 associated with the grid inductance Hi, and a variable shunt capacity 46 representing the input capacity of the tube and comprising mainly the reflected grid-to-anode capacity of the tube, together with the stray capacity and the grid-tocathode capacity of the tube. This input capacity varies with the gain of the tube which is controlled by modulation to effect a variation of the strength or magnitude of the oscillations and the modulation output from the oscillator.

With the arrangement shown, the tube 5 will operate in a self-oscillating condition because of energy feed-back across the inductance 2| through grid-to-place capacity from the anode circuit to the grid circuit l2. When the tube operates, the anode current flow through the resistor 22 decreases because of an increase in negative bias on the oscillator grid 9 established by the grid current flow through the grid resistor I 9, until a steady state of oscillation is established. The oscillator is thus self-excited and tends to oscillate at a fixed frequency determined largely by the circuit constants in the anode circuit.

Modulation of the oscillator is provided by feed back from the anode circuit to the grid circuit through externalmeans comprising the feedback circuit inductance 3| which is coupled to both the grid circuit and the anode circuit and the balanced transmission line 32 and the moduaeeaeos lation source connected therewith. The tap 33 is at the inductive center of the feed-back inductance 3i which is closely coupled inductively with the anode circuit inductance 2!, and is directly coupled to the high potential grid circuit terminal it through the lead 36, whereby icedback current from the anode circuit may flow therethrough to the grid circuit.

In the modulator unit it a movable electrode or capacitor plate 38 is located between the fixed plates 35 and 3% and pivoted as indicated at 39, for oscillation as indicated, between the fixed plates, to vary the air gaps on 'eachsicle thereof inversely, thereby correspondingly varying the capacity of the fixed electrodes with respect thereto and the electronic impedance of the circuits connected with the plates 35 and 236 when as in the present example, the capacitor plate at is connected to ground M. Through the connections 32 between the plates 35 and 36 and the terminals 33 and 3d of the feed-back inductance 3!, the electrical impedance to ground from'the center tap it through each half of the inductance 3! is correspondingly inversely varied in response to movement of a stylus or other movable actuating element 50 for the plate $8.

With this arrangement, the feed-back induct ance 3! is provided with two normally balanced feed-back paths to ground or cathode which produces balanced positive and negative feed-back through feed-back inductance which may be varied differentially to modulate the oscillator. This is by reason of the fact that as the flow of energy or current with-respect .to the tap 53 through one winding portion of the inductance 3i increases, the flow through the other winding portion decreases proportionately, thereby effecting a push-pull action to increase and decrease the strength of oscillations of the oscillator 5. The resulting anode current variations represent the amplified signal from the modulation source which is derived at the modulation frequency across the impedance element 22 from the terminals 7 and B.

The foregoing modulation system may provide any suitable means at the modulation source in connection with the feed-back inductance 39 for conveying feed-back current or energy from the anode circuit through said inductance difi'erentially, to provide differential in-phase and counter-phase inductive feed-back or energy to the grid circuit thereby to vary the amplitude or strength of oscillations of the oscillator.

If the modulation source ii is subjected to the undesired modulation action in operation, as for example, when the operating element c is arranged as the stylus element in the end of a phonograph tone-arm 52 as shown in Figure 2, to which attention is nowdirected, it may have a non-uniform frequency characteristic in the low Figure 2, and are connected with the terminals 33 and 38 as shown in Figure 1.

When the stylus 50 is actuated by constant amplitude recording, for 'example, as in present phonograph records, below 500 cycles and above 2000 cycles, tone arm resonance may cause the percentage of modulation to exceed a desirable limit and result in a high percentage modulation Within the range of frequencies in'which the tone arm resonance occurs. This same result or eiicct may occur in other forms of reproducing equipment, that is, with other forms of modulating devices at the modulation source 5 and in turn may require correction in the particular frequency range in which the over-modulation or undesired frequency characteristic occurs. The modulation system of the present invention may be adapted to meet the requirements for modulation frequency correction in any of the above cases, as

will be seen from a consideration of this present per cent modulation as shown. This is a normal response curve for a phonograph pickup arrangement as shown in Figure 2 when connected with a modulated oscillator as in Figure 1, without provision of frequency response or modulation control means in accordance with the invention.

The rapid rise at the lower frequency end of i the range, particularly below 50 cycles, is the reexample, may excite the tone arm stylus to produce suchhigh modulation percentages as in dicated by'the portion of the curve ti, that cross-modulation of the high frequency portions of the reproduced signal by the low frequency modulation referred to may occur.

In accordance with the invention, effective degeneration or inverse modulation frequency feedback is provided at low frequencies, or in a desired frequency range, so that the percentage of modulation may be reduced to a value such that audio frequency range when the tone arm resonance becomes effective, tending to produce dis= no appreciable cross-modulation results. In the present example, the percentage of modulation is reduced to a value at the tone arm resonant i'requency' such that the cross-modulation is substantially zero.

The curve 62 shows the resulting output which may be effected by this means. It will be noted that the lower end of the curve 82, as indicated at 63, provides selectively low percentage modulation rise which is within a desired limit for preventing cross-modulation of the higher ire-. quencies in the signal output from the system. It will also be noted that the curve $2 is at a generally higher level, of the order of two to one, than that of the curve 52', which indicates the t the overall signal output may be increased greatly without attendant undesired over-modulation at the low frequency end of the response range, whereby .the system may efiectively be used in connection with the reproduction of phonograph records with a normal tone arm arrangement as shown, for example, in Figure 2.

Referring again to the circuit diagram of Figure 1 more particularly, the oscillator 5 is maintained in a steady state of oscillation by the normal feed-back through the grid and anode capacity as hereinbefore referred to, and the modulation from the source 6 is applied thereto by inverse or differential variation of the external feed-back path provided between the anode and grid circuits through the feed-back inductance 3!, the pickup circuit 32 and the differentially variable capacity provided between the movable pickup element 48 and the fixed elements 35 and 36.

The frequency corrective network utilizes a third feed-back path and modulation frequency feed-back as distinguished from the oscillator frequency feed-back for the modulation between -the anode and the grid circuit. The negative or inverse modulation frequency feed-back is applied to the grid by the impedance or resistor 11 in the cathode circuit which is given a desired frequency response characteristic by means of the shunt capacitor H3 or other suitable means as will hereinafter be described.

It will be seen that the resistor I! is located at a point in the oscillator circuit whereby it couples the grid and anode circuits and is in effect a portion of the output impedance 22 located in the anode circuit, whereby anode current variations resulting from modulation appear across the terminals of the impedance or resistor IT. This variation is applied to the grid circuit through the inductance l3, and because of the polarity of the connections, the resulting potential variation at H is in counter-phase or degenerative with respect to the input signal or modulation appear: ing on the grid 9. This circuit arrangement operates to reduce the percentage modulation due to the modulation voltage feed-back being in such phase that the voltage gain of the oscillator is varied in counter-phase to the modulation envelope, as will be described in connection with the operation of the system.

To provide for frequency compensation or correction of the modulation effect in the present example in the tone arm resonant range, the shunt capacitor I8 is of such value that it forms an effective by-pass for the resistor I! at all of the higher frequencies, for example above 500 cycles, Since the grid resistor IQ for the oscillator is returned directly to the cathode IS, the 'bias on the grid 19 is not changed by the variation in modulation potential at the resistor i1. However, when the anode current changes, the change also occurs across the resistor I! at frequencies below that frequency at which the impedance of the capacitor I8 becomes too large appreciably to shunt the resistor H, in this case, below 500 cycles. The resistance value of the latter resistor may be made a predetermined percentage of the resistance value of the resistor 22 so that a predetermined degree of degeneration or inverse modulation frequency feed-back may be obtained, and the degree of frequency compensation may further be adjusted by variation of the resistor [1. By this arrangement the low frequency modulation percentage may be reduced to a point where it becomes possible to increase the overall sensitivity of the modulated oscillator system as shown in Figure 1, without the possibility of cross-modulation.

Suitable amplification of the reproduced signal from the modulation source 8 may be introduced between the oscillator tube 5 and the output terminals 1 and 8, and the inverse modulation frequency feed-back correction network l6 may be provided with additional degenerative or inverse feed-back from one or more amplifier stages connected with the oscillator, as shown, for example, in Figure 4, to which attention is now directed, and in which the same reference numerals as used in connection with Figure 1 are applied to like circuits and circuit elements.

In the circuit of Figure 4, the modulation source and oscillator circuit per se is the same as in Figure 1 and functions in the same manner to produce self-oscillations and a modulation signal output at the terminals 1 and 8 from the source 6. The feed-back modulation frequency correction network it in circuit with the cathode l5 comprises a resistor 65 connected between cathode and the ground lead II and shunted by a resistor or other suitable impedance 66 in series with a capacitor 61.

The oscillator output signal through the output capacitor 21 is applied to the grid 68 of an amplifier tube 69 havin cathode 10 connected through a self-bias resistor H to the ground lead 84. The self-bias resistor H is provided with the usual by-pass capacitor 12 to ground and the output anode 'I3,is connected across an output.

impedance or resistor 14 to the output terminal 1 through a coupling capacitor 15. The output terminal 8 is connected to ground as in Figure 1. Anode potential is applied to the anode 13 through a positive anode potential supply lead 16 which is by-passed to ground by a by-pass capacitor indicated at l1. Bias is applied to the grid through a resistor 18.

Additional inverse feed-back is applied across the resistor 65 in the cathode circuit of the oscillator 5 through 'an inverse or negative modulation frequency feed-back connection lead connected between the cathode end of the resistor 65 at a terminal 8| and a tap 82 on the output impedance 16, through a series coupling capacitor 83 in the lead 80. With this arrangement a portion of the signal appearing across the output impedance I4 is applied substantially in phase with the anode potential modulation frequency variations in the resistor 65 resulting from the modulation, and tends to enhance the inverse feed-back or degenerative effect upon the modulation si nals on the grid circuit, thereby further to control the output response of the modulated oscillator in a desired frequency range.

In the present example, a modulation control or corrective circuit is provided which is effective to suppress undesired frequency response in the system in a band of frequencies, for example, between 500 cycles and 2000 cycles or in any similar frequency pass band, being a modification of the corrective network arrangement of Figure 1 for other purposes. As in the previously described circuit, the modulation frequency characteristic is corrected by inverse feed-back in a circuit external to the self-excitation and modulation circui s.

In the present example, resistor' 66 and the capacitor 61 form a shunt path for the modulation frequency feed-back impedance 65 which has a controlled frequency characteristic such that below a certain frequency, such as 2000 cycles for example, the impedance of the path formed by said resistor and capacitor becomes increasingly effective to permit the inverse modulation frequency feed-back to be applied to the grid cir-f cult, thereby suppressing the signal output in a range below that frequency.

The capacitor 83, however, is of such value that at the lower limit of thesuppression range desired, such as at 500 cycles in the present example, it becomes increasingly eflective in impedance value to prevent the flow of feed-back current through the circuit 80, thereby preventing reduction of the gain of the system below that frequency and cutting off the effectiveness of the by-pass arrangement 86 and 67 for the additional feed-back from the amplifier. It is obvious that other suitable bandpass control networks may be devised for providing frequency characteristic correction in any desired band or range as may be required.

In any case, from the foregoing consideration of the circuits of Figure 1 and Figure 4, it will be seen that the feed-back modulation fre quency correction network is placed in the oathode circuit of the oscillator so that it eifectively couples the grid and anode circuits substantially independently of the difierentially controlled modulation circuit. Furthermore, with this arrangement, the inverse feed-back is amplified by the gain of the oscillator tube itself and therefore the correction eiiect is greatly enhanced over that which would be possible by use of ordinary series circuit limiting or other known means.

The operation of the system shown in Figure 1 may briefly be considered as follows:

Assumin the oscillator to be energized, feedback of energy from the anode circuit through the normal grid-to-anode capacity path maintains the oscillator in a steady state of oscillation at a frequency determined by the anode and grid circuit constants. In the present example this may be assumed to be of the order of 1445 kc. The average anode current flowing through the output coupling impedance 22 and the feed-back correction impedance il in a cathode circuit, assumes a normal and constant value, and with substantially no variation in average anode current resulting from no excitation a variation of the modulation source capacitor 35--36t8, the modulation signal output at theterminals' i and 3 is zero. Energy from the anode circuit at the oscillator frequency is prevented from appearing at the output terminals by reason of the bypass capacitor 2e and the filter 28 connected with the terminal 3.

Modulation feed-back current at the frequency of the tuned anode circuit, that is, at the normal oscillator frequency, flows by inductive coupling through the feed-back inductance 3i substantially equally on either side of the intermediate tap it. Modulation feed-back current through the inductance 3i flows in a path which may be traced from the center terminal d3 through the connection lead its? to the grid circuit inductance l3, thence through the ground connection it and the movable electrode 58, thence through the electrodes or plates 35 and 36, the cable connections 32 back to the terminals 33 and 3d of the inductance 38.

It will be seen that when the return path through the leads 3? and the modulation source 6 is balanced, that is when the impedance to ground in connection with the terminals 33 and 3d are equal, the opposing potentials induced in the grid circuit through the coupling with the feed-back inductance 38 are equal and opposite in phase and cancel, whereby the steady state of oscillation is maintained without change.

Variation of the impedance element at the l control end or the modulation circuit, 32 causes a variation in the impedance between ground and the terminals 33 and as, thereby causingan unbalance in the potentials applied to the terminal 63 and the grid circuit connected therewith difi'erentially, thereby causing a flow of iii-phase or counter-phase feed-back energy to the grid circuit, and increasing or decreasing the strength of oscillations.

This is by reason of the fact that when the feed-back of energy through the feed-back inductance Si is varied, and increase in feed back from the anode circuit in one direction will aid and in the opposite direction will oppose, the normal reed-back from the. plate to the grid, thereby varying the amplitude of oscillations and the signal output at the terminals l and d in response to modulation control of the variable impedance means at the modulation source d.

Variation in reed-back of the oscillator signal energy causes the input capacity as across the grid circuit to vary, thereby varying the frequency of response of the grid circuit with respect to the fixed frequency or the oscillator circuit. The variation of t of the grid circuit, however, has substantially no efiect upon the fre quency of the oscillator which is maintained substantially constant by the tuned anode circuit comprising the inductance 2t and the-reflected capacity 3E! as hereinbefore referred to. Variation of the of the grid circuit by variation of the input capacity only varies the magnitude or strength of the oscillations. Therefore, in this system, modulation is effected by varying the strength or magnitude of the oscillations by varying the input capacity of the oscillator tube in response to differential variations of feed-back of energy at the oscillator frequency from the anode circuit to the grid circuit, with respect to the normal feed-back for a steady state, self-excited condition of oscillation.

The anode current is correspondingly varied at the modulation frequency and provides a corresponding modulation potential variation across the output impedance element 22 and the modulation control feed-back impedance ll, the voltage across the latter impedance introduces inverse or degenerative modulation frequency feed-back in the grid circuit of the oscillator to reduce the percentage modulation applied thereto and the resulting signal output at the terminals 7 and t3 correspondingly.

Correction of the frequency characteristic by negative or inverse modulation frequency feedback takes place in the grid circuit. when applied to the network 5 5 in the cathode lead, as follows: The percentage modulation is reduced by feedbackirom the plate-to-anode circuit'to the grid circuit due to the fact that the modulation voltage is fed back in such phase relation that the voltage gain of the tube is caused to vary in counter-phase to the modulation envelope. Since the effective grid-to-anode capacity, or input capacity, appearing across the grid circuit is essentially proportional to the voltage gain of the oscillator tube, fluctuations in the voltage gain of the oscillator tube occasioned by negative modulation feed-back limits the maximum swing or variation of the efiective tuning capacity across the grid circuit, and therefore reduces the percentage modulation.

The feed-back voltage across the impedance ll is modified to apply a desired correction to the modulation amplitude thereby to compensate for any undesired frequency characteri tic resulting from the modulation at the source 8; In the present example, as hereinbefore referred to, the capacitor It forms an effective bypass for the inverse feed-back modulation frequency potentials above a certain low frequency range so that the degenerative eifect of the correction network becomes effective only below, for example, 500 cycles, and increasingly eflective in the range of the tone arm resonance frequency, thereby preventing over-modulation andpermitting an overall increase in the signal output by adjustment of the system to optimum amplification, as indicatedby a comparison of the curves 6i and 62 in Figure 3.

In the circuit of Figure 4, additional gain in the modulation signal output'is provided at the terminals 1 and 8 by introduction of the amplifier stage associated with the amplifier tube 69, and additional corrective inverse feed-back is obtained from the amplifier stage output circuit by reason of the feed-back connection between the tap 82 and the cathode end of the oscillator cathode impedance 65. By proper relation of the impedance elements 83, 66 and 61 a band-pass effect for frequency response correction may be applied. Furthermore, by making the tap 82 adjustable as indicated, the degree of feed-back from the amplifier stage may be controlled and likewise the impedance 65 may be made adjustable as shown in connection with the similar impedance IT in the cathode circuit of Figure 1.

In any case, the feed-back modulation frequency correction network is applied to the oscillator system at the cathode circuit to provide for amplification of the correction eflect by reason of the gain in the oscillator tube itself.

The present invention is, therefore, of importance in connection with the reproduction of phonograph records and the like, through the usual tone arm and pickup arrangement, as it will be seen that it may be made highly effective to control the modulation within any desired frequency range, thereby to impart to the output signal a desired frequency characteristic devoid of any undesired modulation effects by reason of operation of the modulation source and its associated elements, such as tone arm resonance, for example.

Furthermore, while the invention has been shown and described in connection with a present preferred form of modulated oscillator, and is particularly efiective in connection therewith, it may be applied to any modulation control system for a self-excited electronic-tube oscillator provided with means for efiecting a variable control of oscillation strength or modulation effect by inverse variation of feed-back.

What is claimed as new and useful is:

In a self-excited electronic-tube oscillator system, the combination of means for tuning said system to a predetermined constant frequency of oscillation, modulator means for difierentially varying feed-back of energy at the oscillator frequency to modulate said system in a predetermined modulation frequency range, means for applying thereto a controlled negative modulation frequency feed-back for limiting the efiect of variation of said oscillator frequency feed-back to control the percentage modulation in a predetermined portion of said modulation frequency range, said last named means including an oscillator cathode circuit and an impedance network in said circuit, amplifier means for the modulated signal output of said system having an output impedance, and a negative modulation frequency feed-back connection from said last named impedance to said network for further controlling the percentage modulation of said system in a predetermined portion of said modulation frequency range.

PAUL WEATHERS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,101,688 Rechnitzer Dec. 7, 1937 2,371,373 Badmaiefl' Mar. 13, 1945 2,412,023 Woll Dec. 3, 1946

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