Патент USA US2411494код для вставки
Nov. 19, 1946. w. H. BLISS `2,411,494 RADIO RECEIVER Filed Aug. 18, 1942 mvENToR - ëßuss /Lovm/ ArroRNEY Patented Nov. 19, 1946 2,411,494 UNITED STATES- PATENT GFFICE 2,411,494 - RADIO RECEIVER Warren H. Bliss, Orono, Maine, assigner to Radio Corporation of America, a corporation of Dela Ware Application August 18, 1942, Serial No. 455,187 26 Claims. (Cl. Z50-_20) 2 This disclosure relates broadly to apparatus for scanning a given frequency band of the radio spectrum, locating any new signal which appears The relative speeds of rotation of the condenser 3 and tape 6 must be such that the loop of tape 6 makes one complete circuit while the condenser therein and controlling a device for making a 3 sweeps through a complete cycle of operation record of such a signal. twice. During each cycle of _operation of con In monitoring the radio spectrum for enemy denser 3, the receiver 2 will sweep through the and illegal stations, a transmitter which sudden desired portion of the radio spectrum to be ly goes into action sends a short message at high studied and will be ready for the next sweep. speed and then shuts down quickly is very hard The detected output from the receiver 2 is fed to detect. A primary object of the present inven through leads 41 and audio transformer 9 into a tion is to provide a method of and apparatus for pulse reshaper consisting of gas triodes I2 and detecting the signals from such unknown sta I5, sometimes known as “Thyratronsf’ This tions. pulse reshaper I2, I5 is sometimes referred to as An important feature of the present invention a trigger circuit which requires an initiating pulse lies in an arrangement which enables the receiver 15 to change the condition of equilibrium for a dura to scan or hunt continuously over a given band tion depending in part upon the constants of the of frequencies, and when a new signal is found circuit elements and in part upon the electrode voltages. , that was not there previously the receiver stops hunting and remains on the frequency of the new The term “reshaper” has here been employed signal so that it may be recorded. In order to because the circuit I2, I5 converts the shape of accomplish this result, a panoramic representa an initiating pulse into a substantially rectan tion of all signals in the band is set up, registered gular pulse for use by the recorder coil in a man or stored for reference or comparison purposes. ner described later. This registration may employ any suitable re These triodes I2 and I5 have plate resistors II cording system such as a magnetic tape. The in 25 and I4 and cathode resistors i6 and I9. A mag coming signals are then continuously compared netic recording coil -I is connected in series with with those registered on the tape. The sudden the cathode resistor I5 of triode I2. Normally, appearance of any new signal is quickly detected, that is, in the absence of an initiating pulse, tube which action causes it to be recorded in another I2 is non-conducting and tube I5 is conducting. apparatus as a continuous record available for 30 An initiating pulse reverses these conditions of study. current conduction of these two tubes. The grid A more detailed description of the invention of gas triode I2 receives the incoming pulses from follows in conjunction with a drawing whose transformer 9 over resistor I0. The grid of triode single figure illustrates a specific embodiment of l5 is connected to a supply of negative bias C~ this invention. 35 through a large resistor I8 which is shunted by a Referring to the drawing, unit 2 is any conven small condenser I'I. A condenser I3 is connected tional radio receiver with a motor driven Vernier between the plates of the two gas triodes, and its tuning condenser 3 which is connected in parallel value determines the duration of the reshaped with the main tuning condenser of the receiver. pulse. The motor which drives the condenser 3 is labeled 40 A magnetic pick-up coil 8 is located half way 5. It is preferred that the receiver sweep over around the tape from the recording coil 'I and is a desired portion of the radio spectrum, and that connected to an armature 35 of relay 3l having it follow a standard saw-tooth variation in oppositely disposed contacts 36 and 31,- by means which the forward sweep is very slow compared of which the coil 8 can be connected either to the to the return sweep. The main tuning condenser 45 grid of vacuum tube pentode amplifier 45 or to across which the Vernier 3 is connected may be the B+ plate supply through resistor 33. Pen the frequency determining element of a local tode 40 has the usual resistors and condensers oscillator in a, superheterodyne receiver, or where for class “A” operation. Its plate is coupled via a multi-stage tuned radio frequency receiver is condenser 28 to the grid of a vacuum tube triode employed, it is preferred that all tuning con 50 2l. This triode 2| has its plate connected densers of the different stages vary simultaneous through connection 46 to the cathode of gas tri ly by a unicontrol means. ode I2, and the cathode of tube 2| is coupled A magnetic clutch 4 is so arranged that the directly to the control grid of gas triode or motion of condenser 3 may be suddenly stopped “Thyratron” 2B. Resistors 23 and 22 are also at any position in response to energizing current 55 used for the grid and cathode, respectively, of in the coil of the actuating magnet of the clutch. vacuum tube 2 I . The motor 5 also drives a loop 6 of magnetic tape. Resistors 24 and 2l form a potential divider 2,411,494 d 4 between the B+ lead and ground to maintain a high negative grid bias on gas triode 2% by ele vating the cathode potential above ground. The plate circuit oi’ this tube contains in series the actuating coil ci relay 3l anda switch 3Q. ln the absence ci”` energizing current`.through the windingof relay 3i, contact pairs SZ-dß, 3G, 35, and Sil, ¿iii will be normally open, while contacts ing every sweep. Because each station has a given frequency, the signals from each station will always appear at the same position on the panorama. As the receiver continues to hunt over therspectrum, each station will have a rep resentative magnetic pulse on the tape and no new pulsesl will be recorded, since each incoming pulse will be applied at a point where it was pre ¿i5-3? will be closed. Contact 32 is connected viously registered. At this time, switch 30 should to the magnetizing coil of clutch @i and serves to 10 4be closed. Now, if new signals, of a different fre complete the circuit of this coil with the B+ lead quency than those previously registered, suddenly through a resistor 2S. IThe closure oi contacts appear in the Vspectrum being scanned, these will Sii-«555i causes the signal recorder 2d to commence formy new recorded spots, as will 4be explained recording. Recorder 25 is connected to the re later. ceiver 2 by means of leads ¿il and records't’n'e In order to compare the panoramic radio signals from the receiver. This recorder may be record with the constantly incoming signals, a any conventional type of device for permanently pick-up coil B and a comparator device are used. recording radio signals and operates to record Since the tape is two sweep cycles in length and only when contacts Sil-fifi are closed by the ac recorder1 coil 'l and pick-up f8 are spaced one cycle tion of relay ë l. 20 apart, the pulses picked up by coil il will be at the The operation of the system of the invention same phase as those coming from the receiver will now be described in greater detail: Switch through the reshaper or trigger circuit I2, I5. Sil, whose function is described later, is normally Triode 2l compares these incoming pulses with open (as shown). The radio receiver-2 is made those already recorded on the magnetic tape 6, to swing its tuning over a given frequency band of 25 since the incoming reshapedy pulses from the the radio spectrum by means of the small motor cathode of gas triode I2 are applied to the plate driven condenser 3 which operates in conjunction of this triode 2|, while the recorded pulses (as with the main tuning device of the receiver in picked up by coil ê and amplified by pentode 40) such a way as to cause it to scan over the given are applied to the grid of triode 2l. The pulse band of the radio frequency spectrum in an ap from the receiver applied to the plate of tube 2l proximately _linear manner. At the end of each must be of opposite polarity to the pulse simulta sweep, the tuning of the receiver very quickly neously applied to the grid of tube 2i by the pick reverts to its original position in preparation for up coil S through pentode lill. IThis condition can another sweep, or putting it in other words, the be assured by properly poling the terminal con tuning jumps back and starts over again. For nections of the pick-up coil 8. Since the pulse purposes of explanation, a sweep cycle of one sec ond duration may be assumed. As the receiver sweeps over a portion of the radio spectrum, each radio signal encountered will appear as_‘a- pulse of voltage in the output leads el’. , applied to the plate of tube 2i is always positive, the pulse applied to the grid oi tube 2l should always be negative and of a Value sufficient to prevent space current flow through tube 2l. 40 When a new signal suddenly appears in the ob These pulses are recorded on the‘loop of mag netic tape 6 which is exactly two- sweep cycles in length. Due to the fact that pulses from the re served band, the plate of triode 2i will receive a reshaped pulse for this new signal but since it has not been registered previously on the mag ceiver 2 vary widely in both shape and magnitude, netic tape the grid of this comparator tube 2i a reshaper or trigger circuit consisting of gas tri 43 will receive no matching or counterbalancing odes i2 and lâ is provided. Normally, tube |52 is negative pulse on its grid. Since the plateand non-conducting and tube i5 is conducting. grid pulses of stations already recognized are of opposite polarity and cancel each other, the ap-v plication of a pulse to the plate only of tube 2i lll, it passes through transformer S and is applied to the grid of tube l2, thus firing or initiating 50 caused by the appearance of a new signal will produce a pulse of plate current which flows conduction in tube i2, as a result of which cur rent is set up through the recording coil l. When through cathode resistor 22. tube i2 starts conducting, it changes the con This pulse of current in tube 2l, which occurs dition of charge on condenser i3, thus reducing only for a new signal, will be passed on as a volt the voltage on the plate of tube I2 below the arc age swing to trip or ñre gas triode 23. The firing sustaining value, as a result of which tube l5 of triode 2t energizes the coil of relay 3l which ceases conduction momentarily. The tubes l2 quickly closes contacts 25E-¿i3 and Sii-fifi. The and I5 regain their original status after a time closure of the former contacts causes magnetic interval depending upon the value of condensers clutch ¿l to arrest the motion of auxiliary tuning i3 and il. ïn this_ manner a comparativelyy 60 condenser 3, thus leaving receiver 2 tuned to the square pulse of current is obtained from the cath frequency of the station which has just been ode of tube »l2 for energizing the recorder coil l found. The operation of relay 3l by virtue of for each received pulse, irrespective of its char the closure of contacts 3e, ¿lil also causes recorder acter. Thus, all pulses received on the receiver 20 to go into operation and start registering the When a pulse from the receiver appears in leads 2 irrespective of their shape and magnitude, are recorded on the tape alike. Since the magnetic tape 6 is synchronized with the frequency sweep of the receiver, a panorama of the radio signals in the chosen band of the radio frequency spectrum is set up or registered cn the tape as soon as the tape has made a few revolutions. In the case of keyed continuous waveY signals, several sweep cycles may be neces' sary tc set up the complete panorama because the signals of a given station will not be present dur newly found signal. The operation of relay 3i also opens contacts 37-3‘5 and closes contacts ¿iii-35 to apply direct current to pick-up coil il for magnetic erasing. While the registered signals on tape ß are being erased, the signal recorder 2€? is Vreceiving the newly found signals. The relay Si remains en ergized until switch 3Q is opened manually at the will of the operator after the new signals have been recorded for a desired interval of time. The panorama of signals may then be set up again 2,411,494 5 6 on the magnetic tape after which switch 30 can be reclosed to put the system in readiness for hunting another new signal. and adapted to open the link between said motor and said receiver, means responsive to the de tected output of said receiver including a trigger It should be noted that application of pulses circuit and a. magnetic tape recorder coupled to to the grid of tube 2| in the absence of voltage Ol said trigger circuit for producing recordings rep applied to the plate of this same tube will have resentative of the signals received within said no effect on the signal recorder 20. Also, gas spectrum, a permanent recorder, an electron dis triode 2G is normally biased well below the firing charge device circuit operative solely by a signal point (i. e., the critical point at which conduc received within said spectrum but which does not `tion starts), but when this tube 25 is triggered by 10 register with the recordings on said tape for op the pulse from a new signal through the inter eratively connecting said last recorder to the out mediary of vacuum tube 2l, it maintains a state put of said receiver, a relay coupled to said elec of conduction until it is reset to its normal condi tron discharge device circuit and operative solely tion of non-conduction by opening switch 3S. upon the flow of space current through said de Relay 3l and magnetic clutch 4 must be high vice, a pair of contacts for said relay one of which speed devices so that the hunting or scanning is connected to the energizing coil of said mag action of the receiver can be stopped instantly; netic clutch and the other of which is connected otherwise the receiver tuning would pass on to a to a source of potential, whereby operation of said different frequency than that of the newly found electron discharge device circuit operates said station. This stopping action is preferably as 20 relay and causes the energization of said clutch sured by suitable circuits for compensating or with a resultant fixation of the receiver response correcting for undesirable overshooting of the re ceiver tuning because of mechanical factors. In order to compensate for the possibility that mag netic clutch 4 may not be able to arrest the mo tion of condenser 3 quickly enough, when a new signal is found, so that the tuning of the re ceiver will not be swept beyond the frequency where the new signal is found an additional iixed condenser lill of suitable value is arranged to be switched in parallel with the Vernier scanning condenser 3 at the same instant that the mag netic clutch Il is energized. The rotation and design of condenser 3 must be such that its ca band. 3. In a radio receiving system, means for con tinually and repeatedly changing the receiver re spense band over a predetermined portion of the vradio frequency spectrum, means responsive to the detected output of said receiver for producing indications representative of the signals received within said spectrum, and means operative solely in response to a new signal received within said spectrum which does not register with the fre quency positions corresponding to sa-id iirst indi cations for producing an indication of another kind and for stopping movement of the receiver pacitance varies linearly from its maximum to its ‘ response band. minimum value during the sweep cycle and then li. In a radio receiving system, a tuning element snaps back quickly to its maximum value at the controlling the receiver response band, a motor end or the sweep cycle. The value of condenser having a drive shaft linked to said element for 50 can be set to exactly equal the amount of ca continually and repeatedly changing the receiver pacitance overshoot of condenser 3. The use of 40 response band over a predetermined portion of an additional pair of normally open contacts the radio frequency spectrum, a magnetic clutch ¿iS-£9 on relay 3l 'will switch in the compensat between said shaft and said receiver, an output ing condenser 5i) at the proper instant. Another circuit for said receiver, a trigger circuit coupled way of obtaining this compensation is to employ to said output circuit for converting the shape of known types of automatic frequency control to fr' a voltage pulseappearing therein to substantially pull the receiver into exact tuning with the newly rectangular wave form, a, magnetic tape having found station when stopped in the vicinity of a recorder coil in circuit with said trigger circuit the carrier of the unknown station. for operation from said rectangular wave form A disadvantage of this last method of com pulse, said magnetic tape having such length and pensation resides in the fact that if the newly being driven at such- speed that it travels one found station has a frequency close to a strong complete revolution for an integralI number of known station, the receiver might be pulled into cycles of travel of the tuning element, a pick-up tuning the known station rather than the newly coil for said magnetic tape positioned at a prede found station. termined location on said tape, a normally inop What is claimed is: erative signal recorder also coupled to the output 1. In a radio receiving system, means includ circuit of said receiver, and a comparator device ing a motor for continually and repeatedly in circuit both with said trigger circuit and said changing the receiver response band over a pre pick-up and responsive to signal pulses from said determined portion of the radio frequency spec rigger circuit which do not register with previous trum, means responsive to the detected output (il) recordings for rendering said signal recorder op of said receiver' including a trigger circuit and a erative and for energizing said magnetic clutch magnetic tape recorder coupled to said trigger to disengage said motor drive shaft from said circuit for producing recordings representative tuning element. of the signals received within said spectrum, a 5. A receiving system in accordance with claim permanent recorder, and an electron discharge 65 4, including a circuit operative in dependence device circuit operative solely by a signal received upon the operation of comparator device for cor within said spectrum but which does not register recting the tuning of said receiver to assure the with the recordings on said tape for operatively stopping of said receiver on the frequency within connecting said last recorder to the output of said spectrum which corresponds to that of the said receiver. signal causing the operation of said comparator 2. In a radio receiving system, means including device. a motor for continually and repeatedly changing 5. In a radio receiving system, means for caus the receiver response band over a predetermined ing the receiver to repeatedly scan a predeter portion of the radio frequency spectrum, a mag mined portion of the radio frequency spectrum, netic clutch between said motor and said receiver means for producing indications representative of .2,411,494 8 'the signals within said spectrum, and means for `automatically recording solely those signals with termined portion of the radio frequency spectrum, means for producing indications representative of in said spectrum which were ynot present during the previous scanning operation. paring the indications with the indications pro- . the signals within said spectrum, means for com '2. In a radio receiving system, an antenna duced in previous scanning operations, a recorder, a relay for controlling the operativeness of said therefor, means for causing the receiver to re ne ,tedlv scan a predetermined portion of the recorder and ior controlling said clutch, an elec radio frequency spectrum, means for producing tronic circuit for controlling said relay, said elec indications representative of the signals within tronic circuit being responsive solely to new sig said spectrum, a recorder, and circuit means re 10 nals within said spectrum which have not caused sponsive solely to new signals within said spec indications during the preceding scanning opera trum which do not register with the frequency tions for operating said relay to thereby opera positions corresponding to said indications for tively connect said recorder to the output of said operatively connecting said recorder to the out receiver and to thereby simultaneously energize put of said receiver and for simultaneously stop 15 said clutch to stop the scanning of said receiver _ping the scar-„ning of said receiver approximately approximately at the frequency position of the new signals. at the frequency position of the new signals. 8. The method of operating a radio receiving ifi. In aradio receiving system having a tuning system having a circuit for detecting the signals element, an antenna therefor, means including a impressed thereon which includes the steps of 20 motor and magnetic clutch between said motor automatically and cyclically varying the tuning and a tuning element of said receiving system for of the receiver over a predetermined range of causing the receiver to repeatedly scan a prede frequencies, and utilizing any newly detected termined portion of the radio frequency spectrum, means for producing indications representative of signal received by said receiver in said range which was not present during an immediately the signals within said spectrum, means for com preceding cycle of variation for automatically paring the indications with the indications pro stopping said cyclical variation at the point at duced in previous scanning operations, a recorder, a relay for controlling the operativeness of said which. the new signal is detected. recorder and for controlling said clutch, an elec The method of operating radio receiving system which includes the steps of automatically 30 tronic circuit for controlling said relay, said elec tronic circuit being responsive solely to new sig cyclically varying the tuning of the receiver over l a predeten nals within said spectrum which have not caused „ed frequency range, comparing the indications during the preceding scanning opera time phase of signals appearing in successive tions for operating said relay to thereby opera cycles of variation, detecting any new signal re tively connect said recorder to the output of said ceived by said receiver in said range which was receiver and to thereby simultaneously energize not present during an immediately preceding said clutch to stop the scanning of said receiver cycle ci variation, utilizing the detected new sig approximately at the frequency position of the nal to stop variation in tuning, and automati new signals, and reactance means also under con caily recording solely the intelligence conveyed by 40 trol of said relay for assuring that the tuning of said new signal. the receiver does not sweep beyond the frequency l0. The method ci operating a radio receiving position of the new signals. ` system which includes the steps of automatically l5. The method of operating a radio receiving and cyclically varying the tuning of the receiver system which includes the steps of repeatedly over a predetermined frequency range, and uti scanning a predetermined portion of the radio lizing any new signal received by said receiver in said range which was not present during an im frequency spectrum, causing signals emanating mediateiy preceding cycle of variation for automatically causing the recording of the new signal Aand simultaneously stopping the variation of the from identiñable transmitting stations in said p0r tion of the spectrum to produce one type of indi cation in said receiving system, and causing a new signal in said portion of the spectrum which ema nates from a newly found transmitting station receiver at substantially the point at which the new signal is detected. ’ 1l. in a radio receiving system, an antenna therefor, means' for causing the receiver to re peatedly >scan a predetermined portion of the suddenly appearing during the scanning of said spectrum to produce another type of indication, and automatically recording solely the new signal to the exclusion of the other signals emanating radio frequency spectrumv` means for producing from the identifiable transmitting stations and indications representative of the signals within occurring simultaneously with the new signal. said spectrum, a recorder, circuit means responsive solely to new signals within said spectrum which 16. The method ci operating a radio receiving do not-register with the frequency positions corre system which includes the steps of uniformly, sponding to said indications for operatively con 60 automatically and repeatedly scanning a prede necting said recorder to the output of said receiver termined portion oi the radio frequency spectrum, and for simultaneously stopping the scanning of producing indications representative of the sig said receiver approximately at the frequency posi nals Within said spectrum, utilizing a new signal tion of the new signals, and means for assuring appearing in said portion of the spectrum which that the tuning of the receiver does not sweep was not present during the previous scanning op beyond the frequency position of the new signals. eration and for which'there is no corresponding A system in accordance with claim fl, in indication to automatically stop the scanning oi cluding means for assuring the stopping of the re the receiving system at that position which sub ceiver tuning at the position corresponding to th stantially leaves the receiving system tuned tothe frequency of the newly found signals. new signal, and automatically recording solely said i3. In a radio receiving system having a tuning new signal. ' element, an antenna therefor, means including a i7. The method of operatinga radio receiving motor and a magnetic clutch between said motor system which includes the steps of automatically and a tuning element of said receiving system for and repeatedly scanning a predetermined portion Vcausing the receiver to repeatedly scan a prede of theV radio .frequency spectrum, detecting the sig 2,411,494 10 nals appearing in said portion of the spectrum, device coupled to the output of said trigger cir cuit, and an electromagnetic relay under control of said electron discharge device circuit, said re lay having contacts connected to said signal re and utilizing a new signal appearing in the spec trum which was not present during the previous scanning operation to automatically stop the scanning of the receiving system at that po-sition which leaves the receiving system substantially corder. 23. lin a radio receiving system, a tuning ele ment for said receiving system, a magnetic clutch, tuned to the new signal. 18. The method of operating a radio living system which includes the steps of automatically and repeatedly scanning a, predetermined portion of the radio frequency spectrum, producing indi cations representative of the signals within said portion of the spectrum, automatically comparing the positions of said indications appearing in suc a motor linked to said tuning element through said magnetic clutch for continually and repeat edly changing the receiver response band over a predetermined portion of the radio frequency spectrum, means responsive to the detected out put of receiver for producing indications rep resentative of the signals received within said spectrum and for comparing said indications with the indications representative of signals re ceeding scanning operations, and automatically recording solely a new signal in the spectrum which was not present during the previous scan ceived during previous scanning' operations, and ning operation. means operative solely in response to a new signal 19. The method of operating a radio receiving' system which includes the step of automatically scanning a predetermined portion of the radio frequency spectrum at a constant and relatively slow rate, automatically restoring the tuning of received within said spectrum which has not caused an indication during the preceding scan ning operation for energizing said magnetic clutch to thereby stop movement of said tuning element. 24. In a radio receiving system, a receiver hav the receiver to its starting condition at a fast ing a tuning element, an antenna coupled to the rate immediately after the scanning operation in 25 input of said receiver, means including a motor order to commence rescanning of the same range and a magnetic clutch linked to the shaft of the of frequencies, automatically repeating said scan motor for moving said tuning element and caus ning operations, detecting a new signal in the ing the receiver to repeatedly scan a predeter spectrum which was not present during the pre mined portion of the radio frequency spectrum, vious scanning operation, and utilizing said new means for producing indications representative signal to automatically stop the scanning of the system at the frequency position of the new sig of the frequency positions of the signals within said spectrum, a recorder coupled to the output of said receiver, a relay for controlling the eiîec tiveness of said recorder and for controlling the energisation of said clutch, and electronic circuit nal, and continuously recording the intelligence conveyed by the new signal. 20. In a radio receiving system having a cir cuit for detecting the signals impressed thereon, means responsive solely to a new signal within said spectrum which has not caused an indica means for continually and repeatedly changing the receiver response band over a predetermined tion during the preceding scanning operation for portion of the radio frequency spectrum, means operating said relay to thereby operatively con responsive to the detected output of said receiver 40 nect said recorder to the output of said receiver for producing indications representative of the and to thereby energize said clutch for simulta signals received within said spectrum, a signal re neously stopping the movement of said tuning corder, and means responsive to a new signal element approximately at the frequency position received within said spectrum which has not of the new signal. 25. A frequency spectrum monitoring system caused an indication during the preceding scan ning operations for operatively connecting said recorder to the output of said receiver. comprising a radio receiver having a tuning ele ment for tuning said receiver over a band of fre duencies, means including a motor having a shaft 21. The method of operating a radio receiving system which includes the steps of repeatedly linked to said tuning element for periodically and automatically causing said receiver to scan said scanning a predetermined portion of the radio frequency spectrum, causing signals emanating hand from one end to the other at a rate of the order of once per second, signal storage means from identifiable transmitting stations in said portion of the spectrum to produce one type of for producing indications representative of the signals within the band being scanned, an elec tronic circuit responsive to a new signal received by said receiver and which does not correspond in time position to any of said indications for indication in said receiving system, causing a new signal in said portion of the spectrum which ema nates from a newly found transmitting station suddenly appearing during the scanning of said spectrum to produce another type of indication, automatically stopping said motor approximately at the frequency position in and utilizing said new signal to stop the scan ning of the system at approximately the fre quency position of the new signal. toring the output of said receiver. 2€. The method of operating a radio receiving system which includes the steps of automatically and cyclically varying the tuning of the receiver 22. In a radio receiving system having a circuit for detecting the signals impressed thereon, means for continually and repeatedly changing the receiver response band over a predetermined spectrum at 60 which said signal appears, and means for moni “ over a predetermined frequency range, establish portion of the radio frequency spectrum, means responsive to the detected output of said receiver for producing indications representative of the signals received within said spectrum, a signal ing successive patterns representative of the sig nals received during successive cycles of variation, automatically comparing the time phase of the signals of a pattern with the time of occurrence recorder, and means responsive to a new signal 70 of signals received during a preceding pattern, and detecting solely a new signal received by said received within said spectrum which has not receiver in said range which does not correspond caused indications during the preceding scanning in time phase with a signal of said pattern. operations for operatively connecting said record er to the output of said receiver, said last means including a trigger circuit, an electron discharge 75 WARREN H. BLISS.