NW. 12, 1946. c. MQS|NNETT 2,411,008‘ TRANSDUCER SYSTEM ' Fiil>ed Nov; 28, 1944 CAPACZMNCE AMPLIHER 05'C/LLA TOE’ 7D REZWFIER 26 I ‘ii/570° CAPACITANCE _ AMPLIFIER j . M OSC/L LA 70/? "“ REC7/F/ER came/72m: INVEN TOR. +8 CHESTER M. SINNETT _ v BY A TTOkA/EV 2,411,008 Patented Nov. 12, 1946 UNITED STATES PATENT OFFICE 2,411,008 TRANSDUCER SYSTEM ‘ Chester Sinnett, Westmont, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application November 28, 1944, Serial No. 565,462 7 Claims. ( Cl. 179_100.4) 1 lation of high frequency oscillations, and more particularly to improved frequency modulated (FM) oscillation systems for deriving from re actance variations into electrical currents repre sentative of such variations. ganizations whereby my invention may be car ried into effect. In my application Serial No. 459,375, ?led September 23, 1942, I have disclosed systems for translating physical displacements of the mobile 10 electrode of a capacitative reactance into‘ FM oscillations, a simple form of discriminator-recti 2 itself, however, as to both its organization and method of operation will ‘best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit or My present invention relates generally to trans ducer systems of the type employing angle modu ~ In the drawing: Fig. 1 shows an embodiment of the invention; Fig. 1a, showsddeally a resonance curve of :the discriminator section of the detector; Fig. 2 shows a simpli?ed modi?cation wherein the FM detector electrodes are embodied in the ?er being employed to derive ‘from the FM oscilla capacitance ampli?er tube; and tions electrical waves representativeof the afore said physical displacements. In my said appli 15 Fig. 3 shows a further simpli?cation wherein a special discriminator circuit is eliminated. cation a small condenser was employed whose Referring now to the accompanying drawing, mobile electrode was stylus-actuated, and the wherein like reference characters in the differ condenser was connected directly across the oscil ent ?gures designate similar circuit elements, the 'lator ‘tank circuit. The capacity variation across the tank circuit is limited by the normally small 20 embodiment .of Fig. ‘1' is shown applied to a con. denser or capacityv pickup device. However, there maximum capacitance value of the pickup con is no restriction to that form of transducer, since denser. the latter. may be a condenser microphone. Ad It is one of the primary objects of my present ditionally, the reactive input element may be any invention to provide a method of amplifying the pickup capacitance variations prior to utilization 25 form of reactance provided with a mobile element for providing reactance variations representative of the latter in varying the oscillatortank-ci-rcuit of physical changes of force, space, temperature frequency. and the like. Assuming for the .purposes of my Another important object of my invention is present patent application that the input element to convert a small capacitance variation by purely the capacity pickup device of a record repro electronic means into a large similar variation, 30 is ducing system, the pickup generally comprises a and to employ the large variation :for angle ?xed electrode -.l and a spaced mobile electrode 2. modulating an oscillator. The spaced electrodes land .2 provide a con Another object of my invention is to ‘provide in denser whose capacity varies in .accordance with combination with a high frequency oscillator, a transducer of the reactance type whose variations 35 the motion of a stylus secured to the mobile elec trode 2. The pickup is schematically represented. are utilized to vary the tuned circuit frequency It is not a part of this invention, and reference of a reactance tube whose input electrodes are is made to my applicationSerial No. 414,305, ?led coupled across the oscillator. October 9, 1941, now Patent No. 2,376,456, dated .Still another object of my invention is to pro vides. variable condenser adapted to .be coupled 40 May 22, 1945, for a preferred pickup device. Displacement of electrode ,2 causes a change in across the tuned circuit of a reactance tube whose the capacitance of the pickup. Displacement of input capacitance is connected to vthe tank cir a stylus‘ laterally, as ‘when scanning record cuit of a high frequency oscillator, and the tuned grooves, results in a change of the position of circuit simultaneously acting as .a discriminator of FM oscillations. ' A more speci?c object of my invention is to provide an FM phonograph system whose con dense-r pickup unit has its capacitance variations ampli?ed by causing the 'latterto vary the tuning 45 electrode 2 with respect to ?xed-electrode >l . The ?xed electrode l is represented as the grounded-or low potential side oft-he condenser, while the mobile electrode 2 functions as the high potential side of the pickup device. Whereas in the system-of my application Serial of the resonant plate circuit of a reactance tube 50 No. 459,375 the pickup ‘device was connected di whose input capacitance is connected across the rectly-across the resonant tan-kcircuit of the high oscillator tank circuit of the system. frequency oscillator, in the ‘present system The novel features which I believe to be char capacitance changes of the pickup device are acteristic of ‘my invention are set forthwith par ticularity in the appended claims; the invention 55 ampli?ed by (means of an electronic device which 12,411,00é . 4 3 I have termed a “capacitance ampli?er.” The latter electronic device consists of an electron discharge tube 3 which is shown, by way of speci?c course F0 may be located on a suitable point of the opposite ?ank of the resonance curve. suitable direct current source through a coil 1, Those skilled in the art of FM communication are fully aware of the fact that the oscillations produced in the tank circuit I5 are of substantially constant frequency in the absence of capacitance variation of the pickup I-2. However, varia and the coil is shunted by a condenser 8. tion of the pickup capacitance causes the am example, as a triode. The tube 3 includes a cathode 4, a control grid 5 and a plate 5. ‘The plate 6 is connected to the +B terminal of a The pli?ed capacitance I4 to vary the frequency of The 10 tank circuit l5, and, of course, the frequency low potential side of tuned circuit 1-8 is re- 7 variation or deviation will correspond to the turned to ground for high frequency currents capacitance variations at the pickup I-2. Var by condenser 9. Numeral I0 denotes the cou iations in frequency of the local oscillations will pling path between the mobile electrode 2 and result in developing across circuit 25-26 high the high potential side of circuit 1-8. The 15 frequency oscillations whose amplitude will vary cathode 4 may be connected to ground through in accordance with the frequency variations oc resonant circuit 1-8 is tuned to substantially the operating normal oscillator frequency F0. a biasing resistor H which is bypassed for high frequency currents by condenser I2, and the con trol grid 5 is returned to ground through a re sistor l3 which acts as a direct current return 20 path for the control grid. .It is to be clearly understood that the coupling path In may be of any desired form. For ex ample, it may be a coaxial cable whose grounded sheath would be connected to the ?xed electrode l. Merely by way of example, and in no way restrictive upon the invention, F0 may be given a value in the megacycle range such as 30 mega cycles (mc.). The control grid 5 of tube 4 has a curring at tank circuit l5. This is the well known frequency discrimination action which has been described in my aforesaid co-pending patent ap plication. Any suitable type of recti?er may be coupled across circuit 25-26, as by coupling condenser 21, for the purpose of rectifying the high fre quency oscillations of variable amplitude. The recti?ed voltage produced during the recti?ca~ tion step corresponds to the capacitance varia tions of the pickup element l-2. The recti?er device coupled to the discriminator circuit 25-26 may be of any well known construction, as, for predetermined capacity with respect to the 30 example, a simple diode. Where the mobile elec trode 2 is varied by a stylus scanning sound grounded cathode 4, and numeral [4 denotes this grooves of a record, the voltage produced by the input capacity, or inherent interelectrode capaci recti?er coupled to circuit 25-26 represents the tance, of the input electrodes of tube 3. The audio frequency voltage corresponding to the re capacity [4 is designated in dotted lines to repre sent the fact that it is inherent, and is not an actual physical condenser across the tank cir cuit of the high frequency oscillator. The control grid 5 is connected to the high potential side of tank circuit I5 of the high frequency oscillator by corded sound. In the case where'condenser l-2 is a condenser microphone the output of the sys tem Will be the audio frequency waves corre sponding to the sound waves varying the capaci tance of condenser l-2. In order to provide an ampli?cation of the rela means of a direct current blocking condenser I6. The low potential side of . tank circuit I5 is tively small capacity changes of condenser 1-2 grounded. Tank circuit l5 consists of coil I‘! and and shunt condenser I8, and the tank circuit is normally tuned to the frequency F0. The oscillator tube is designated by numeral l9 and is shown as a pentode type tube, although my invention is not limited to that type of tube. Since the present invention is concerned mainly the tube 3 and its associated circuits are utilized. Since the resonant circuit 1-8 of tube 3 is tuned to F0, it follows that tube 3 will have a plate circuit whose plate impedance is essentially re sistive when circuit 1-8 is tuned to F0. It can be shown that in any tube, such as tube 3 of Fig. 1, whose plate impedance consists of a reso with the circuit details between the reactive input , nant circuit tuned to the frequency of the high element l-2 and the oscillator tank circuit 50 frequency voltage applied to the grid of the tube, l'l-l8, the remainder of the system comprising the input capacitance M will have its magnitude the oscillator and discriminator-recti?er will be depend upon the effective magnitude of the plate impedance of tube 3. At resonance the input capacity C1 of a vacuum tube ampli?er is given generally referred to. The oscillator tube cathode 20 is connected to a suitable point on coil [1, control grid 21 being connected by direct current blocking condenser 22 to the high potential side of tank circuit I5. A suitable grid leak resistor 23 returns grid 2| to ground. Plate 24 is connected through coil 25 to the +3 terminal of a suitable direct current source. Coil 25 is shunted by condenser 26, and the resonant circuit 25-26 is tuned to a frequency which is su?iciently different from the normal oscillator frequency F0 to provide frequency dis criminator action. In Fig. lo I have shown an ideal resonance curve of circuit 25-26. The peak frequency of the resonance curve is chosen to be Fo+f, the value of 1‘ being such that F0 falls by: where M is the ampli?cation factor of the tube; Cgk is the capacity between the grid and cathode; and Cgp is the' capacity between cathode and plate. This relation applies for a resistance load. Since at resonance a tuned circuit possesses unity power factor, the relation will hold for the cir cuit shown in Fig, l for tube 3. If now the fre quency of the resonant plate circuit of tube 3 is varied relative to F0 the effective impedance of the plate circuit varies. On either side of reso nance (F0) the value of Cl decreases, and for the speci?ed circuit is given by: at the midpoint of the most linear section of one of the ?anks of the resonance curve. If, for ex 70 ample, F0 is chosen to be 30 me, then .the dis Due to this effect the input capacity IA of tube 3 criminator circuit 25-26 will be tuned to a peak is re?ected into the oscillator tank circuit in frequency sufficiently above 30 me. so that the magni?ed form relative to the capacitance of latter value will fall at a linear point of the lower ?ank of the resonance curve shown in Fig. 1a. Of condenser l, 2. 2,5191 mos . , . 6 5 . lations into _ amplitude V modulated‘ oscillations. ‘This change in effect" of plate impedance of fl‘lii’eflatter areiappliedto‘anode 30 of‘ diode rectié tube 3v will be clearly magni?ed as a correspond; ing variation in the, magnitude of the input capacitance 14, and the‘ variation of capacitance I4 will follow the frequency variation of circuit 'l'-—-8. It is for ‘this reason that the capacity variation of capacitance M will frequency modu "late the" high frequency oscillations produced at tank ‘circuit [5; The extent of frequencydevia ?er’i’ril, AFthroi-igh condenser 45-' which has a small capacity)‘ "; - ' " ' The anode 30 is connected to ground through a series path consisting of 'choke coil 3| and» load resistor 32. Load resistor 32 is shuntedby con denser. 33 which bypassesv high frequency cur rents. ‘ ‘The recti?ed voltage across-resistor 32 is am at tank circuit l5 will'be a function of the 10 appliedv through condenser 34 to the following audio frequency network. The fact that: circuit amplitude of physical displacement of mobile electrode 2, while the rate of frequency variation 7, _8 is‘ tuned to Fo-l-f in no way interferes with ‘the function of the circuit» in ‘acting as the plate will depend upon the rate of physical displace ment. In the case of recorded sound the extent of frequency variation at tank circuit IE will de impedance 'of tube 3''. pend ‘upon the amplitude of the recorded sound, aridthe rate of deviation will depend upon the and- down» the ?ank'of‘the resonance curve of tuned circuitT, 8. This causes a change in voltage audio' frequencies per se] The generic term “anglemodulated” is'employed herein to indicate the fact that the variations of the-high ‘fre > Changes in pickup capacity modulate the high frequency oscillations, but the ampli?ed oscillator voltage also swings up is across the‘diode load resistor 32. ' a ‘quency-energy' at tank circuiti? could be phases modulated, since frequency and phase modula tiorr have characteristicsin common. In Fig. 2 there is shown a modi?cation of‘ the invention wherein the recti?er device coupled "25 across discriminator circuit 25—~2i5 has its elec trodes included within the tube envelope of the capacitance ampli?er. Additionally, the leads it’ , Tube 3’ is fundamentally a high frequency ampli?er receiving its energy from oscillator tube vl9.3Therewill appear across the tank circuit: 1, ‘8"anipli?ed energy derived from the oscillator. The input capacity of tube >3'vis,‘however, con nected‘across tank circuit l1, l3. ‘ From the above ‘relation for vC'i'the tuning o-f'thelplate circuit of tube 3’ has an effect on its input capacity. Even when circuit 1, 8 is tuned o? resonance with Fe it can be: seen fromtheabove are an unshielded pair of twisted wires. It will further be noticed that control grid 5 of tube 3' 30 formula thatv the eifectiveinput capacity becomes a variabie'or a magnitude larger than the tube’s normal‘ grid to cathode capacity. The variation in‘ frequency of the oscillator is- ampli?ed‘ by tube 3', and-with its plate-circuitoff resonance large changes in voltage occur across circuit 1.8., The capacity. variations in ‘circuit ‘l, 8 dueto the pick is connected directly to the high potential end of oscillator tank'coil i1. Adjacent a portion of the ‘cathode 4 ‘of tube 3’ there is located the diode anode 3i’! which is returned to the grounded “cathode through a path consisting of the choke coil 3! and load resistor 32, the latter resistor ~upfaresmall, but the changes in voltageare large ‘ being bypassed for; high frequency currents by since. tube 3’- ampli?es‘ ‘the oscillator‘ output condenser 33. The anode 38 is connected through direct current blocking condenser 21 to the plate _ 'end of discriminator coil 25. The tube 3’ may be a diode-triode of the 6Q7G type, although any other well known type of multiple duty tube may be employed. The ' recti?ed audio " frequency voltage across resistor 32 may be transmitted by coupling, condenser 31; toany suitable audio fre quency ampli?er network. The action described in connection with Fig. 1 applies equally well to the functions of the system of Fig. 2. Here again the inherent input capacity of tube 3’ is Varied in accordance with the variations in magnitude of condenser [-2, and the input capacitance of the triode section of tube 3' is provided across the oscillator tank circuit as described for the system of Fig. 1. In Fig. 3 I have shown a further simpli?cation of the invention. In this case discriminator cir cuit 25—26 is completely dispensed with, ‘while the resonant plate circuit of tube 3' is caused to perform frequency discrimination in addition to its function as described in Fig. 1. In this . "voltage at Foif. l ' Whi'le I. have indicated and described several ' systems for carryingimy invention-into effect, it .40 . will be apparent to one skilled in the art that'my , ‘ invention is. bylno means limited to the particular > organizations shown‘ ‘and described,»but‘ that many modi?cations-may be made: without-.de parting from theiscop'e of my, invention. : What I claim is: v r '1. In combination, an oscillator having a reso nant tank circuit tuned to a desired frequency, a reactance device having a displaceable element responsive to a physical force for varying the reactance of said device, and the improvement which comprises an electron discharge tube hav-v ' ing at least a cathode, control grid and plate, a , second resonant circuit connected beween the cathode and‘plate tuned substantially to. said de sired frequency, means electrically connecting the grid to cathode capacity of said tube across ‘ said tank circuit, means connecting the reactance 60 device across said second resonant circuit for providing frequency variations of the latter in modi?ed system the resonant plate circuit 1, 8 response to displacements of said element there of tube 3' is tuned to the peak frequency Fo+f, by to cause corresponding variations of said grid while the oscillator tank circuit ll, l8 of tube i9 to cathode capacity, and means for deriving from is tuned to F0. The variations in capacitance of condenser i, 2 cause corresponding variations 65 oscillations variable in frequency, due to said ca pacity variations, a Voltage representative of said in plate impedance of tube 3' thereby producing physical force. ' ampli?ed capacitance variations across tank cir 2. In combination with a high frequency oscil cuit IT, IS. The FM oscillations across tank cir lator producing oscillations of a predetermined cuit I1, is are applied to grid 5, and there will be developed in the plate circuit of tube 3' FM oscil 70 frequency, a condenser having a mobile electrode responsive to forces to be reproduced, a tube hav lations which are applied to circuit 1, 8. Since ing a resonant plate circuit tuned substantially circuit 7, 8 is tuned to Fo-l-J‘ and the FM oscilla to said frequency, said condenser being connected tions have a mean or center frequency F0, it ' directly across said resonant plate circuit to vary follows that circuit 1, 8 acts in the manner of a frequency discriminator to convert the FM oscil 75 the frequency thereof in accordance with move 2,411,008 7 8 mentsof the ,mobile, electrode, means responsive to corresponding variations in'the input capaci tanceof said tube for varying the frequency of resonant tank circuit tuned to a desired high fre said oscillations, and means for deriving from the frequency-variable oscillations a voltage whose variations are, representative of said electrode movements. , 3. In combination with a high frequency oscil lator producing oscillations of a predetermined frequency, a condenser having a mobile elec trode responsive to forces to be reproduced, a tube having a resonant plate circuit tuned sub quency, a reactance device having a displaceable element responsive to a physical force for vary ing the reactance of said device, an electron dis charge'tube having at least a cathode, control grid and plate, a second resonant circuit con nected between the cathode and plate tuned to a frequency differing by a small value from said desired frequency, means electrically connecting 10 the grid to cathode capacity of said tube across said tank circuit, means connecting the reactance device across said second resonant circuit for stantially to said frequency, said condenser being providing frequency variations of the latter in connected directly across said resonant plate cir response to displacements of said element thereby cult‘ to vary the frequency thereof in accordance 15 to cause corresponding variations of said grid to with movements of the mobile electrode, means cathode capacity, and recti?er means in circuit responsive to corresponding variations in the in with said second resonant circuit for deriving put capacitance of said tube for varying the fre from oscillations variable in frequency, due to quency of said oscillations, means for deriving said, capacity variations, a voltage representative from the frequency-variable oscillations a voltage 20 .ofsaid physical force, whose variations are representative of said elec 6. In combination with a high frequency oscil trode movements, said mobile electrode having a lator tube producing oscillations of a predeter record reproducing stylus coupled thereto, and mined frequency, a condenser pickup having a said deriving means including a recti?er whose mobile electrode responsive to record grooves to input circuit is said resonant plate circuit, 25 be reproduced, a tube having a resonant plate 4. In combination with a high frequency oscil circuit tuned oif resonance with said frequency, lator producing oscillations of a predetermined said pickup condenser being connected directly ‘frequency, a condenser having a mobile electrode responsive to forces to be reproduced, a tube hav across said resonant plate circuit to vary the fre quency thereof in accordance with movements of ing a resonant plate circuit tuned substantially 30 the mobile electrode, means responsive to corre to said frequency, said condenser being connected sponding variations in the input capacitance of directly across said resonant plate circuit to vary said tube for varying the frequency of said oscil the frequency thereof in accordance with move lations, a recti?er whose electrodes are included ments of the mobile electrode, means responsive in the tube envelope of said tube, and a condenser to corresponding variations in the input capaci 35 coupling said resonant plate circuit to said recti tance of said tube for varying the frequency of ?er. said oscillations, means for deriving from the fre 7. In combination with a high frequency oscil quency-variable oscillations a_voltage whose vari lator, a tube whose input capacitance is coupled ations are representative of said electrode move across a frequency determining circuit of the ments, said deriving means comprising a recti?er 40 oscillator, a resonant circuit tuned away from the whose electrodes are included in the tube en oscillator frequency by a small frequency value velope of said tube, and a frequency discrimina connected in the tube plate circuit, and a modu tor circuit coupling said) oscillator and recti?er lating reactance device connected directly across and tuned to a frequency close to said predeter said resonant circuit, mined frequency. , l . 5. In combination, an oscillator tube having a 45 CHESTER M. SINNETT.