Title:
Reciprocal circuit
United States Patent 2403540


Abstract:
My invention relates to the production of the reciprocal of a signal or wave and particularly to the production of such a signal for use in a system for secret communication. In the copending application of Alda V. Bedford Serial No. 456,578, filed August 29, 1942, and entitled Secret communication...



Inventors:
Meneley, Carl A.
Application Number:
US48430443A
Publication Date:
07/09/1946
Filing Date:
04/23/1943
Assignee:
RCA CORP
Primary Class:
Other Classes:
327/360, 380/260, 380/274
International Classes:
H04K1/02
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Description:

My invention relates to the production of the reciprocal of a signal or wave and particularly to the production of such a signal for use in a system for secret communication.

In the copending application of Alda V. Bedford Serial No. 456,578, filed August 29, 1942, and entitled Secret communication system, there is described a communication system in which the speech or other communication signal S is multiplied by a coding signal K and in which the product SK is transmitted. At the receiver the incoming signal SK is multiplied by the reciprocal of the coding signal K to obtain the original signal S, that is, SK 1/K=S. According to the above Bedford application, the coding signal K may be generated by means of a code disc which is cut to make its periphery have the wave form of the signal K while the decoding signal 1/K may be produced by a different disc which is cut in accordance with calculated values to make its periphery have the wave form of 1/K.

In a later application of Alda V. Bedford and Frank P. Wipff, Serial No. 480,716, filed March 26, 1943, and entitled Reciprocal circuits, there is described one type of electrical circuit for producing the reciprocal of a signal or wave such as the wave K.

An object of the present invention is to provide an improved method of and means for producing the reciprocal of a signal or wave.

A further object of the invention is to provide an improved electrical circuit for converting an applied signal having a certain wave form into a signal having the reciprocal wave form.

A still further object of the invention is to provide an improved communication system for secret signalling.

In a preferred embodiment of my invention the reciprocal of a wave K is obtained by means of an electrical circuit in which the wave K is clipped on both its positive cycle and on its negative cycle to produce a rectangular wave, and in which the wave K and the rectangular wave are added together with one of them reversed in polarity, preferably after the peaks of the positive and negative cycles of the wave K have been "squashed" or flattened somewhat. My reciprocal circuit contains no capactive or inductive reactances (the blocking capacitors in the circuit presenting a negligible impedance) and, therefore, functions the same for any applied signal regardless of its frequency or wave form.

In utilizing the reciprocal circuit in a communication system, a coding signal K is generated at the transmitter in any suitable manner as by S 2 means of a coding disc, the speech signal S is multiplied by the signal K, and the signal SK is transmitted to the receiver. At the receiver there is another signal generating device for producing the signal K; it may be a coding disc like that at the transmitter, the two coding discs being run in synchronism. The signal K generated at the receiver is supplied to the reciprocal circuit to obtain the decoding signal 1/K which-is then multiplied with the received signal SK to obtain the original signal S.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a block diagram of signalling apparatus embodying my invention, Figures 2 to 5 are graphs which are referred to in explaining the invention, Figure 6 is a circuit diagram of a reciprocal circult designed in accordance with one embodiment of the invention, Figure 7 is a circuit diagram of another reciprocal circuit designed in accordance with another embodiment of the invention, and Figure 8 is a circuit diagram of a multiplier circuit that may be utilized in the circuit of Fig, 1.

In the several figures, similar parts are indicated by similar reference characters.

Referring to Fig. 1, the invention is shown applied to radio apparatus that may be switched, for operation either as a transmitter or as a receiver.

The several switches are shown in the position for transmitter operation. This is the operation of the apparatus that will first be described. A microphone and a speech amplifier are shown at 7 and 8, respectively. The signal S is applied through a switch 9 to a multiplier unit 14 which may be of the same design as that described in the above-identified Bedford application or which may be of the type shown in Fig. 8 and described and claimed in application Serial No. 484,303, filed on the same day as the present application in the name of Frank P. Wipff and entitled Multiplier -circuits. The code signal K may be produced by means of a code disc 16, a mask 17, a light source 18, a condensing lens 19, and a photoelectric cell 20. The signal K is supplied through amplifiers 21 and. 22 and through a switch to the multipler unit 14. The resulting multiplier output signal SK is supplied through an amplifier 26 and through a switch 27 to a radio transmitter 28 or to a wire line, if preferred.

The receiver for decoding the signal SK may 5 be the same apparatus, as in Fig. 1 with switches 9, 23 and 21 thrown to their contact positions R.

The signal SK is supplied from a radio receiver 31 through the switch 9 to the multiplier 14. The coding signal K produced by the disc 16 is supplied through the amplifier 21 to a reciprocal circult 32 designed in accordance with my invention and shown in detail in Fig. 6. The reciprocal circuit 32 supplies the decoding signal 1/K through the switch 23 to the multiplier 14. The resulting outpt of multiplier 14 is the original communication signal S since SKX 1/K=S. The signal S is amplified by the amplifier 26 and supplied through the switch 27 to headphones or to the loudspeaker 33.

The code disc 16 at the transmitter and the similar code disc at the receiver are held in synchronism and in the proper phase relation by suitable synchronizing means. For example, at the transmitter a 600-cycle per second current from a source 36 may be supplied through a switch 37 to a synchronous motor 38 which rotates the code disc 16. The 600-cycle current also modulates a radio transmitter 39 for the transmission of synchronizing signals to the receiver. At the receiver the switch 37 is in its contact position R whereby the received 600cycle current is supplied from a radio receiver 41 to the synchronous motor 38. In some cases it may be preferable to transmit the synchronizing signal over a wire line. My method of obtaining the reciprocal wave 1/K will now be explained with reference to the 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 which is the sum of the flattened wave m of reversed polarity shown in Fig. 3 and the rectangular wave n of Fig. 4. The "squashed" or flattened wave m may be obtained by passing the wave K through a circuit that changes its resistance with a change in applied voltage.

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 amplitude of 60 volts, for example, is applied through a blocking capacitor 51 and a resistor 52 to a copper oxide rectifier unit 53 which functions as a non-linear resistor having the property of decreasing in resistance as the applied 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 impedance whereby there is only a slight variation in the current flow through the unit 53.

The unit 53 may consist of a pair of copper oxide rectifiers 53a and 53b connected to conduct current in opposite directions.

The voltage appearing across the non-linear unit 53 is the voltage m of Fig. 3 having the flattened wave form. This voltage is amplified by a cathode biased vacuum tube 54 and appears across an anode resistor 56 and a portion of the anode resistor 57-57' of a second amplifier tube 58.

The rectangular wave n is produced, in this 7( particular example, by applying the output of the tube 54 through a blocking capacitor 59 and a high impedance resistor 61 to a pair of diodes 62 and 63 which are connected to conduct in opposite directions. Output resistors 64 and 66 7 of comparatively low resistance are connected in series with the diodes 62 and 63, respectively.

A biasing voltage drop for opposing current flow through diode 63 is produced across the resistor 66 by connecting a source of voltage (not shown) thereacross, a resistor 67 being in series with the voltage source. The diodes 62 and 63 clip the applied wave m symmetrically about its A.-C. axis because a voltage which causes current flow through the diode 62 and resistor 64 is built up across the capacitor 59 by the positive cycle pulses flowing through the diode 63. Thus, the diodes 62 and 63 become conducting on alternate cycles when the signal voltage exceeds the D.-C. voltage drop across the resistors 64 and 66, respectively. The resulting rectangular wave n 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 57-57' 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 relative amplitudes, the resulting signal will be substantially a true reciprocal of the wave K. The 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 I/K necessarily has a definite limit. The waves ni and n may be mixed with the correct relative amplitudes by adjusting a variable tap 1i on the anode resistor 57. The correct shaping of the flattened wave m may be obtained by selecting a non-linear resistor unit 53 having a suitable voltage-resistance characteristic and by adjusting the value of the resistor 52.

As previously noted, the above-described reciprocal circuit is purely resistive so that its operation is independent of frequency. The instantaneous voltage output of the circuit is always the reciprocal of the instantaneous applied voltage. It follows that if the reciprocal circuit is adjusted to produce the reciprocal of an applied signal having one wave form, the circuit will then always produce the reciprocal of an applied signal regardless of its wave form.

There are various ways of determining when the circuit has been adjusted to give substantially a true reciprocal. One way is to connect the reciprocal circuit into the signalling system of Fig. 1 and, while transmitting speech or music, adjust the resistor 52 and the variable tap 71 at the receiver until the speech or music has a minimum of distortion.

Fig. 7 shows a reciprocal circuit that is the same as that of Fig. 6 except that a pair of diodes 72 and 73 have been substituted for the copper oxide rectifiers 53a and 53b. The diodes 172 and 73 may be properly biased by adjusting a pair of variable taps 74 and 76 on resistors 77 and 78, respectively. When properly biased, the two diodes will be operated along the lower knee of their characteristic curve and in the proper region to shape the wave m in the desired manner. It will be noted that the bias applied to the cathode of the diode 72 is of positive polarity and corresponds to the positive bias applied to diode 63 of Fig. 6. Also, the diode circuit 720 73 operates the same as the diode circuit 62--63 except that the adjustment of the bias and the magnitude of the applied signal are such that the signal is not clipped.

It will be understood that the invention is not limited to the particular circuits illustrated I ' -since the waves m and n may be derived from -the wave K in various ways and since the two - waves may be combined by means of a variety of circuits.

According to the above-identified Wipff application, the multiplier circuit 14 may utilize one of several known types of circuits that have the property of providing an instantaneous output voltage which is proportional to the square of the instantaneous input voltage for a reasonable voltage swing. Such circuits will be referred to as "squaring circuits" and will be designated "Q" where referred to in the description and drawings. In the multiplier circuit illustrated in Fig. 8, the waves S and K to be multiplied are each separately squared in two squaring circuits Qi and Q2. In a branch circuit the signals S and K are added, then the sum S+K is squared in a third squaring circuit Q3 and the squared signal is reversed in polarity. The three outputs of Q, Q2 and Q3 are then added to obtain the sum as shown in the following equations: Output of Qi= (S + A)2= S2 +A2 +2SA Output of Q2= (K + A)2= K2+A2 +2KA Output of Qa= -(S+K+A)2=-S2-K-A-2SK-21 A-2SA Sum output= -A2-2SK The value "A" above is the D.-C. component or bias that is applied to the squaring circuits to cause the voltage swing always to be on the desired portion of the squaring circuit characteristic curve.

Referring more specifically to Fig. 8, the vacuum tubes 80, 81 and 82 are utilized as the squaring circuits Qi, Q3 and Q2, respectively. These tubes must have plate current vs. grid voltage characteristic that follows substantially a square law. Vacuum tubes of the types 6J5, 6SN7, 6C5, and other triodes have suitable characteristics.

The plate resistors for suqh tubes should be kept 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 81 through resistors 85 and 86, respectively. A suitable negative bias is applied to the grids of the tubes 80, 81 and 82 through grid resistors 87 and 88. The resistors 83, 84, 85, 86, 87 and 88 all have the same comparatively high resistance, such as between 0.1 megohm and 1 megohm, so as not to load the comparatively low impedance driving source too much. However, the resistance is not made so high as to permit' excessive phase shift due to grid-cathode capacity. Since the pairs of resistors 83-87, 85-86, and 84-88 act as voltage dividers for the applied signals, the voltages applied to the grids of tubes 80, 81 and 82 will be S S+K K ' - and respectively.

The grid bias (-C) on tubes 80, 81 and 82 should be about to the middle of the straight portion of the grid voltage vs. mutual conductance 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 polarity of the output of tube 81 and should have approximately unity gain and very low distortion.

The outputs of the tubes 80 and 82 and of the reversing tube 89 are added by supplying them to a common junction point 91 through adding resistors 92, 93 and 94. These adding resistors may have a resistance of from 0.05 megohm to 0.1 megohm, for example. If everything is properly adjusted, only the desired product :term--2SK and the D.-C. component A2 appear at the junction point 91. In practice it is necessary to adjust the multiplier circuit carefully for minimum residual signal when either the signal S or the signal K is removed.

In the several figures, circuit values are given, merely by way of example, in ohms, megohms, and microfarads.

I claim as my invention: 1. The method of producing the reciprocal of a signal that reverses in polarity at intervals with respect to -an alternating-current axis whereby the signal has cross-over points on said axis, said method comprising the steps of producing a sighal that has a'relatively large amplitude substantially at the time of occurrence of said crossover-points, and which reverses in polarity at said cross-over points, producing another signal that has a wave form approximately similar to that of the first-mentioned signal, and adding the two produced signals with one of them reversed in polarity and having the proper amplitude with respect to the other produced signal for producing a sum voltage that is substantially the desired reciprocal signal.

2. The method of producing the reciprocal of a signal that reverses in polarity at intervals with respect to an alternating-current axis whereby the signal has cross-over points on said axis, said method comprising the steps of producing a signal that has a relatively large amplitude at the time of occurrence of said cross-over points, and which reverses in polarity at said cross-over points, producing another signal that has a wave form approximately similar to but flatter than that of the first-mentioned signal, and adding the two produced signals with one of them reversed in polarity and having the proper amplitude 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 comprises the steps of producing a signal of substantially rectangular wave form having positive and negative cycles corresponding to the positive and negative cycles of the first signal, producing another signal that is generally similar in wave 0 form to the first-mentioned signal, and adding said rectangular wave to said other wave with one of the waves reversed in polarity and with the two waves having such relative amplitudes ,~ that said reciprocal signal is obtained.

4. The method of obtaining the reciprocal of a signal having a certain wave form which comprises the steps of producing a signal of substantially rectangular wave form having positive and negative cycles corresponding to the positive and negative cycles of the first signal, flattening the positive and negative cycles of the first signal, and adding said rectangular wave to said flattened wave with one of the waves reversed in polarity.

5. An electrical circuit for obtaining the reciprocal value of a voltage applied thereto comprising means for producing a substantially rectangular wave having positive and negative cycles corresponding to the positive and negative cycles of a voltage applied to said circuit, means for producing a wave having flattened positive and negative cycles corresponding to the positive and negative cycles of the voltage applied to said circult, and means for adding said rectangular wave to said flattened wave with one of the waves reversed in polarity.

6. The invention according to claim 5 wherein the impedance of the circuit is substantially purely resistive at the frequencies of the applied voltage.

7. An electrical circuit for obtaining the reciprocal of a signal voltage applied thereto comprising a non-linear resistive device which decreases 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 producing a substantially rectangular voltage wave having positive 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 reversed in polarity.

8. An electrical circuit for obtaining the reciprocal of a signal voltage applied thereto comeprising a non-linear resistive device which decreases 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 alternating-current axis to produce a rectangular voltage wave having positive 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 reversed in polarity and with their relative amplitudes such as to produce a sum voltage that is the desired reciprocal signal.

CARL A. M1ENELEY.

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