Radio signaling system
United States Patent 2388576

S Clamo. (Il. 277-52) Our present invention relates to radio signalIng systems, and more particularly to modulated carrier signaling systems which employ a free Quency located in the lower part of the aud~ile srequency range as a modulation component. One of the main objects of our invention...

Seeley, Stuart W.
Rankin, John A.
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327/98, 340/13.25, 361/182, 455/228, 455/701
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S Clamo. (Il. 277-52) Our present invention relates to radio signalIng systems, and more particularly to modulated carrier signaling systems which employ a free Quency located in the lower part of the aud~ile srequency range as a modulation component.

One of the main objects of our invention is to provide a method of radio sgna lingover distances reater than that at which aural monitorina is s sin o t h e r i n o u r s y st e m o b e t esible; in or system ~ here is secured ati ory signaling with eceived signois that ar 20 weaKer than the noise level. The risenl s t5 brj droamno  a a s eever oirz thi in the 550 to 1700 s 200% effective beyond the range at which static hnd local noises would cause too much intrue frronce for aural monitoz-ing.

One of the important objecýt o our preDsent 1i invention is.to provide a signallng system. wich c 3 effective between any radio bro.adcmazti ytra¾ . mitter and any number of ree!vMi-ng loýationn; o signal being superinrmposed on the mproseam mc, unition of the transmitter caier, and d..c... "0 ay p nroperly designed receiver, or b.y an artc,ment connected" to any satisfactory radio acciver which ýs tuned to the transmite:r, another imprant object of oL; InvtentiOn i , provide a l ow freouenoy alarmz ut, u dapted C Ž cr aahment to any st0ndard broaden"t ra zilvra, which separates the 2ow ,freaueny toue from the progream material by meas of a e0 - C3ra35 tire band pass filter, and, after amplficat";on foi lowed by amplitude E-M1tatiou, caIser a sha....y N;tand mechanical resonator t;o bo actuated on cc2o.anr to the tone whereby an alarm , circuit L rendered oparative, Anot"her obiject of our invention imo o pOvfe a sys'tna for uti.iz ing a10 low frequency moduln- 25 Ion component of a reeived mnodu'ated carrier uave; the system comprising n amplitude li-o il-er which feeds the low frequency tone sigma to n degenerative input circuIt followed by a bamnd pas Miter reonant to the low frequency, and '49 e itrd toe bei ampia laigi , prior to actuation of a reed tuned to the, low frequency tone, follown time delty to at the work circut relay agrainst, spuns-ious zp-ation caused by strong off-reso nWn tones or Tae d longed busts of severe static or other interfeeonce impulses.

Stil other objects of w inventi on are yzove generally the sensitvity, range and den'dJbaillty of radio alarm systems, and more ~I3 es'Pecialy to provide a highly efficient and economical form of radio signaling system, The novel features which we believe to be characterlstic of our Invention are set forth with particularity in the appended claimns; the Invenrtion itself, however, as to both its organization and method of operation will best be uaderstood by reference to the followvng description taken in connection wirh the drawing in which we have inicated diapammaticaly a circuit organiztlon whereby our invention may be carried Inhto efect.

eferring, now, to the accompanyiring drawing i s e ot e l s c r " k a r othre are shovtnm the circuit details of the alarm uit which is adapted for connection to a standroadose eve r the 550o hreyese (hc.) ~ alitude mcdulation broadcast bnd. Zn ordser to wae a all baoadcasters in. a g iven r a of a, imiending emergency, such as an air atec, one treasmiter of the area is desognaed s the ey station. Thi station, at,; a roper signal from the milit0ary or police authorties, bTQroadcst a w7arningl to6 h stations 60 '0 oft the ai. 72nccc, nil other stations in the a:so, must emplog sv=ealc broadcast receiver 'to monitor the Ley sttlaon modulation tbou3hout its ojprction. Such san ural monitoringl syszerequires a srpzoill monitor technician to "lis-n' co:astazatly to the 'ey station programs; the le-e 2 of the lE atter roy s tel stu eothers in n studio if tha m-onior receiver is not kept n ý spe-cianl r'oom. 7he Cmost serious imitation of a'n monitoring s:stm is that it !- lmited a range by static, local noise and the ic, .OU oals n of eonalng Opeartes with. 20,45 efetrZ m· 7 belyond the rouage nt which a. u-: mncniorhs a :eirble. Zn our present systmS' it i n'ot aeGCSoy to employ a. sp31a attendat to monitosr the Ley stantion program mwaterial. Th ony eiet mnte to put our systaem into operation in ~oraJnction with a regular broad-ý cast tracn mIttetr s a, oonssant-frequency, low frequencv tone generator. For examPle, W have sa.tsfactorily used ae 40 Cycle tone. This tone i supgrm'ossd on the program modulation of the ey siotn, and- detected by the 40 cycle alarm unit shown n the drawivg. The 40 cycle tone a- amptudeo, which Is superrimposed dn týi program matenral that normally modulates the bre1,dcr1st transmitt, y Ie ajs roduce 20% modulation.N At the 20% modu atlon level the 40 cycle tone will not be 'heard in n Ithe ordinary home type, sotndard broadcast racsever. This permits continuous applicatlon if deaired, without annoyance to the listeninag public. It must be noted, however, that contiLtuous modulation at the" level wll substsanlly reduce the power of the transmitter so far as its program handling capa bility is concerned. In the present disclosure let it be assumed that the low frequency tone is applied to the carrier only when it is desired to signal to the other broadcasters in the area that a radio blackout is to be enforced. Of course, the invention is not limited to this use, nor to the standard broadcast band. The invention may be applied to any desired frequency range so far as the carrier is concerned, and may be used for signaling information to receivers located in any desired place.

Numeral I denotes the diode demodulator, or second detector, of a superheterodyne receiver.

The usual intermediate frequency (I. F.) transformer 2 feeds the I. F. energy, whose frequency value may be at 465 kc., to the diode detector.

Load resistors 3 and 4 are bypassed for I. F. currents by condenser 5. The condenser 6 bypasses the resistor 4. The receiver feeding the detector diode I should be one whose sensitivity, selectivity, stability and freedom from frequency drift are sufficient so that complete dependence can be placed on the operation of the device when the necessary low percentage, low frequency modulating tone is mixed with the program material from the received station. For use in locations where the received signal is strong enough for aural monitoring a good broadcast receiver will be satisfactory. At distances farther from the receiver a good "communications" receiver may be necessary.

In order to limit noise impulses which may exist in the demodulator output, a noise limiter tube 7 has its input grid 8 connected by adjustable tap 9 to the risistor 4. Tube 7 may be of the 77 type. The slidable tap 9 functions as E. volume control device. The plate of tube 7 is connected to the +B terminal of the power supply network (not shown) through resistor 9'.

The screen grid of tube 7 is connected to any desired point on resistor 10 by slidable tap i I; the resistor 10 is connected from the lower end of resistor 9' to the grounded cathode of tube 7.

Bypass condenser 12 is connected from tap 1 to ground. The noise limiter tube functions to limit the noise output by plate current cut-off of tube 7. The level at which limiting starts may be adjusted by the limiter control tap 11.

The noise limiter tube 7 functions to "clip" the noise peaks. Subsequent to limiting action, the modulation energy is transmitted to the grid 13 of the degenerative input amplifier tube 14. The condenser 20 shunts resistor 9' to ground. The condenser is connected from the limiter plate to ground in order to limit the frequency response of the limiter output circuit. The components of audio frequency that are of particular interest are low frequencies lying between 20 to 40 cycles; in this particular case 40 cycles. Hence, the condenser 20 is employed partially to remove the higher audio frequencies. The normal audio channel of the broadcast receiver is connected to the junction of resistors 3 and 4. In this way the entire range of audio frequencies will be transmitted through the condenser 3' to the usual audio volume control potentiometer 4'. The usual audio amplifiers and reproducer will follow the volume control device 4'.

The grid 13 of tube 14 is connected to ground by grid return resistor 11. The grounded sheath 18 shields the input lead to grid IS. The cathode 16 is connected to a point of -90 volts through an unbypassed resistor 24. The plate 15 of tube 14 is connected to a point of +325 volts through series resistors 25 and 26; the condenser 27 bypassing resistor 26 to ground. The degenerative input amplifier is designed to be extremely flexible. As long as the tone signal at the input grid 13 is greater than /4 volt peak to peak, and the entire applied audio components do not exceed 100 volts peak to peak, operation will be satisfactory. An additional reason for employing a degenerative input amplifier is to eliminate the possibility of distortion and cross-modulation. In order to remove the program components and pass the 40 cycle tone, there is employed a band pass separating filter. The filter consists of a pair of parallel resonant circuits 29 and 30 connected in shunt relation. The condensers 28 and 31 are chosen to provide the proper coupling between the limiter output and circuit 29, as well as between circuits 29 and 30. Circuit 29, tuned to the 40 cycle tone, is connected from the junction of 28--3 to ground. Circuit 30, similarly tuned to 40 cycles, is connected at its high alternating potential side to the grid terminal of condenser S3. Condenser 32 connects the low potential side of circuit 30 to ground. The grid of tube 33 returns to the junction of resistors 34 and 35 through the coil of circuit 30.

The filtered 40 cycle tone is amplified by tube 33, whose plate returns to the plate supply lead 36 through a resistive load 37. The resistor ST is tapped by bypass condenser 39. Condenser 40, of low impedance to program frequencies, bypasses the upper end of resistor 37 to ground.

The amplified 40 cycle tone is applied to a second limiter tube 41. The grid 42 of the latter is coupled through condenser 43 to the upper end of resistor 17. Grid return resistor 48 connects grid 42 to ground. The cathode of tube .4 connects to ground through resistor 45. The plate of the limiter tube is connected by resistor 46 to the ground line 38; an intermediate tap on resistor 40 is connected to supply line 36 by resistor 47.

The second limiter prevents excessive amplitude being applied to the reed actuating circuit. In this way there is provided additional security against having the system operated by exceedingly strong off-resonant tones or prolonged bursts of severe static or other interference.

The output of limiter 61 is applied to the reed amplifier 48, the grid 49 of the latter being coupled.by condenser 50 to the plate of tube  . The cathode of tube 46 is connected by resistor 02 to ground, while grid 49 is returned to ground by resistor 51. In the plate circuit of tube 46 there are arranged in series the winding B of a reedactuating electromagnet and choke coil 3S. The choke 53 is employed to prevent any 40 cycle curSrent (the plate current of reed amplifier 40) from flowing back through the +B supply impedance, and causing overall regeneration. The lower end. of winding B is bypassed to the cathode of tube 40 by condenser 54. The upper end of choke §3 is bypassed to ground by condenser 55. The reed actuated by the electromagnet is designated by numeral 56.

Since the relay and reed §1 form no part of this invention, and since the construction thereof is very well known to those skilled in the art, they are schematically represented. It will be understood that the reed 56 is usually constructed of spring steel, and has the lower end thereof fixed. The reed is vibrated when the current flowing through the win'ding B is of 40 cycles.

Hence, the reed is a highly selective device. The upper end of reed 6I is provided with a contact point. This contact point is arranged in spaced relation to a second contact point. The spaced contact points are denoted by numeral 57. When the contact points 67 touch, a circuit is completed through them for energizing a subsequent relay. The contact point 67 secured to the reed 56 is connected through lead 68 to the control grid 59 of the following direct current (D. C.) amplifier tube 60. The grid 59 is connected by the low leakage condenser 61 to the line 36.

The second contact point 57 is connected through resistor 62 to the adjustable arm 63 of a time delay circuit. The adjustable arm 63 is arranged to be selectively connected to a condenser 64, a condenser 65 or a condenser 66. The lower end of each of these condensers is connected to the -90 volts line 67. The upper terminal of each of the time delay condensers has a contact to which may be connected the adjustable arm 63. It is indicated on the drawing that when arm 63 connects condenser 64 in circuit, then there is a time delay of 5 seconds. The connection of 63 to condenser 65 introduces a delay of 10 seconds,.and connection of arm 63 to condenser 66 introduces a delay of 20 seconds.

The lower end of resistor 62 is connected through resistor 70 to the positive energizing line 36, and the grid condenser 61 has its lower terminal connected to the lower end of resistor 70.

The control grid 59 of tube 60 is connected by resistor I7 to the negative biasing line 67, while the cathode of tube 60 is connected to ground.

A switch 80 is arranged in shunt relation to the grid return resistor 71. The switch 80 is arranged for simultaneous adjustment with a second switch 81 whose,function it is to control the energization of the winding As. One end of the winding may be connected through switch 81 to ground, while the other end is connected to the screen grid energizing lead 82, which provides the screen grid voltage for the screen grid of amplifier tube 60.

In other words, when switch 81 is closed the winding As is energized. A second winding Ai is arranged in the plate energizing circuit of tube 6g. We have schematically represented windings Ai and As as controlling the schematicallyrepresented, circuit-closing armature 90. Here, again, the armature 90 and the energizing electromagnets therefor are schematically represented, since they are well known to those skilled in the art. For the purpose of this applicationit is believed sufficient schematically to represent these elements. It is sufficient for this application to point out that armature 90 will be drawn towards the core around which winding Ai is wound. Upon the attraction of armature 80 the 115 volt outlet 100 will be in electrical connection, with the 60 cycle alternating current source. Any electrical device may, therefore, be plugged into the outlet 100, and will be energized.

For example, an electric lamp, bell or siren de- 6 vice may be inserted in outlet 100. When armature 90 is drawn towards electromagnet Ai, the armature contact 91 will come into electrical contact with contact point 92 which is connected by lead 93 to one side of the outlet 100. 6 As long as switch 81 is open, the electromagnet As will not be energized from the +S source.

However, when switch 81 is closed electromagnet winding As is energized, and it is so arranged with respect to armature 90 as to pull the armature 7 away from that position in which there is electrical contact between 91 and 92. In other words, the alarm circuit will now be-broken. Concurrently, with closing of switch 81 switch 80 is closed. This renders the amplifier 60 ineffective, 7 and thus causes de-energization of the electromagnet winding Ai. We have, therefore, designated switches 80- 81 as "Alarm release push button." It has been previously pointed out that the limiter 41 acts to prevent operation by exceedingly strong off-resonant tones, or prolonged bursts of severe static or other interference. Further protection against spurious operation is obtained by the time delay control circuit. This time delay circuit can be adjusted so that the work circuit relay winding Ai will be energized only after 5, 10 or 20 seconds of sustained tone at the correct 40 cycle frequency. Associated with the reed 56 is a pair of electrical contacts 57 which are closed solely when reed vibration has built up to a sufficient amplitude. These contacts 57 are connected to the time delay circuit which energizes the electrical relay winding Ai after a predetermined time of continuous reed vibration. It will be understood that the work circuit relay is of the "latching" type with sufficient contacts so that several functions may be performed. These may include the ringing of an alarm bell, switching of the loudspeaker of the receiver, and any other functions. The reed 56 should be tuned to the exact operating frequency. High capacity filter and bypass condensers should be used throughout the system, not only to reduce hum components but also to prevent regeneration at extremely low frequencies of 1 to 10 cycles per second through common coupling in the +B supply. To explain the functioning of the time delay control circuit in further detail, when the device is inoperative (reed not vibrating) contacts 57 are open and the grid 59 will be biased to -90 v. relative to the cathode which is at ground. This means that condenser 61 is charged to +325 v. -(-90 v.), or 415 v., with the positive charge on the +B side and the negative charge on the grid side. Condenser 64, 65 or 66, depending on the location of switch 63, (assume condenser 65 here being used) is charged to an equal voltage as condenser 61 (415 v.). The negatively charged plate of condenser 65 is connected to the -90 v. line, while the positively charged plate is connected to resistor 62.

When the reed 56 is caused to vibrate, contacts 57 are closed thereby connecting the two condensers 61 and 65 together through resistor 62.

The latter is included only to reduce the sparking of contacts 57. Contacts 57 close each /o of a second, and as explained above connect the two condensers 65 and 61, but the polarities of the 5 two connected plates of the condensers are opposite. Hence, from the standpoint of condenser 61 the charge across it is reduced. During the time (each cycle) that contacts 57 are open, con-' denser 65 again charges up to its initial value of J0 415 v., due to the short time constant of condenser 65 and resistor 70 (1 meg.X720 mnif.=.00072 second) However, condenser 61 does not reach its initial 5 charge due to the long time constant of condenser 61 and resistor I7 (20 meg.X1 mf.=20 seconds).

Therefore, on each contact of contacts 57 the charge across condenser 61 is decreased by a small amount until the bias on grid 59 reaches 0 zero bias, and plate current flows in the D. C. amplifier 60 thereby closing the relay in its plate circuit.

Due to the large difference in value between each of condensers 64, 65 or 66 and condenser 5 61 the amount of change of charge on condenser SI resulting from each closing of contacts 57 is small. Therefore, several seconds of time are required to reduce the charge on condenser 61 sufficiently to permit grid 59 to reach zero bias.

The times indicated on the drawing adjacent each of condensers 64,65 and 66 are approximately correct. This means, for example, that with condenser 65 in circuit the contacts 57 are closed 10 sec.X40 cycles/sec.=400 times, before the relay 90-91 is closed.

Field tests with this type of alarm unit are highly satisfactory. In one test a 50 kilowatt transmitter operating at 750 kc. (WSB) was received by equipment located 290 miles away (airline separation) at WJAX, Jacksonville, Fla. Broadcast reception in this area is haphazard. Using the present system of low frequency signaling 100% operation was had. The receiving equipment was located near the center of the city where high intensity local noise is common. In another test perfect performance of the system was had in an airline distance of 145 miles between transmitter and receiver. In this test the daytime static was sufficiently severe to make the signal completelSy unusable from a monitor point of view, but had no effect on the low frequency signaling system. In another series of field tests, conducted in the New England area, perfect performance was obtained on a 50 kilowatt transmitter under severe noise conditions at 355 miles. Perfect performance was obtained in the interference area between two synchronized transmitters both transmitting the 40 cycle tone. The synchronized transmitters were WBZ and WBZA.

While it is not essential to the present invention, the following list of illustrative constants is given in order to aid those skilled in the art to construct the present system: Rio------------------------ohms-e'-------------------------do ,,, , E, ...-- .....................--- megohms Rae--------------------------- ohms-RB s.----- ----------- .-do....R---------------------.......................--....---do---....

37--------------------------- do-35.......... -----------------------------do--...

R34----------.............-------------------do ---R4--.........----.--.. ---- megohm-R~e~ .....------ ---..... -------ohms-R ----------------....----.......do.... 6 s-------------------------.......... do--...

Ri ... --- ---------.-megohmS--------- ------- ohms-. Ra--...----.............-----...do--.. o0 -------------------------megohm-_ R .--------- -----------megohmsC------------------- microfarads-Co-------------- icro-micro-mirofarads-_ C---------------------microfarads-C- ..-------------------------do-Of- rdf Ca..----.----------------------.do-....

a. . .. .. .. ____ ....__ 25,000 250,000 10 39,000 47,000 30,000 156,000 5,600 68,000 1 50,000 560,000 578,000 1 820 10,000 1 20 1,000 20 0.1 0.1 15 8 0.05 0.01 0.01 S CM___--u-_-- _ -_...-o Ca ------------------- do C . -------- ---micro-microfaradsCos-. ---------------do--...

Ca -.--- ........do-....

Le3 _-----------------.. .henries-_ 1 300 720 2,500 42 While we have indicated and described a system for carrying our invention into effect, it will 10 be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of our inventions as set forth in the appended claims.

15 What we claim is: 1. In a signaling system adapted to receive carrier wave energy modulated with a low frequency, a demodulator for providing energy of said low frequency, means for limiting the amplitude of 20 the demodulated energy to prevent operation by undesired noise impulses; a band pass filter, tuned to said low frequency, connected to the limiting means, an alarm circuit, means for controlling the operation of the latter, and a highly selec25 tive device, responsive directly to the filter output low frequency energy, for rendering said controlling means effective and an adjustable time delay means connected to said selective device, said time delay means consisting of a plurality 30 of condensers of different sizes providing a time delay range of 5 seconds to 20 seconds.

2. In combination with a system adapted to derive a low audible frequency control tone from a radiated modulated radio carrier, a band pass filter tuned to the control tone frequency, an alarm circuit, a highly selective mechanical resonator, tuned to said control frequency, coupled to said filter output for direct energization by the 40 control tone energy, means responsive to actuation of the resonator for controlling said alarm circuit, a time delay control means, consisting of a plurality of delay elements of respectively different time delays and means for selecting a desired one of the delay elements, for preventing Sspurious actuation of the alarm control means, an amplitude limiter means preceding said filter to decrease the amplitude of noise impulses, and a second amplitude limiter means following the Sfilter to prevent operation by undesired impulses.

3. In combination, a source of signal energy of the order of 40 cycles, a band pass filter tuned to the signal frequency coupled to the source, an alarm circuit including a reed-controlled, circuitclosing device, a plurality of condensers of respectively different magnitudes to provide correspondingly different time delays, a selective device for connecting a desired one of the condensers to said circuit-closing device, said reed being tuned to the signal frequency, means for 60 vibrating said reed with solely the filter output energy, and an amplitude limiter and a degenerative amplifier arranged in cascade between said source and filter input terminals to prevent spurious operation of the reed by undesired 65 impulses.



C------------------do C-------------.------------ do---C50 --...--- ......------------ - .. .do .---44 ----------------7 --------- ---.------------------------- ------fl- -, -------- Co ------------------------ do---