US2409012A - Radio receiving system - Google Patents

Description

Oct. 8, 1946.

w. H. BLISS 2,409,012

RADIO RECEIVING SYSTEM Filed Sept. 24, 1942 s Sheets-Sheet 1 Ju spusrspooru: I? la CoMPnmroe i RECEIVER Mow/rap Pfflornuc TUBE 01.490017 mrwwwmn I #2 PUL SE WARREN BLISS #2 (fa/F552 ATTORNEY 0d. 8, 1946. w uss RADIO RECEIVING SYSTEM Filed Sept. 24, 1942 3 Sheets-Sheet 3 fscoko 0F A/EWJ/wm. 011/ 729, 5

| l l l l I M17701, Oar ur OFPULSE smsparopw s 4 I l I l F/AML OUTPUT OF'PULSE' ssmsknroen ENVENTOR WARREN H. BLISS BY Hg Z W ATTORNEY Patented Oct. 8, 1946 2,409,012 RADIO RECEIVING SYSTEM Warren H. Bliss, Orono, Maine, assignor to Radio Corporation of America, a corporation of Delaware Application September 24, 1942, Serial No. 459,504

11 Claims.

This invention relates broadly to apparatus for scanning a given frequency band of the radio spectrum, locating any new signal which appears therein, and controlling a device for making a record of such a signal. Reference is herein made to my copending applications Serial Nos. 455,187 and 457,282, respectively, filed August 18, 1942, and September 4, 1942, for other inventions of mine relating to similar subject matter.

In monitoring the radio spectrum for enemy and illegal stations, a transmitter which suddenly goes into action, sends a short message at high speed, and then shuts down quickly is very hard to detect.

An object of the present invention is to provide a method of and apparatus for detecting the signals from such unknown stations.

In brief, the present invention is a scanning receiver for detecting telegraph stations within the range of frequencies scanned by the receiver and recording the signals therefrom. For achieving the results of the invention, there is provided a superheterodyne receiver which is swept or tuned rapidly over a given band of frequencies by means of a sweep device. The output of the receiver is arranged to be in the form of keyed tone whose modulations correspond to the modulations of the received signals. This keyed tone is supplied to a comparator device in which the signals of the known stations in the band of frequencies scanned by the receiver are neutralized or blanked out, so to speak, by locally generated pulses. To achieve this result, there are provided as many local blank-out pulse generators as there are different known stations in the band to be scanned. Thus, the presence of a new'signal suddenly appearing in the frequency spectrum being scanned will have no compensating or blank-out pulse and can be registered on a short loop of magnetic tape which is exactly two sweep cycles in length. This new signal from an unknown station can then be continuously recorded by means of a second receiver whose tuning is automatically adjusted by a motor control circuit to the particular position in the frequency band being scanned in which the new signal appears.-

In order to prevent the second receiver from responding to noise pulses which are random in nature and time of appearance, the motor control circuit for the second receiver is controlled by a comparator device. This comparator device will enable the motor control circuit to function, pro-- vided the new signals are repeated during consecutive sweeps of the scanning superheterodyne receiver.

A feature of the invention lies in the system of comparator device which passes onto the motorcontrol circuit for the recording receiver only those new 'signals'which persist in consecutive sweeps of the scanning receiver.

Another feature of the invention comprises the circuit arrangement for stopping the tuning of the recording receiver at the particular point of the frequency spectrum being scanned at which the newly found signals appear.

A more detailed description of the invention follows in conjunction with'drawings wherein:

Fig. 1a and Fig. 1b, taken together, illustrate diagrammatically a specific embodiment of the invention;

Fig; 2 graphically illustrates the manner in which the tuning of the recording receiver is stopped at the particular portion of the frequency spectrum being scanned at which the newly found signals appear and Fig. 3 graphically illustrates the operation of the 360 phase shifter of Fig. 1.

Referring to Figs. 1a and 1b in more detail, there is shown a supe'rheterodyne receiver I indicated diagrammatically in box form. The heterodyne oscillator to the receiver (not shown) has its frequency controlled by a parallel tuned circuit 2. The receiver 1 is designed to receive telegraph signals, and is provided with a suitable tone keying circuit for supplying interrupted audio tone whcse'interruptions correspond to the modulations of the interruptions of the received telegraph signals; These keyed tone signals appear in the output circuit 3 of the superhete'rodyne receiver for subsequent use in the comparator device 4 to be described in more detail later. For causing the receiver to periodically scan a predetermined portion of the radio frequency spectrum, there is provided a reactance tube circuit 5 of a Well-known type, whose anode is connected to the tuned oscillatory circuit 2 of the heterodyne oscillator of the receiver, and whose control grid is connected to the output of a sweep device 6 which generates saw-tooth waves. Since the reactance tube circuit 5 is well known in the frequency modulation art for providing a'variable'reactance output dependent upon the modulating voltages applied to the grid of the device. it is not believed necessary to describe this circuit further.

The sweep device 6 for generating sawtooth oscillations comprises, in the particular embodiment shown, a gaseous tube 1 which is arranged to rapidly discharge the condenser 8 through the space path of the tube after the charge on the condenser 8 has reached a critical value. A vacuum tube pentode 9 serves to charge the condenser 8 linearly. A sixty cycle synchronizing source, labeled as such, assures the breakdown of the gas tube 1 at a desired point in the cycle of operations. The output of the sweep device 6 is connected through lead H) to the grid of the reactance tube 5 to provide sixty cycle saw-tooth waves per second to the reactance tube 5. As a result of this, the superheterodyne receiver will be caused to scan at a substantially uniform rate a predetermined portion of the radio frequency spectrum, after which the receiver will return to normal and repeat the scannin operation at the rate of sixty complete scanning operations per second. The range of the band of frequencies to be swept or scanned by the superheterodyne receiver is determined to a large, extent by themagnitude of the saw-tooth variations of the sweep circuit, and the position of the band in the spectrum is adjustable by the usual controls in the receiver.

A multiplicity of blank-out pulse generators with their respective associated phase shifters are herein designated by the legends #1 pulse generator and #2 pulse generator. Although only two generators are shown, there are usually provided as many pulse generators as there are known stations in the frequency band to be scanned whose signals are to be blanked out.

If, in the operation of the system, there are more pulse generators than known stations, the excess number of pulse generators may be rendered inoperative by a suitable adjustment in a manner to be described later. All blank-out pulse generators are connected in parallel to the lead 21.

Since the pulse generators are identical in construction and operation, it is deemed necessary to describe only one of these, let us say #1 pulse generator.

This pulse generator is supplied with energy from the sixty cycle synchronizing source over leads I I through reversing switch I2 and phase shifter l3. The phase shifter consists of a sixty cycle transformer whose primary windingis connected to the reversing switch I2 and whose secondary winding has shunted across it a condenser and a variable resistor 24. The junction of the condenser and variable resistor is connected to the grid of a gaseous discharge device l4 (generally referred to as a 'I'hyratron) The center point of the secondary winding of the sixty cycle transformer is connected to a bias resistor l5 which supplies negative bias for the grid of the glow tube l4. The condenser l6 across the'glow tube I4 is arranged to be charged through the variable resistor 11 and to be discharged through the space path of the tube at a sixty cycle rate. The voltage from condenser I8 is supplied to a trigger circuit i8 composed of a pair ofvacuum tube triodes l9 and 20.

Normally, tube 20 is in the state of conduction and tube l9 non-conducting. When a pulse is impressed on the grid of tube I!) from tube 14, the state of conduction of the two tubes 19 and 20 is reversed. These two tubes, in effect, comprise a trigger circuit which always tends to be restored to the condition where H! is non-conductive and 28 is conductive. This restoration takes place at a short interval after conduction starts in. tube l9, and is determined by the values of resistor 62 and condenser E3. When tube I9 becomes conducting, its plate potential immediately drops for the duration of the conduction period, as a result of which a pulse of negative polarity is fed back via lead 21 to the comparator 4 At this time, when tube l9 becomes conducting, tube 20 will become non-conducting by virtue of the regenerative nature of the feed-back circuit of the trigger arrangement. The value of condenser 63 and the value of resistor 62 determines the time of conduction of tube I9 and hence the time of duration of the pulse impressed on lead 2|. Cathode resistor 64 furnishes the common bias for the grids of both tubes l9 and 20.

The particular time of concurrence or position in the sweep cycle of the pulse obtainable from th trigger circuit I8 is controlled by adjustment of the phase shifter 13 and more specifically by the adjustable resistor 24 in the phase shifter. Since the adjustment of the resistor 24 of the phase shifter will only provid a variation in position of the pulse obtainable from the trigger l8 over substantially one-hair" or 180 of the sweep cycle, it thus becomes necesssary to provide means for changing the position of the pulse in the sweep cycle over the entire cycle and this is accomplished by means of the reversing switch l2 which by its operation enables the adjustable resistor 24 of the phase shifter to control the position of the pulse obtainable from the trigger circuit IQ for any desired portion of either of the two halves of the cycle. It should at this time be understood that the adjustment of the position of the pulse obtainable from the pulse generator is important because this position must be made to correspond to the position of the signal received from a known station in order to be able to blank-out the signal from the known station in the comparator 4. The adjustment oi the duration of the blank-out pulse obtainable from the pulse generator is also important because the duration of the blank-out pulse must correspond to the duration of the signal received from the known station. The appearance of the blank-out puls in the lead 2| will cause a relativel negative pulse to be applied to the comparator 4 for reasons which appear hereinafter.

As for the comparator 4, this circuit comprises a pair of push-pull vacuum tube amplifiers 25 and 26 operating as class B; that is, normally biased to a point of anode current cut-off. The grids of these amplifiers are connected to opposite terminals of the secondary winding of a single input transformer 21 which is designed to pass the keyed tone appearing in the output circuit 3 of the receiver. The combination of resistors I58 and [BI give the proper bias for the grids of the tubes 25 and 25 by raising the cathode potential to a desired amount above ground potential. By making the cathode positive, I in effect make the grid negative since the grid is tied down to ground potential. The anodes of the tubes 25 and 26 of the comparator 4 are connected in push-pull to the opposite terminals of audio output transformer 28, also designed to pass the tone. In the operation of the comparator 4, the vacuum tubes 25 and 26 will normally pass the alternate half cycles of the tone appearing in the transformer 21. However, the application of a negative blank-out pulse from one of the pulse generators over lead 2| will supply an additional negative bias to the grids of the tubes 25 and 26 which will prevent these tubes from passing current during the application of the blank-out pulse even in the presence of tone signals on input transformer 21. It will thus be seen that by means of the pulse generators it is possible to prevent the comparator 4 from passing current at any particular time and for any desired duration in any sweep cycle.

parator 4 and which are representative of a new or unblanked signal. s

The magnetic tape '65 is continuously driven in the directionindicated by the'arrow by motor M which is synchronously controlled by the same sixty cycle source which is associated with the sweep device 6. Loop 66 is exactly two sweep cyclesin length; that'is, the relative speeds of scanning ofthe receiver l and rotation of loop 66 must be such that the loop of tape '66 -makes one complete circuit while the receiver 'l sweeps through a complete sweep cycle twice. During each sweep cycle of the receiver I, a desired portion of the radio spectrum to be studied will be scanned and the receiver will be ready for the next sweep A magnetic pick-up coil Q61 'isvlocated half way around the tape 66 from the recording coil 65 and spaced one complete sweep cycle apart along thetape. k n v s The pick-up coil 61 is connected to a switch 58 which in one position (namely, the left) can erase the signalsrecorded on the tape 66, while in the other position (namely, the right-hand position) passes thepicked up signals to the am 'plifier 69. n The output of the amplifier 69 is in the form of amplified pulses which have been picked up by the pick-up coil 61. 7 These amplified pulses are passed on to a second comparator 90 where they are rectified in tube 10 and applied to the anode ofatriode H. The

purpose I of comparator 90 is to prevent noise pulses from being passed on to operate the motor control circuit 180. This second comparator includes 'a pair of rectifier tubes 10 and 12, and also a vacuum tube triode I l. I p

Any newly foundsignal will appear in the output of trans'iorrner 128 of the comparator! in the form of keyed tone pulses. This newly found signal will be rectified in rectifier 12 of the second comparator and i'mpressed upon the grid of the vacuum tube triode I I, at thesame time that the signal is applied to the recorder coil 65. If this newly found signal has appearedin a previous sweep cycle, it will have been registered on tape 66. recorder coil65 corresponds in time to an impulse registered on thetape 66 and simultaneously picked-up by coil B1,it will be seen that both rectifiers l and 12 will at the 'sametime be impressing positive potentials on both the grid and anode electrodes of tube H. This condition will cause the tube H to pass current, as a result of which a pulse of voltage will be taken off the cathode of triode ll and passed on to the motor control circuit 80. The comparator #2 serves to prevent'noise from entering the motor control pulse corresponding to thisnoise'which will pass rectifier "Hand be impressed upon the'g'rid of vacuum'tubell, but there will not be'a correspondingly-timed pulse picked up by coil 61, am-

plified by19, rectified by tube and impressed on the'anode ofll. The application of'apulse solely t'othe grid' or to the anode of tube II will not cause this tube to-pass current since thecircuit elements are s'o'constructe'd and designed that there is required the application of pulses When an impulse impressed on to the anode and to the grid of tube H simultaneously in order to cause this tube to pass current. Thus, only when a newly found signal appears (which of course will be repeated during consecutive sweeps of the receiver) will the comparator!!!) pass current to the motor control circuit.

The motor control circuit comprises two gas triodes (Thyratrons) I8 and 19, both of which are normally non-conductive. It should be noted that the path to operate the grid of tube 18 comprises connection 15 and includes the armature and normally closed contact of relay ll. Relay 1'! is in the cathode circuit of gaseous conduction tube 18 while relay 16 is in the anode circuit of gaseou conduction tube 19. The application of a positive'pul'se'from the comparator 9D to the lead 15 will cause a voltage to pass through the contacts of the relay 1! and ignite tube 18 of the motor control circuit. This will cause tube 18 to pass current and operate relay ll. The operation of the relay" will break the path from the lead 15 to the grid of the tube 18 and will connect this path to the grid of tube 19. At the same time, the operation of relay 1''! will connect ground to lead 14 to operate the spottingmotoF M, which, in turn, causes the conventional recording receiver 9| to become operative and change its tuning over the same band of irequencies which is being scanned by the superhet-erodyne receiver I. v s

The manner in which the tuning of the receiver 9| over the band of frequencies being scanned is stopped at the exact point at which the newly found station appears in the spectrum 'being scanned will now be described. Receiver 9| is initially tuned to one end of the band of frequeneies being investigated. The tripping of the motor control circuit 80 and the consequent operation of the motor M starts the receiver 9| and causes the frequency of the receiver 9| to which it is initially tuned) tochange under the drive of the spotting motor M. The motor-M also drives the phase shifter BL over shaft I82 simultan'eously with the change in tuning of receiver 9|. This phase shifter Bl is a 360"motor driv'en'phase shifter which issynchronously controlled over leads 8?! from the same sixty cycle synchronous y source which synchronizes the sweep devices and the pulse generators, as'well as synchronizing the motor 'M'. The phase shifter 8| includes a pair-of potentiometers 92 and '93 with continuous'resistance elements and sliders S4 and 95' th'at c'an'rotate 360. The resistance' elements are tapped every The two potentiometer'sare ganged and arranged so that, if corresponding taps are'in line, the "sliders 9'4 'and '95 are mutually perpendicular. A voltage from synchronizing leads "82 is applied through a transformer "91 to terminals C and D on pctentiometer S3, andthe same magnitude of voltage applied through 90 phase shifter 98 and transformer "99 to' terminals A and B onthe other unit 92 butshifted 90 with respect to the voltage applied to terminals C, D. The output of the '360-'phase shifter!!! taken from the sliders 9'4 "and is fedthro'ugh transformer'flfito the pulse generator. r

The vectordiagram' of Fig. 3-shows the voltagesat the taps ofthe twore'sistiveelements. Points 0 "and O 'onbothelementsare all at the same potential or referenc point'as show cn the vector diagram. As slider 94 "rotates, the magnitude but not the phaseb-f the voltage which itpicks upvaries' fromvector 0A to vectorOB .erator and #2 pulse generator.

'7 and backagain to A. This is an amplitude variation with no variation in phase. Likewise the voltage from slider 95 varies from O0 to ODand back to OC.

The output taken through transformer 96 is the vector sum of the slider voltages. This out-- put, indicated as vector OR will have a locus of variation as indicated by the dotted line on Fig. 3. As the common shaft I82 of the potentiometers 92 and 93 is rotated, the phase of vector OR, representing the output voltage, will advance continuously in the direction of th arrow from its initial or reference position 00 and for one complete revolution of this shaft I82 the phase of the output voltage will change by 360.

A short pulse generator labeled #3 Pulse generator" is provided which is identical in construc- .tion and operation to the circuits of #1 pulse'gen- Initially, the pulse developed by this #3 pulse generator apnearing in lead 83 has a phase as indicated bythe pulse in line B of Fig. 2. As the phase shifter 8| is driven by the motor M, the position of the pulse in the output lead 83 of the #3 pulse generator advances over the sweep cycle until such time as it coincides in time with a pulse of the newly found station. This is indicated in line C of Fig. 2. The pulses in lead 83 coming from #3 pulse generator are impressed on the grid of tube 19 of the motor control circuit 89. As long as the pulse of the newly found signal appearing in lead does not coincide in time with the pulse appearing in lead 93, tube 19 will remain nonconductive. When, however, a pulse in lead 83 coincides in time phase with a pulse appearing in lead 15 from the output of comparator 90, it will be seen that both pulses will be impressed on the grid of gas triode I9 simultaneously. Since these pulses are positive, the simultaneous appearance of both pulses in leads 15 and 83 will ignite tube 19, as a result of which relay 16 will operate. The operation of relay 16 will break the anode circuit of tube 78, thus extinguishing the arc in tube 18 and causing relay '1! to release. It

is assumed, of course, that the manually op- L .erated switch 84 supplying positive polarizing potentials to gas tubes 18 and 19 is normally closed during all this time, it being opened only when it is desired to restore the complete circuit to normal. The release of relay 11 will open its right-hand contact, thus stoppin the motor M at a point which corresponds to the point at which ;the newly found signal appears in the frequency spectrum. The tuning of the receiver 9| will be stopped at this point in the radio frequency spectrum. The standard code recorder 85 which operates continuously from the time receiver 9| is made to be operative by the motor M, will now record the signals of the newly found station collected by receiver 9| over antenna I92. Putting it in other words, it willbe seen that since the although receiver 9| is tuned to the frequency of ,the newly found signal within the spectrum being scanned, thesuperheterodyne receiver I will continue scanning.

To restore the entire system to normal, in order to hunt for another new station, the registrations on the magnetic tape 66 should be erased tuning of recording receiver 9|.

by throwing switch 58 to the left. The opening of switch 84 in the motor control circuit will restore the motor control circuit to normal. By means of a manual adjustment (not shown) in the receiver 9|, this receiver can be returned to the point of origin of the frequency spectrum being scanned by the receiver I. It should, of course, be understood that when the receiver 9| is made to be operative to be tuned over the frequency band by motor M, the width of the tuning band of the receiver 9| corresponds to the frequency spectrum scanned by superheterodyne receiver l. The 360 phase shifter 8| must also be restored to normal and this can also be done manually by means not shown. In order to place this system in operation again, the pulse generators #1 and #2 are reset to blank out the signals corresponding to the known stations during which time the switch 68 should be in a position to continue erasing and after which time the switch 68 should be thrown to the amplifier position. The switch 84 in the motor control circuit should now be closed.

The purpose of the monitor circuit 55 is to enable the operator to make suitable adjustments in the system. This monitor consists of a cathode ray oscilloscope 54 having horizontal deflection plates 56 and vertical deflection plates 51, together with an associated rectifier 5B for applying rectified pulses to one of the vertical deflection plates 5?. The anode of the rectifier 5B is connected to the armature of a switch 59 by means of which the rectifier can be connected either directly to the keyed tone output circuit 3 of the superheterodyne receiver by means of lead 60, or directly to the output of the comparator device 4 by means of lead 6|. Thus, the attendant is able to observe at a glance by suitably operating the switch 59, the appearance of the pulses in the output of the superheterodyne receiver 3 and also the appearance of the pulses in the output of the comparator 4. If the system is properly adjusted, the keyed tone pulses appearing in the output of the superheterodyne receiver I will not appear in the output of the comparator 4, except for the signals corresponding to those from the unknown or newly found station.

A general description of the operation of the system as a whole will now be given, Sweep device causes the frequency of hunting receiver to sweep over a given predetermined band width. Thepanorama of signals in this band will be observed on cathode ray monitorili the receiver sweeps repeatedly over the band, By means of phase shifters l3 and pulse generators #1 and #2. and additional such units as needed, the pulses in the output of receiver representing the presence of normal signal from known stations are cancelled out in comparator 4. The output of comparator 4, which is registered on magnetic loop 68, will then consist only of noise pulses. if present, and any new signals which may appear. In the case of random noise pulses, comparator 99 will not respond. In the case of signals from a new station, hoWeven-these are registered on the tape and comparator 90 will function because of the presence of simultaneous pulses from the receiver 1 and pick-up coil 61. The comparator 90 passes the newly found signals on to motor control unit which places spotting motor M into operation for simultaneously rotating phase shifter 8| and moving the As the motor M rotates, the phase of the local pulse generated by #3 pulse generator .will advance and finally be in time phase with the received pulse coming over lead 15 at which time motor M will stop, thus causing the tuning of the recording receiver 9li to stop in the proper position for this last receiver to pick-up the new signals and record them in apparatus 85., Receiver 91 will remain tuned to the frequency of the new signals until an attendant, or other means notv indicated, changes the tuning. In summation, the oper- ,ation consists of the following fundamental steps: (1) A given portionof the radio spectrum is repeatedly scanned for the presence of signals, (2) all signals in the selected portion or the spectrum are investigated and balanced out by locally generated pulses, (3) the appearance of new signals is investigated, and if they persist (4) a new receiver is automatically tuned to the frequenc of the new signals so that they may be recorded, and (5) the effect of noise peaks is eliminated.

It should be understood that it is within the contemplation of the invention to change the scanning rate and the width of the portion of the radio spectrum to be scanned at the will of the operator. The scanning rate can most easily be changed by changing the fundamental frequency of the sweep device and the pulse generators.

Whatis claimed is:

1. In a telegraph receiving system, a first receiver, means for periodically tuning the receiver over a selected band of frequencies, a local pulse generator, means for combining the pulses from said generator with the output of said receiver, whereby signals from a known station appearing in the output of said receiver may be blanked out, a second receiver, a recorder in circuit with said second receiver, and means responsive to new signals in the output of said first receiver for causing said second receiver to tune to the frequency of the new signals.

2. In a telegraph receiving system, a first receiver, means for periodically tuning the receiver over a selected band of frequencies, a-local pulse generator, means for combining the pulses from said generator with the output of said receiver, whereby signals from a known station appearing in the output of said receiver may be blanked out, a second receiver, a recorder in circuit withsaid second receiver, and circuit means responsive to new signals in the. output of said first receiver which persist for a plurality of consecutive sweep cycles of said first receiver for automatically tuning said second receiver to the frequency of the new signals.

3. In atelegraph receiving system, a first receiver, means for periodically tuning the receiver over a selected bandof frequencies, a local pulse generator, means for combining the pulses from said generator with the output of said receiver, whereby signals from a known station appearing in the output of said receiver may be blanked out, a moving magnetic loop synchronously driven with relation to the scanning of said receiver for registering new signals in the output of said receiver, a comparator circuit responsive solely to a new signal which appears in said receiver and to the registration of said new signal from a previous cycle of scanning for producing a voltage pulse of desired polarity, a second receiver adapted to be tuned over said selected band of frequencies, a motor to change the tuning of the second receiver, a motor control circuit for said motor, a connection from said comparator to said motor control circuit, said motor control circuit being so constructed and arranged as to be responsive, to a voltage pulse in said connection for operating saidmotor to change the tuning of said second receiver over said selected band of frequencies.

4. In a telegraph receiving system, a, first receiver, means for periodically tuning the receiver over a'selected band of frequencies, a local pulse generator, means for combining the pulses from said'generator with the output of said receiver, whereby signals from a known station appearing in theoutput of said receiver may be blanked out, a moving magnetic loop synchronously driven with relation to the scanning of said receiver for registering new signals in the output of said receiver, a comparator circuit responsive solely to a new signal which appears in said receiver and to the registration of said new signal from a previous cycle of scanning for producing a voltage pulse of desired polarity, a second receiver adapted tobe tuned over said selected band of frequencies, a motor to change the tuning of the second receiver, a motor controlcircui-t for said motor, a connection from said comparator .to said motorv control circuit, said motor control circuit being so constructed andv arranged as to be responsive to a voltage pulse in said connection for operating said motor to change the tuning, of said second receiver over said selected band of frequencies, another local pulse generator, a rotatable phase shifter for said last .pulse generator, a shaft for driving said phase shifter from said motor simultaneously with the change in tuningrof said second receiver, said motor control circuit including circuit elements responsive to the simultaneous appearance of a voltage pulse on said connection and a voltage pulse from said last, local generator for stopping said motor at a point which leaves said second receiver properly tuned to pick up the new signals.

5. In a radio signal receiving circuit, a first receiver, means for periodically tuning said first receiver over a selected band of frequencies, a second receiver adapted to be tuned over said same selected band of frequencies, and means for changing the tuning, of said second receiver over said band in synchronism with said first receiver including a signal comparator circuit which is responsive solely to the presence of signals to be detected which appear a predetermined number of times over successive cycles of tuning of the first receiver.

6. In a telegraph radio receiving system, a, first receiver, means for periodically tuning the receiver over a selected band of'frequencies, local pulse generator means having circuit elements for adjusting the position and width of the pulse generated in each cycle, connections for combining the pulses from said local generator means with the output of said receiver, whereby signals from known stations in said band can be blanked out, a second receiver adapted to be tuned over said same selected band, and means for changing the tuning of said second receiver over said band in synchronism with said first receiver including a signal comparator circuit which is responsive solely to the presence of signals to be detected which appear a predetermined number of times over successive cycles of tuning of the first receiver.

7. In a radio receiving system, means for periodically tuning the receiving circuit over a selected band of frequencies, means for converting the received signals to keyed tone output signals, an amplifier for passing the tone signal output of said receiver, a plurality of pulse generators,

means for synchronously operating all of said pulse generators in timed relation with said periodic tuning means, said pulse generators having means for adjusting the position and the width of the pulses generated thereby in each tuning cycle, a connection from each of said pulse generators to the input circuit of said amplifier for supplying said amplifier with pulses of such polarity as to bias said amplifier beyond anodecurrent cut-01f during the occurrence of said pulses, whereby keyed tone output signals from said receiver corresponding to signals received from known stations can be prevented from passing through said amplifier, a recording system for registering signals from new stations received by said receiver, a, signal comparator circuit responsive solely to signals registered on said recording system and to correspondingly time phased new signals appearing in succeeding sweep cycles of said first receiver for producing a pulse of voltage, a second receiver adapted to be tuned over said selected band of frequencies, a motor to change the tuning of said second receiver, a motor control circuit for said motor, a connection from said signal comparator circuit to said motor control circuit, said motor control circuit being so constructed and arranged as to be responsive to a voltage pulse in said connection for operating said motor to change the tuning of said second receiver over said selected band of frequencies.

8. A radio receiving system for detecting unknown radio telegraph transmitting stations comprising a receiver, means for continually and repeatedly changing the receiver response band over a predetermined portion of the radio frequency spectrum, an element for storing a charge representative of a signal pulse received by said receiver from the telegraph station to be detected within said spectrum, an electron discharge de vice having an anode electrode and a control electrode, a circuit for impressing upon one of said electrodes of said discharge device a positive voltage pulse representative of the stored charge, and a circuit from the output of said receiver to said other electrode of saiddischarge device for impressing thereupon a positive voltage pulse representative of a subsequent signal pulse received by said receiver from the telegraph station to be detected, means for normally biasing said control electrode to such a value that said discharge device becomes responsive solely upon the simultaneous application of voltage pulses to said anode and control electrodes, and a normally non-conductive translation device coupled to said discharge device and arranged to become conductive upon the passage of currentin said discharge device.

9. A radio receiving system for detecting unknown radio telegraph transmitting stations comprising a receiver, means for continually and repeatedly changing the receiver response band pressing upon one of said electrodes of said discharge device a positive voltage pulse representative of the stored charge, and a circuit from the output of said receiver to said other electrode of said discharge device for impressing thereupon a positive voltage pulse representative of a subsequent signal pulse received by said receiver from the telegraph station to be detected, means for normally biasing said control electrode to such a value that said discharge device becomes responsive solely upon the simultaneous application of voltage pulses to said anode and control electrodes, a trigger circuit in the form of a gaseous grid-controlled tube, and a connection from the grid of said gaseous tube to the cathode of said evacuated electron discharge device, whereby the conductivity of said evacuated device ignites said gaseous tube.

10. In a radio receiving system, a receiver which is tunable over a selected band of frequencies, a second receiver. a signal storage element, a signal comparator circuit coupled both to said first receiver and said storage element for comparing the time phase of recurring signals received by said first receiver, circuit means including a motor for changing the tuning of said second receiver, means for coupling said circuit means to the output of said signal comparator circuit, said signal comparator circuit being so constructed and arranged as to pass a pulse to said circuit means for operating the same only when received recurring signals appear a predetermined number of times within a certain interval of time.

11. In a radio receiving system, a receiver which is tunable over a selected band of frequencies and which is normally tuned to a predetermined frequency within said band, means autornatically responsive to recurring signals within said selected band of frequencies and emanating from a remote point for changing the tuning of said receiver over said band, a signal comparator circuit for comparing the time phase of said recurring signals, said signal comparator circuit being so constructed and arranged as to prevent operation of said means until said signals appear a predetermined number of times within a certain interval of time, and means including a local pulse generator and a rotatable phase shifter in circuit therewith for stopping the change in tuning of said receiver at the frequency position of said signals.

WARREN H. BLISS.

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