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July 9, E46. c. A. MENELEY ‘ ¿43,540 RECIPROCAL CIRCUIT Filed April 23. 1945 2 sheets-sheet 1 Juñy , 1946. c. A. MENELEY 2,403,546 REGIPROCAL CIRCUIT Filed April 23, 1943 2. Sheets-Sheet 2 | i | i i l l L «ln»i - 5. . www1.2¢: Nm. l nu Gttorneg Patented July 9, 10946 2,403,540 UNITED STATES PATENT ro»l=i=1'c_E`--¿j ’ RECIPROCAL CIRCUIT Carl - Radio A. Corporation Meneley, Princeton, of America, N.l~:incorporation J., as'signor to off ' ' ’ I ' Delaware Application April'23, 1943,»'Se1‘ial'NoL 484,304'1' l s claims; (ci. ricami 1 My' invention relates to the production of the means ‘,ofga coding ¿disc,~thespeech signaly S- is multiplied'by the signal K,1and the signal SKf-is transmitted ,to the receiver. At theV receiver reciprocal of a signal or wave and particularly to the production of such a signal for use in a sys-f tem for secret communication. ` , « In the copending application of Alda V. Bed ford Serial No. 456,578, filed August 29, 1942, and entitled Secret communication system,l there is described a communication system in which the speech or other communication signal S is mul tiplied by a coding signal K and in which the 10 product SK is transmitted. At ithe receiver the incoming signal SK is multiplied. by the recip rocal of the coding signal K to obtain the orig inal signal S, that is, SK >< 1/K=S. According to the above Bedford application, the coding signal there is another signal generating device for pro.-r ducing ¿the signal K; it may -be a coding disclike that at the transmitter, the two coding discs‘be ingv runin synchronism. The _signal'K generated at 'the` receiver is supplied to the .reciprocal cir cuitfto »obtain the decoding _signal `1/K,-which~is then multiplied With the received signal SK to obtain'the-Qrieinal signals-f: i . 1 -, f, - ¿11; ¿The invention Lwill be better understoodfrom ther following description taken >vin connection with vthe accompanying'drawingsin which - ¿» " 15 ,Figure `1 is a vblock diagram _of signalling appar, K may be generated-by means of aîcode disc which is cut toy make-its periphery have the wave ratus ¿Figures embodying 2 to 5 are' mygraphs inventiom which ~ 'arereferredto » ' -f form of the signal K lwhile the decoding signal in explaining the» invention; '_i l __ - » ' A l/K may be produced by, a differentA discwhich . Figure 61isa circuit diagram of a reciprocal cir is cut in accordance with calculated values to 20 cuit designed inaccordance with one embodiment make its periphery have the Wave form of l/K. of the-invention, i Y» - v v- » _ In a later application of Alda V. Bedford Yand A. 'Figure 7 >is a circuit diagram ofanother recip~ Frank P. Wipñî, Serial No.-480,716, ñled March rocal circuit designed in` accordance with another . 26, 1943, and entitled Reciprocal circuits, there is embodiment ofthe invention, and . A, described one type of electrical circuit for pro ducing-the reciprocal ofa signal or wave such --Figureß 'is a` circuit‘diagramiof-a multiplier cir cuit Vthat may be `utilized in- .the circuitioffFig. ¿1; as the wave K. f y r' , . . ~In-the- several' ñgures; similar parts are indi „ An object of the present-invention isl to pro-A cated by’ similar reference characters. g f ducing the reciprocal of a signal or Wave. ‘ A further object of the invention is to provide an improved electrical circuit for converting an „ » ~ « >.i--Referring `„to Fig; 1,V the invention is shown ap-` vide an improved method of and means for pro 30 plied to radio apparatus that may-be >switchedgfoz‘nf, . operation either as a transmitter or as a receiver. Theseveral switches are shown in the'ppsi-tionior applied signal having a certain wave form into a transmitter operation, signal having the reciprocal wave form.- ' . the apparatus that `will iflrst Vbe described. «,A ' A still further object of the invention is'to pro vide an improved communication'syst’em for se cret signalling. " ‘ - vThis , is f the t operation f oi 35 microphone and a speechampliner are-showniat ' In a preferred embodiment of my invention the >reciprocal of a wave K is-obtained yby> means of f'lyand v8,-‘respectively.,` 'The .signal S is applied through -a switch.k 9 »to a multiplier unit. i4, which may be of; the samev designv as thatdescribedyin the above-identified. Bedford application or which an electrical circuit in which'the îwaveK is clipped 40 maybe of the type vshownin Fig. -8_ and described on both its positive cycle and onïits negative and `claimed ingapplication Serial No. 484,303, ñled cycle to produce a rectangular wave, and in which on the same day as the present applicationin the wave K and the rectangular'waveare added the namepf >Frank-P. Wipff and entitled Multi together with one of them' reversed in polarity, plier Fcircuits. _'The code Isignal K may :be profl preferably after the peaks‘of‘the positive >and ducedfby; means of a'code disc I6, a maskA |71, a negative cycles of the wave K have been light source'lß, a condensing lens I9, and a pho “squashed” or flattened somewhat. My recip toelectric-Y cell '2_0., ,The signalrK is, supplied rocal circuit contains nocapactive lor-ginductive -through amplifiers 2l and. 22 and through a reactances (the blocking capacitors inthe‘circuit switch to the multipler unit '14. The resulting presenting a negligible impedance» and,; there 50 multiplier-.output signal -SK is supplied through fore, functions the same for )any applied signal anv ampliñer 2B*- and through a sWitchÍZl ltoa. regardless of its frequency or waveform. e , . radio-v transmitter 28 or to a wire line, if .pre In utilizing the reciprocal lcircuit in a commu-‘ nicationy system,;a coding signalK--is generated at the transmitter in any suitable manner'fas'by ' ‘ ferred.` , - ‘ ’ ' ' ' ' ' The receiver Vfo-rfdïecocling the gsignalSK may C be the same» apparatus. as¿_rin Eig. :1.,with switches 2,403,540 3 4 9, 23 and 21 thrown to their contact positions R. The signal SK is supplied from a radio receiver 3| through the switch 9 to the multiplier I4. The of comparatively low resistance are connected in series with the diodes 62 and 63, respectively. A biasing voltage drop for opposing current ñow coding signal K produced by the disc I6 is sup plied through the amplifier 2l to a reciprocal cir cuit 32 designed in accordance with my inven thereacross, a resistor 61 being in series with the through diode 63 is produced across the resistor 66 by connecting a source of voltage (not shown) rocal circuit 32 supplies the decoding signal l/K voltage source. The diodes 62 and 63 clip the applied wave m symmetrically about its A.-C. through the switch 23 to the multiplier I4. The resulting output of multiplier I4 is the original communication signal S since SK >< l/K=S. The signal S is ampliñed by the amplifier 26 and supn across the capacitor 59 by the positive cycle pulses ilowing through the diode 63. Thus, the tion and shown in detail in Fig. 6. The recip plied through the switch 21 to headphones or to the loudspeaker 33. The code disc I6 at the transmitter and the similar code disc at the receiver are held in syn chronism and in the proper phase relation by suitable synchronizing means. For example, at the transmitter a 60G-cycle per second current from a source 36 may be supplied through a switch ,31 to a synchronous motor 38 which ro tates the code disc I6. The GOO-cycle current axis because a voltage which causes current flow through the diode 62 and resistor 64 is built up diodes 52 and 63 become conducting on alternate cycles when the signal voltage exceeds the D.-C. voltage drop across the resistors 64 and 66, re spectively. The resulting rectangular wave 1L is amplified and reversed in polarity by the tube 58. The wave n and the flattened wave m add in the portion of the anode resistor 51-51' that is common to the tubes 54 and 58 to produce the desired reciprocal wave I/K shown in Fig. 5. If the wave m is flattened correctly and if the waves m and n are added with the correct rela also modulates a radio transmitter 39 for the tive amplitudes, the resulting signal will be sub transmission of synchronizing signals to the re ceiver. At the receiver the switch 31 is in its 25 stantially a true reciprocal of the wave K. The contact position R whereby the received 600 only substantial departure from a true reciprocal signal will be where the wave K crosses the A.-C. axis. Here the reciprocal value is infinity whereas the maximum amplitude of the wave may be preferable to transmit the synchronizing 30 I/K necessarily has a deñnite limit. The .waves signal over a wire line. m and n may be mixed with the correct relative My method of obtaining the reciprocal wave amplitudes by adjusting a variable tap '1l on I /K will now be explained with reference to the cycle current is supplied from a radio receiver 4I to the synchronous motor 38. In some cases it graphs of Figs. 2 to 5. The graph of Fig. 2 represents the wave K while the graph of Fig. 5 represents substantially the reciprocal wave I/K the anode resistor 51. The correct shaping of the flattened wave m may be 'obtained byselect ing a non-linear resistor unit 53 having a suit which is the sum of the flattened wave m of able voltage-resistance characteristic and by ad reversed polarity shown in Fig. 3 and the rec tangular wave n of Fig. 4. The “squashed” or justing the value'of the resistor 52. ' As previously noted, the above-described re ciprocal circuit isV purelyresistive so that its flattened wave m may be obtained by passing the wave K through a circuit that changes its 40 operation is independent of frequency. > The in-` stantaneous voltage 'output of the circuit'A is resistance with a change in applied voltage. always the reciprocal of the instantaneous ap The rectangular wave n may be produced by clipping the positive and negative cycles of the wave m at the voltage levels e and f, respectively, for example, near the alternating-current axis of the signal and then amplifying the clipped signal. Fig. 6 shows, by way of example, one reciprocal circuit that -may be employed in practicing the invention. The wave K, having a peak-to-peak plied voltage. It follows thatîîf the reciprocal circuit is adjusted to produce the reciprocal of an appliedI signal having one wave form, the cir cuit will then always produce the lreciprocal of an applied signal regardless of itsv wave form. There are various ways of determining when the circuit has been adjusted to give substantiallya amplitude of 6() volts, for example, is applied 50 true reciprocal` ‘ One way is to connect the re ciprocal circuit into the signalling system of through a blocking capacitor 5I and a resistor Fig. 1 and, while transmitting speech or music. 52 to a copper oxide rectifier unit 53 which vfunctions as a non-linear resistor having the property of decreasing in resistance as the ap plied voltage increases. The resistor 52 is of high enough resistance so that the driving source for the non-linear resistance unit 53 is of high adjust the resistor 52 and the variable tap 1I at the receiver until the speech or music has» a minimum of distortion. . Fig. 'Z shows a reciprocal circuit that is the same as that of Fig. 6 except that a `pair of di odes 12 and 13 have been substituted for the impedance whereby there is only a slight varia copper oxide rectiñers 53a and 53h. The diodes tion in the current iiow through the unit 53. The unit 53 may consist of a pair of copper 60 ‘i2 and 13 may beproperly biased by adjusting a pair of variable taps 14 and 16 on resistors 11 oxide rectifiers 53a and 53h connected to conduct and 18, respectively. When properly biased, the current in opposite directions. two diodes will be operated along the lower knee The voltage appearing across the non-linear of their characteristic curve and in the proper unit 53 is the voltage m of Fig. 3 having the flat tened wave form. This voltage is amplified by a 65 region to shape the wave m in the desired manè ner. It will be noted that the bias applied. to cathode biased vacuum tube 54 and appears the cathode of the diode 12 is of positive polar across an anode resistor 56 and a portion of the ity and corresponds tothe positive bias applied anode resistor 51-51’ of a second amplifier to diode 63 of Fig. 6. Also, the diode circuit 12 tube 58. 13 operates the same as the diode circuit 62-53 The rectangular wave n is produced, in this except that the adjustment of the bias and the particular example, by applying the output of the magnitude of the applied signal ’are such that tube 54 through a blocking capacitor 59 and a the signal is not clipped. ' i ` ‘A " ’ high Yimpedance resistor 6I to a pair of diodes It will be understood that the invention is 62 and 63 which are connected to conduct in opposite directions. Output resistors 64 and 66 not limited to the particular circuits illustrated '2,403,540 6 5 adjusted; only.v .the f 'desiree product iterm'igzsx and the D.-C. component A2. appear'at'the 'junc tion-point 9|. In practice it is- necessary "toad ïju'st'the multiplier circuit carefully for minimum vsince the lwaves m 'andr-n, maybe derived from "the wave K in various-'ways andìsince the two 'waves may be combined by means of a variety of circuits. '_ , l residual _fsig'nal when' >eitherïthe signal Sor'ïthe According to the above-identiñed Wipff appli signal K is removed: , = -' cation, the multiplier circuit I4 may utilize' one ' '-‘In the several figures, circuit values- are'rgiven, of several known types of circuits that have the. merely by-way-of example, in ohms, megohms, property of providing an instantaneousoutput voltage which is proportional to the square of "and microfarads. ' the instantaneous input voltage for a reason' 10 _»I `claim as my invention: - - f. . "1‘ ’The methcd 0f ' VI'Qdlißîng the reciprocal’v of able voltage swing. Such circuits will be >referred to as “squaring circuits,” and will be designated :a lsignal that-reverses in polarityat intervals with “Q” where referred to in the description ’anfd 'respect to >an alternating-current ` axis whereby drawings. In the multiplier circuitïillustratedin the »signal has'cross-ov'er'points yon saidfaxis, said K are added, then the sum S-|-K is squared inta~ third squaring circuit Qa and the squared signal ¿ over’points, and' whichireverseis in polarity rat said cross-over points, producing another'si'g'nal Fig. 8, the waves S and K to ¿be lmultipliedl >are ,15 niethodlcomprising the stepsof producing'a'si'g‘ each separately squared in two squaring-circuit's» "rial'that `has ai relatively largev amplitude >sub stantially at the’time of occurrence -of said; cross Q1 and Q2. In a branch circuit the signals S and is reversed in polarity. 'I‘he three outputs of Q_i, 20 that has a wave form approximately,` similar to that of the first-mentioned signal, and adding Ch and Q3 are then added to obtain the sum as shown in the following equations: the two produced signals with one of them re versed in polarity and having the proper ampli- _» 1' +2.84 tude with respect to the other produced signal 25 for producing a sum voltage that is substantially the desired reciprocal signal. . 2. The method of producing the reciprocal of The value “A” above is the D.-C. component or a signal that reverses in polarity at intervals withv bias that is applied to the squaring circuits to respectr to an alternating-current axis whereby cause the voltage swing always to be on the de sired portion of the squaring circuit character 30 the signal has cross-over points on said axis, said method comprising the steps of producing a sig istic curve. ` nal that has a relatively large amplitude at the Referring more speciñcally to Fig. 8, the vac uum tubes 80, 8| and 82 are utilized as the squar ing circuits Q1, Q3 and Q2, respectively. These tubes must have plate current vs. grid voltage 35 characteristic that follows substantially a square law. Vacuum tubes of the types 6.15, BSN?, 605, and other triodes have suitable characteristics. The plate resistors for such tubes should be kept' 40 small, not more than about 1000 ohms. The signals S and K are applied to the grids of tubes 80 and 82 through resistors 83 and 84, respectively. Both of the signals S and K are applied to the grid of tube 8| through resistors 45 85 and 86, respectively. A suitable negative bias is applied to the grids of the tubes 80, 8| and 82 through grid resistors 81 and 88. The resistors B3, 84, 85, 86, 81 and 88 all have the same com paratively high resistance, such as between 0.1' to megohm and 1 megohm, so as not to load the time of occurrence of said cross-over points, and which reverses,v in polarity at said «cross-over points, producing another signal that has a wave form approximately` similar to but ilatter than that of the first-mentioned signal, and adding the two produced-signals with one of them re versed in polarity and having the proper ampli- ,k tude with respect to the other produced signal ' for producing a sum voltage that is substantially the desired reciprocal signal. ' 3. The method of obtaining the reciprocal of -a, signal having a certain `wave' form which com prises the steps of producing a signal of` substan tially rectangular wave form having positive and negative cycles corresponding to the positive and negative cycles of the >first signal, producing an other signal that is generally similar in vwavev form to the first-mentioned signal, and adding ,Y . comparatively low impedance driving source too said rectangular wave to said other wave _with much. However, the resistance is not made so high as to permit~ excessive phase shift due' to grid-cathode capacity. Since the pairs of re ' one of the waves reversed in polarity andV with the two waves having such relative amplitudes that said reciprocal signal is obtained? sistors 83-81, 85--86, and 84-88 act as voltage ' 9, signal having a certain wave form which com dividers for the applied signals, the voltages ap plied to the grids of tubes 80, 8| and 82 will be S-l-K K 2 Silldî respectively. , 4. The method of obtaining the reciprocal of prises the steps of producing a signal of substan tially rectangular wave form having positive and 60 negative cycles corresponding to the positive and negative cycles of the ñrst signal,- flattening the y positivejand negative cycles of the '.ñrst signal, The grid bias (_C) on tubes 80, 8| and 82 and Aadding said rectangular wave to saidilat should be about to the middle of the straight por tened wave with one of the `waves reversed in , tion of the grid voltage vs. mutual' conductance 65 polarity. curve. For a 6SN7 or 6J5 with 250 volts on the plate, this is about -14 volts. A vacuum tube 89 serves to reverse the polar ity of the output of tube 8| and should have ap 5. An electrical circuit for` obtaining Vthe recip rocal value of- a voltage applied thereto compris- - ing means for producing a substantially rectan-> gular wave having positive and negative cycles proximately unity gain and very low distortion. corresponding tothe positive and negative cycles 70 The outputs of the tubes 80 and 82 and of the re of a voltage applied to said circuit, vI_neans for versing tube 89 are added by supplying them to producing a wave having flattened positive and a common junction point 9| through adding re negative cycles corresponding to the positive and sistors 92, 93 and 94. These adding resistors may negative cycles of the voltage applied to said cir- ' ~ have a resistance of from 0.05 megohm to 0.1 75 cuit, and means for adding said rectangular wave megohm, for example. If everything isproperlyY V2,403,540 7 Ito 'said iiattened Wave with ,one of- the waves reversed in polarity. ~ » „ ' ' .6. The invention according to claim«5_ wherein lthe impedance of the circuit is substantially purely resistive at the-frequencies of the applied l voltage. u l Y 7. `An electrical circuit for obtaining the recip rocal of a signal voltage applied thereto com prising a non-linear resistive device which de 8. Anelejctrical circuit for obtaining the recip rocal of a signal voltage applied thereto `com aprising a non-linear resistive device which de creases in resistance with an increase in applied voltage, means' for passing said signal voltage through said resistive device whereby its Wave form is flattened, means for .clipping said signal voltage symmetrically with respect to its alter nating-current axis _to produce a rectangular creases in resistance with an increase in applied 10 voltage wavev having positive and negative cycles voltage, means for passing said signal voltage corresponding to the positive _and negative‘cycles through said resistive device whereby its wave -form is flattened, means for producing a sub of said signalwavaand means for adding said rectangular wave to said ñattened wave withone of said waves reversed in polarity and withvtheir relative amplitudes such as to produce a sum voltage that is the desired reciprocal signal. stantially rectangular voltage wave having posi tive and negative cycles corresponding to the positive and negative cycles of said signal wave, and means for adding said rectangular Wave to »said flattened wave with one of said waves re versed in polarity. CARL A. MENELEY.