US2483718A

US2483718A - Phase selective signal transmission system

Info

Publication number: US2483718A
Application number: US651013A
Authority: US
Inventors: Charles B Aiken
Original assignee: Schlumberger Well Surveying Corp
Current assignee: Schlumberger Well Surveying Corp
Priority date: 1946-02-28
Filing date: 1946-02-28
Publication date: 1949-10-04
Anticipated expiration: 1966-10-04

Description

Oct. 4, 1949. c, AIKEN 2,483,718

PHASE SELECTIVE SIGNAL TRANSMISSION SYSTEM CHA/FL E0 .B.AIKEN 045i. 4, 1949. c. B. AIKEN 2,483,7ig

YHASE SELECTIVE SIGilAL TRANSMISSION SYSTEM E'iled Feb. 28, 1946 2 Sheets-Sheet 2 Patented Oct. 4, 1949 PHASE SELECTIVE SIGNAL TRANSMISSION SYSTEM Charles B. Aiken, Houston, Tex., assignor, by

mesne assignments, to Schlumberger Well Surveying Corporation, Houston, Tex a corporation of Delaware Application February 28. 1946, Serial No. 651,013

14 Claims. 1

This invention relates to improvements in signal transmitting systems, such as, for example, systems for transmitting signals produced in electrical well logging, telegraphy, radio and the like. The invention relates particularly to systems for transmitting two different signals simultaneously over a single channel.

An object of the present invention is to provide systems whereby two signals having the same frequency, but of different phase, may be transmitted over a single channel and then separated by means of phase selective networks.

Another object of the invention is to provide systems whereby two signals of diiferent phase may be separated to permit transmission of such signals over a single channel.

An additional object of the invention is to provide alternating current networks that are selective to signals of a selected phase and discriminate against signals in the network that are in phase quadrature to the signals of selected phase.

These and other objects of the invention are .attained by impressing a voltage of a selected phase on a network that carries a signal of the same phase as the impressed voltage and also carries another signal substantially in quadrature to the impressed voltage. The impressed voltage renders the network selective to the signal having the selected phase by discriminating against signals in quadrature to the selected phase and thus permits separation of the selected signal from the other signal. Similarly, when voltages substantially in phase quadrature are impressed on different branches of a system carrying two signals of smaller magnitude and having phases the same as the impressed signals, the two branches are rendered phase discriminating and separate indications of the two signals or their magnitudes or values can be obtained.

More particularly, the objects of the invention are attained by transmitting over the same frequency channel, two alternating currents that differ from each other in phase by 90, and independently keying or modulating these currents so as to provide two signal circuits. At the receiving or detecting end of the system, two branch circuits are provided, one for the reception of each of the signals. Each branch is made phase selective by the use of suitable rectifying or detecting elements. One form of such element embodies the use of a rectifier in conjunction with a locally impressed voltage having'the same phase as the received alternating current or voltage, with the result that this particular received voltage actuates the rectifier to give an output that is proportional to its intensity at any instant, while the second received voltage, which is in phase quadrature, actuates the rectifier to only a small degree, and gives no output of consequence. In the other branch, a local voltage having the phase of the second received signal is impressed, with the result that this signal gives a useful output, while the first received signal is discriminated against. Other forms of phase selective detectors may be used if desired.

Signal transmitting systems of the type described above are useful in many fields. For example, in electrical well logging operations, it may be desirable to make a plurality of determinations simultaneously and to obtain indications of these determinations at the surface of the earth. If a large number of separate conductors are provided in the well logging cables for transmitting the indications to the surface of the earth, the cables are not only expensive but are necessarily less strong, for a given overall diameter, than are cables containing fewer conductors, since a larger proportion of the available space must be devoted to insulation. Moreover, they are more susceptible to damage because of their.

size and there is always the possibility of current leakage between the conductors that aifects the accuracy of the determinations.

With systems of the type embodying the present invention, it is possible to reduce the number of conductors in the cable and at the same time transmit a plurality of indications or signals to the surface of the earth with relatively simple and compact equipment.

The invention is applicable to message transmission, also, as well as to other fields wherein a plurality of signals must be transmitted from a source or plurality of sources to remote points or when only one of two signals in phase quadrature is of interest.

For a better understanding of the present invention, reference may be had to the accompanying drawings, in which:

Figure 1 is a diagrammatic illustration of a typical form of system or network for obtaining two resistivity indications in a bore hole traversing earth formations over a single conductor con nected to equipment at the surface of the earth; and 1 Figure 2 is a diagrammatic illustration of a telegraphic signal transmitting system also embodying the present invention.

As is well understood in the art, it is quite common to conduct a plurality of electrical well logging determinations simultaneously in a bore hole. For example, it may be desired to make resistivity determinations at different depths in a formation by utilizing electrodes in diflerent spaced relationships in order to determine the resistivity of the formations at different zones spaced radially from the bore hole.

The embodiment of the invention illustrated in Figure 1 is an apparatus whereby two resistivity measurements of the earth formations traversed by a bore hole may be conducted simultaneously by supplying alternating current power at the surface, providing a two-wire cable for the current circuit in the hole, and one wire for the voltage pickup circuit. This system includes an oscillator III that supplies low frequency alternating current to the conductors II and I2. The conductor H is connected to an electrode l3 which is located at a sufllcient distance from the active electrodes l4, l5, and I6 so that these latter electrodes are not appreciably affected by the current field of the electrode l3. A distance between the electrodes l3 and H of 10 to 100 feet usually is suitable, the value depending upon the spacing between the electrodes. l4, l5 and IS.

The conductor I2 is connected to the electrodes l4 and I5 at a junction point II. The current through the conductor I2 divides at the junction point l1, part of it flowing through the condenser It, the resistance l8, and the electrode i4 out into the surrounding earth. The other part of the current through the conductor |2 flows through the inductance 20, the resistance 2|, and the electrode IS.

The four circuit elements |8, I3, 20, and 2| are chosen so that the current through the electrode l4 differs in phase by very. nearly 90 from the current passing from the electrode IS. The impedances of thecircuit elements l8, I8, and 2| should be sufiiciently large so that the changes in the resistance of the electrodes l4 and I5 will not affect the phase relations of currents flowing through these electrodes to any important degree.

The electrode I6 is located in the fields of the electrodes l4 and I5. These two fields impress voltages upon the electrode l6 which are brought to the surface by means of the conductor 22. This conductor is connected to the grid 23 of a triode tube 24 forming a part of a single-stage amplifier. The triode 24 has a cathode 25 and a plate 26. A grid leak resistor 21 of high value is connected between the grid 23 and ground and a cathode bias resistor 28 is connected between the cathode 25 and ground. The amplifier also includes the usual plate bypass condenser 29, a decoupling resistor 30, and a transformer 3| having its primary connected between the plate 26 and ground through the capacitance 29. The transformer 3| couples the output of the triode 24 to a rectifier circuit to be described.

Voltage for the rectifier circuit is taken from the oscillator l0 through the transformer 32 having its primary connected to the conductors II and I2 and its secondary connected to the phase shifter 33. One terminal 33a of the adjustable phase shifter 33 is connected in series with the inductor 34, the variable resistor 35 and the grid of the triode 38. The plate 360 of the triode 36 feeds the primary 31a. of the transformer 81 which delivers a balanced voltage from its secondary to the two

diode rectifiers

38 and 39. The end of the primary winding 31a is connected through the capacitor 4| to ground and through the decoupling resistor 40 to positive side of the B voltage supply- The balance of the transformer 31 and the

rectifiers

38 and 39 may be trimmed by means of the potentiometer 43. One terminal of the indicating meter or recorder 44 is connected to the potentiometer 43 by the adjustable slide. The other terminal of the indicator 44 is connected in series with a resistance 45, the secondary 3| a of the transformer 3| and the cathodes of the

rectiflers

38 and 39. A capacitance 46 is connected across the indicator 44 and resistance in the usual way.

With this arrangement, the phase shifter 33 is first adjusted with the circuit of the electrode I5 open, until a maximum reading is obtained on the meter 44 due to the current flowing into the earth from the electrode l4. When this occurs, the voltage fed to the

diodes

38 and 39 from the transformer 31 is in phase with the signal from the transformer 3|, which is proportional to and of the same phase as the signal picked up by the electrode I8 and produced by current emitted from the electrode'l4. For best operation, the local signal introduced through the phase shifter 33, the triode 35 and the transformer 31 should be several times as large as the signal introduced into the common leg of the

rectifiers

38 and 39 by means of the transformer 3|. The local signal preferably should be 3 to 10 times as great as the signal introduced through the transformer 3|.

When the circuit isnow closed through the inductance 20, the resistor 2| and the electrode IS, the signal from the electrode I5 that is picked up by the electrode IE will not give a reading on the meter.

A similar arrangement of circuits is provided for selecting voltages due to the flow of current through the electrode l5, while rejecting signals due to the current through the electrode I4. These comprise the

triodes

41 and 48 and their associated circuits, the general operation of which is identical with those already described and the balanced rectifier system including the

diodes

49 and 50, the transformer 5| and the meter 52. The local signal from the oscillator I0 is supplied to the grid of the triode 41 through a condenser 53 and the resistor 54. The fixed phase shifting network made up of the condenser 53 and the resistance 54 has a fixed quadrature phase relationship to the phase shifting network made up of the inductance 34 'and the resistance 35 so that the phases of the voltages supplied to the grids of the

triodes

36 and 41 are in quadrature. Thus the phase of the voltage introduced into the circuits of the

diodes

49 and 50 by means of the transformer 5| is in quadrature with that introduced into the

diodes

38 and 39 by the transformer 31. As a result, the meter 52 will respond to signals caused by the flow of current from the electrode l5 but will not be affected by those caused by the flow of current from the electrode In the above, it was stated that the phase shifter 33 should be adjusted for maximum response on the indicator 44, when the circuit of electrode l5 was open. Meter 44 then responds to currents flowing from electrode l4, while meter 52 responds to currents flowing from electrode IS.

The reverse state of affairs could have been chosen instead, phase shifter 33 being adjusted for maximum response when the circuit of I4 is open. Meter 44 then responds to currents from l5. If the phase shift range of the phase shifter 33 is large enough, either adjustment may be made at will.

In operation, first consider the case in which .The transformer 31 is connected so that the voltages developed by the secondary windings 31c and 31d are additive. The

diodes

38 and 39 will both conduct on the same alternate half cycles of the alternating voltage delivered by the transformer 31. Each diode will develop pulses of rectified current that will flow through the middle leg of the rectifier system, comprising the condenser 46, the resistance 45, and the meter 44.

However, the pulses of the two diodes are of opposite polarity so that the average value of the current through the middle leg will be zero, and

the meter 44 will show no reading. The alternating current components of these pulses will be bypassed by the condenser 48 and will not affect the meter 44. If the balance of the transformer 31 is not perfect, the potentiometer 43 may be adjusted to bring the meter reading to ends, and that the voltage delivered by the winding 3| a adds to the voltage of 31d and subtracts from that of 310. Since the voltages delivered by the secondary windings 31c and 31d are both larger than that delivered by the secondary winding 31a, both diodes are able to conduct during this particular half cycle, and, for the conditions stated, the total voltage applied to the diode 39 is increased, while that applied to the diode 381s decreased. As a result, the rectified currents from the two diodes no longer balance, and'themeter 44 shows a reading in a direction indicating that the current from the diode '38 predominates; [If the polarity-of the voltage from 3la were to be reversed, the reading of meter 44 would also reverse, since in this case the current through 38 would be larger than that through 39. Now consider the case in which the voltage picked up by the electrode I6 is in such phase as to make the'voltage delivered by the transformer 3| differ by 90 from the voltage delivered by the transformer 31; The voltage from the secondary winding 3ia must then be vectorially added in quadrature to the voltages delivered by the secondary windings 31c and 31d. Since the voltage from the secondary winding 3la is much smaller than that from the secondary windings 31c and 31d, the magnitude of the resultant voltage will be substantially unchanged, since that resultant voltage is the vector sum of two components in phase quadrature, one of which is much larger than the other. The resultant of such a combination is very closely equal to the magnitude of the larger vector. the

diodes

38 and 39 will be unaffected by the presence of the smallsignal from the secondary winding 3la. If this latter signal is increased, but is still smaller than the voltage delivered by either 31c or- SM, the. magnitudes of the resultant voltages impressed upon the

diodes

38 and 39 will be increased somewhat, but by the same amount. The rectified currents developed by the

diodes

38 and 39 will then increase together and. the meter 44 will show no change, since the rectified currents are of opposite polarity. This will Hence the currents through.

- 6 winding Ila is only slightly less than that of the secondary windings 310 or 31d.

The operation of the phase selective circuit comprising the diodes '49 and 58 is entirely analogous to that of the circuit comprising the

diodes

38 and 38 and further explanation is not necessary.

a local voltage of a certain phase. exercises a pronounced phase selective action; Voltages of the correct phase are rectified with maximum efllciency, and the meter responds accordingly, while voltages of quadrature phase are rectified with minimum eiilciency, and the meter shows no reading unless the system is overloaded. For phases in between these two valuesthe'system will respond with an intermediate efficiency, and the meter will show a reduced reading, which will be less the nearer the phase of theincoming signal is to quadrature. I a

The above system is capable of transmitting two diflerent signals of the same frequency over the same channel or circuit, the conductor 22, and separating the signals for observation or recording at a remote point. In this system, the frequency of the oscillator l0 must be quite low for the reason that the phase shift that normally occurs in the cable, including the conductors ll, i2, and I3 must not change more than a few degrees as the cable is lowered into the drill hole. Satisfactory operation of the system is obtained at frequencies in the neighborhood of 16 cycles.

The separation of two signals by phase selection can be accomplished in other ways and with other systems. For example, instead of using full wave rectifiers as illustrated, half wave rectifiers may be used in conjunction with a local signal if desired. Thus, the

diodes

38 and 49 and the associated secondary windings of the transformers 31 and SI, respectively, could be eliminated, if desired. The local signal will then give a steady reading which may be balanced out by the use of an auxiliary direct current circuit, or may be ignored.

Also, if desired, synchronous mechanical rectifiers may be used in such transmitting systems, inasmuch as such mechanical rectifiers are also phase selective devices. Rectifiers of this type are well known and usually consists of a rotary commutator driven in phase with the signal and so connected that the alternating voltage impressed upon the rectifier is converted into a series of unidirectional voltages acting on the output circuit. If such a rectifier is adjusted to give a maximum response to a signal of one phase, it will give very little response to another signal of the same frequency but differing in phase from the first signal by 90.

Such synchronous mechanical rectifiers may be used in a system, for example, for transmitting two telegraphic signals along a single channel. An example of such a short distance telegraph line is illustrated in Figure 2. As illustrated in this figure, a source of alternating current power 60 may supply power to two triodes 6i and 62, each of which is provided with its own phase shifting network so that the grids of the triodes receive voltages that difier in phase by 90?. The triode 6|, for example, may have its grid 61a connected by means of an isolating resistor 63 and an inductance 64 to one pole 65 of the alternating current source 60. The grid 62a of the triode 62 is connected through a resistorlili. and a capacitance 81 to. the pole Bi ot theJIso'urce of still be true when the voltage of the secondary alternating current 80. The midpoint'between It is thus evident that the rectifier, excited with the resistance 63 and the inductance 64 is grounded through the resistor 69, and is connected to the cathode 6 lb through the resistance 10. The cathode and grid of the triode 62 are connected similarly. The plate He is connected in series with the primary 12a of a transformer 12, a capacitance H and the cathode Gib. The plate and cathode of the triode 62 are similarly connected to a primary winding 92a of the transformer 92.

The inductance 64 and the resistor 69, and the capacitance 61 and the resistance 69a act as fixed phase shifters for the triodes 6| and 62, respectively, so that the voltages impressed on the grids 6 la and 52a are in phase quadrature.

The signalling is accomplished by means of the telegraph keys l3 and 14 connected between the grids and cathodes of the tubes GI and 62, respectively, each of which allows a signal to pass when it is open and cutsv all the signal when it is closed. The

resistances

83 and 66 prevent the keys from changing the load on the oscillator 60 so there is no interaction of the two keying systems.

The triodes liand IS-are fed in parallel from the secondary windings 92b and 120 of the transformers l2 and 92, these triodes being at the receiving end of the system. Each of the triodes l and 16 supplies energy to separate full wave, mechanical rectifier circuits, to be described, through the transformers l1 and 18. One rectifier circuit includes the secondary Ila of the transformer TI. The opposite ends of the secondary are connected to the commutators 19 and 80 of the mechanical rectifier, these commutators being engaged by the brushes 8| and 82 which are connected to each other. A common leg from the midpoint of the secondary Ha to the midpoint between the brushes 8| and 82 of each of the rectifier circuits contains a telegraph sounder 83 and a bypass condenser 84.

The other rectifier and telegraph sounder circuit is the same and is connected to the secondary 18a of the transformer 18. This circuit includes the mechanical rectifier commutator 86 and 81' andthe telegraph sounder 88 in the common leg. The rectifier elements 19 and 80 diner by 90 in mechanical phase from the

rectifier elements

86 and 81, with the result that the sounder 83 will respond to one signal only, while the sounder 88 responds to the other.

In order for this system to function properly, it is necessary for the power source 60 and the rectifiers I9, 80, 86 and 81 to maintain a fixed phase relation to each other. If the oscillator 60 is an alternator driven by a synchronous motor and if the

rectifiers

19, 80, 86, and 81 are also driven by a synchronous motor, the two can be maintained in correct phase relation by operating both motors from the same power network. In this way, the mechanical rectifiers are able to separate the two signals in phase quadrature and thereby actuate the sounder 83 or 88 in response to a signal of a corresponding phase.

While, as illustrated, dependence is had upon synchronous motors for driving the alternator and the rectifiers, if adequate stability is not available for keeping these elements in the proper phase relationship other known means may be provided.

For example, a phasing signal may be transmitted at intervals over the telegraph line in. order to correct the synchronism of the two systems. This phasing voltage preferably would be at 45 with each of the signal voltages and hence the outputs of the two sets of full wave rectifiers,

due to the phasing signal alone, would be equal if the phase of the rectifiers is correct with'respect to the phase of the transmitting system. If the outputs were unequal, the difference would be made to actuate a mechanism for bringing the motors back into the correct phase relation. This mechanism would be inoperative except during the time that the phase correcting signal was transmitted. Also, if desired, proper synchronisinmay be attained by providing very stable oscillators in the opposite ends of the system such as,- for example, tuning fork oscillators for transmitting the signal and for controlling the operation of the rectifier system at the receiving end.

The phase relations of the fork at the transmitting and receiving ends could be corrected at moderately long intervals by means of a special phasing signal, the actual correction being accomplished either automatically or manually, as desired. Many other systems of maintaining synchronism between motors at remote points are known in the art and it is thought that it is unnecessary to describe them in detail herein.

The type of signalling described above can also be applied to radio transmission systems. The simplest application involves the use of two modulating signals of the same audio frequency but in phase quadrature to each other. At the receiving end the output of the conventional detector of the radio receiver contains two signals, both having the frequency of the audio modulation used at the transmitter, and differing from each other in phase by 90 degrees. The detector output is impressed upon two phase-selective circuit branches so that the signals can be separated and made to operate independent signal indicating elements in the manner illustrated in Figure 2.

It is also possible to work directly with the phase of the carrier, using crystal controlled oscillators at each end of the transmission channel and phase correcting signals at intervals. Here the two branches of the receiving circuit are adjusted so that one responds with maximum eflioiency to a carrier of a particular phase, and discriminates against a carrier of approximately quadrature phase, while the other branch responds to the carrier of the second phase and discriminates against that of the first. The two components of the carrier are independently keyed or modulated at the transmitter, so that separate signals can be sent.

From the above described embodiments of the invention, it will be apparent that the present invention provides a system whereby two signals of the same frequency but difiering in phase can be transmitted over a single channel and then separated. It has the advantage that the phase selective elements may be at the receiving end of the system, an obvious advantage in well logging operations inasmuch as little space is available in the sonde for complex transmitting apparatus.

While the invention has been described with reference to specific forms of apparatus typical of the invention, it will be understood that the invention is applicable in other fields and may be 7 following claims.

I claim:

1. A signal transmittingsystem comprising a transmission channel, means for impressing on said channel a first electrical signal having a predetermined phase, means for simultaneously impressing on said channel a second electrical sistion, and means for operating said first and nal having a phase substantially in quadrature second rectifiers in phase with said first and with the phase of said first signal, phase selective second signals, respectively, to render each of means including a rectifier and an indicator assaid rectifiers selective to signals having the sociated with said channel and receiving said same phase and nonresponsive to signals subfirst and second electrical signals, and means stantially in quadrature with the selected phase.

electrically connected to the rectifier for com- 6. A signal transmitting system comprising bining with said first and second signals in said means for supplying to a single transmission rectifier local a voltage having agreater magnichannel first and second signals having the same tude than said first signal and the same phase. frequency and phases differing by about 90, first 2. A signal transmitting system comprising a and second rectifying means for receiving sigtransmission line, means for simultaneously supnals transmitted by said channel, and means for plying to said line first and second signals subsupplying electrical energy having the same frestantially in phase quadrature, rectifying means quency and phases, respectively as said first and connected to said transmission line and receivsecond signals but of substantially greater magniing said first and second signals, and means for tude to said first and second rectifying means, impressing on said rectifying means in conirespectively, in combination with said first and bination with said first and second signals a local second signal to render said first and second voltage of greater magnitude than said first and rectifying means selective to said first and second second signals and having the same phase as said 9 signals, respectively.

first signal to render said rectifier selectivet0 7. A signal transmitting system comprising an said first signal and substantially nonresponsive oscillator for supplying to a single transmission to said second signal. channel first and second signals having the same 3. A signal transmitting system comprising a frequency and phases differing by about 90, first transmission line, means for simultaneously sup- 5 and second mechanical rectifiers for receiving plying to said line first and second signals subsignals transmitted by said channel, and means stantially in phase quadrature, rectifying means for supplying electrical energy to said oscillator connected to said transmission lineand receiving and to said rectifiers for maintaining said first said first and second signals, means for impresand second rectifiers in phase with said first and sing on said rectifying means in combination with .0 second signals, respectively, to render said rectisaid first and second signals a local voltage of fiers selective to signals having substantially greater magnitude than said first and second the same phase as therectifiers.

signals and having the same phase as said first 8. A signal transmitting system comprising a signal to render said rectifier selective to said transmission channeLmeanS o impressing p first signal and substantially nonresponsive to the sending end of said channel two superimposed said second signal, second rectifying means conelectrical signals of the same frequency that are nected to said transmission line and receiving s n ially n Pha q a ure with each said first and secondsignals, and means for com- Other, phase selective means at the receiving end 'bining with said first and second signals in said of said transmission channel, said selective means second rectifying means another local voltage 40 compris W r nches, the first branch comhaving the same phase as said second signal but prising phase selective means for selectin One Of of substantially greater magnitude to render said the two received signals and reje ng the second second rectifying means selective to said second received Signal, the second branch comprising signal and substantially nonresponsive to said phase selective means f r s le ting the second first signal, a received signal and rejecting the first received 4. A signal transmitting system comprising a si n ltransmitting station, a receiving station, means A i nal transmi tin System comprisin a for transmitting from said transmitting station transmittin channel trans i t means for over a single channel to said receiving station impressing on the channel two super posed electwo signals having the same frequency and phases l0 trical signals 01' h m f q cy that are subdiffering by about 90, a pair or rectifying means a a y n phase quad ature heach r, at said receivingstation connected to receive said phase selective receiving means having W first and second signals, means for energizing one nch comm nica in With said channel,

of said rectifying means with said first and means for rendering one of said branches sesecond signals and a standard voltage having the ll lective t0 the Phase o n f ai Si nals and dissame phase as one of said signals but of subminating against the other sign and means stantiauy greater magnitude t render one of id for rendering the other branch selective to the rectifying means responsive to one of said sigphase of the other sign l and dis m n in nals, and means for energizing the other rectiagainst d On Signalfying means with said first and second signals M 10. A signal transmitting System Comprising a and a second standard voltage having substantransmitting cha nel. transmitting means for imtially the same phase as said other signal but of pr ssin on th cha tw s p p d elecsubstantially greater magnitude to render said trical signals of the same frequen y that ar other rectifying means responsive to the other substantiallyinphase quadrature with each other, signalto separate said signals according to their 05 phase selective receiving means having two phase. branches communicating with said channel, 5. A signal transmitting system comprising a means for suppl g a alternatin volta e at transmitting station, a receiving station, means the same frequency as said signals and of the at said transmitting station for providing first same phase as one of said signals to one of and second signals of the same frequency and said branches to render said branch selective to having phases differing by about 90 for transsaid one signal and discriminating against the mission over a single channel to said receiving sig als in quadrature with said one signal, and station, a first synchronous mechanical rectimeans for supplying an alternating voltage to fier at said receiving station, a second synchrothe other branch at the same frequency as said nous mechanical rectifier at said receiving stasignals and of the same frequency as the other l 1 signal to render said branch selective to said other signal and discriminating against said one signal.

11. A method of obtaining separate indications of at least one of two superimposed alternating current signals that are substantially in phase quadrature, comprising providing a standard periodically variable voltage having substantially the same phase as one of said signals but of sub-- stantially constant greater amplitude, combining said two signals and said standard voltage so that the amplitude 01' said combined signals and voltage varies in accordance with said one signal but is not substantially affected by said other signal of quadrature. phase, and utilizing said combined signals and voltage to produce effects in accordance with said one signal.

12. A method of obtaining separate indications of two superimposed alternating current signals that are substantially in phase quadrature, comprising providing a first standard periodically variable voltage having substantially the same .phase as one of said signals but of substantially greater constant amplitude, combining said two signals and said first standard voltage so that the amplitude of said combined signals and first standard voltage varies in accordance with said one signal but is not substantially afiected by said other signal of quadrature phase, utilizing said first combined signals and voltage to produce effects in accordance with said one signal. providing a second standard periodically variable voltage having substantially the same quadrature phase as the other of said two signals but of substantially constant amplitude, separately combining said two signals and said second standard voltage so that the amplitude of said combined signals and second standard voltage varies in accordance with said other signal of quadrature phase but is not substantially affected by said one signal, and utilizing said combined two signals and second standard voltage to produce efiects in accordance with said other signal of quadrature phase.

13. Apparatus for obtaining separate indications of at least one of two superimposed alternating current signals that are substantially in phase quadrature, comprising means providing a standard periodically variable voltage having substantially the same phase as one of said signals but of substantially greater amplitude, means for combining said two signals and said standard voltage so that the amplitude of said combined signals and standard voltage varies in accordance with said one signal but is not substantially affected by said other signal of quadrature phase, and means responsive to said combined signals and standard voltage.

14. Apparatus for obtaining separate indications of two superimposed alternating current signals that are substantially in phase quadrature, comprising means providing a first standard periodically variable voltage having substantially the same phase as one of said signals but of substantially greater amplitude, means for combining said two signals and said standard voltage so that the amplitude of the combination varies in accordance with said one signal but is not substantially ailected by said other signal of quadrature phase, means responsive to said combined signals and standard voltage, second means providing a second standard periodically variable voltage having substantially the same quadrature phase as the other of said signals but of substantially greater amplitude,second means for separately combining said two signals and said second standard voltage so that the amplitude of the combination varies in accordance with said other signal of quadrature phase but is not substantially aifected by said one signal, and means responsive to said combined two signals and second standard voltage.

CHARLES B. AIKEN.

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

UNITED STATES PATENTS Number Name Date 579,473 Davidson Mar. 23, 1897 1,813,929 Hough July 14, 1931 2,100,467 Borden Nov. 30, 1937 2,111,352 Blake Mar. 15, 1938 2,298,794 Howell Oct. 13, 1942 2,300,505 Hubbard Nov. 3, 1942 Certificate of Correction Patent No. 2,483,718 October 4, 1949 CHARLES B. AIKEN It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 6, line 49, for the word consists read consist; column 9, line 9, for local a read a local;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 14th day of February, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.