US2257282A - Frequency modulation of signals - Google Patents

Description

Sept. 30, 1941. l

J. E. SMITH EI'AL FREQUENCY MODULATION OF SIGNALS Filed April 27. 1959 4 Sheets-Sheet l Sept. 30, 1941. J. E. sMlTH ErAL FREQUENCY MODULATION OF SIGNALS 4 Sheets-Sheet 2 Filed April 27, 1939 INVENroRs L-Iw Sept. 30, 1941. J. E. SMITH ETAL FREQUENCY MODULATIVON OF SIGNALS Filed April 27, 1959 4 sheets-sheet 3.

BY ATTORNEY.

Sept 30, 1941 J. E. sMl'rH Erm.

FREQUENCY MODULATION OF SIGNALS 4 Sheets-Sheet 4 Filed April 27, 1939 IVENTOR5 modulated sub-carrier.

.Patented Sept. 30, 1941 UNITED s "iA'rls PATENT OFFICE" .actuantev .FREQUENCY MonULA'rIoN `or" SIGNALSA Application April '27, 1939, serial No. 270,332

21 (ci. 25o-2o) i This invention-relates to the variation of carfading and the second as selective fading. These' two effects may be produced at different times or they may occur simultaneously.

To overcome the effects of fading it.has been proposed to modulate the carrier wave by changing the frequency rather lthanthe amplitude,

` since fading produces amplitude variations of the signal. Frequency modulation would thus enable one to discriminate between the signalmodulation and that produced by fading. By using frequency modulation at the transmitter and rigidly limiting the voltage or current at the receiving station, the amplitude modulations produced by fading can be eliminated. For al disclosure lof this generaimethod of reducing fading, reference is made to the patent of Wright and Smith, No. 1,964,375, June 26, 1934.

Also, in producing frequency variation it has been found advantageous 4to vary the frequency of a sub-carrier and then amplitude modulate the radio frequency carrier byV this frequency produce van improvement in the transmission of pictures as compared to systems using amplitude modulations, selective fading effects are still noticeable. Most objectionable is the selective fadingv of the radio frequency carrier, Whenever thisv occurs the received signal appears to be over modulated by the side bands thus producing spurious components in the detected output con-` sisting. ofthe side band separation' frequency and harmonics thereof. `'I'he main spurious `component is theK second harmonic of the sub-carrier U caused `by this beating together of the two side bands.. Otherl even harmonics may be produced but to a less extent.

yIi: is one object of our `invention toovercome the leilfects of 'selective fading by selectively receiving the subcarrier land its second harmonic land utilizing the latter when the former fades.

' Another object of theinvention is to provide an improved 'switch of instantaneous action `for automatically selecting either the sub-carrier o! While the improvements .the secondharmonic thereof, depending upon selective fading conditions..

Anotherobiect is to ,provide an arrangement that automatically switches a desired harmonic frequency of the signal in or out of circuitfupon the fundamental frequency of the signal falling below a predetermined intensity.

Another object is to provide an arrangement 'in which a utilization device is automatically connected to. receive the fundamental frequency when it rises above a predetermined intensity and to receive a desired harmonic thereof when the fundamental falls below a predetermined intensity. v

Another `object is to produce an automatic switch. that will operate at greater speed than the rate of change in selective fading.

Other objects will appear in the following description, reference being had to the drawings` -voltage limiter of Fig. 2.

Fig. fi isadiagrammatic illustrationof the frequency doubler of Fig. 2.

Fig. 5 is a. diagrammatic inustrauon of the eiectron switch of Fla. 2.

Fig. 6 gives the voltage-frequency graphs of the frequency detector.`

Fig. 'l is a diagrammatic illustration of the band pass frequency detector. of Fig. 2.

Fig. 8 gives a series of graphs illustrating the 'operating characteristics of the limiter.

. Fig. 9 is a modified form of receiver.

Fig.. 10 is a modified form of transmitter.

Fig. 11 .is a receiving system for the transmitter of Fig. 10.

Referring to Fig. 1, the picture scanner may `be of any desired kind as indicated at l, but it preferably has a tone output by control of`asource 2 of audio oscillations. The tone output at 3 varies the frequency of asub-carrier oscillator which inV turn modulates a radio' frequency carrier. Various sub-carrier oscillators may be used but we have illustrated one of the heterodvne type. The tone output l of the scanner feeds into double rectifier l and the rectified out- 4 put is applied lso that it acts as a negative bias 1 on tube 5 that varies' with the scanner output.

Two high frequency oscillators i and 'I are adjusted-to oscillate at frequencies differing by a low frequency beat, for example, 800 cycles. Os-

4 a low pass but the va cillator I maintains a constant frequency but oscillator l has its fremi'ency varied' by the vacuum tube stage 5. Tube 5 has its pletecathode circuit in shunt with the high resistance t and condenser 8' which is in series with the condenser t to one side of coil it. The seriesparallel circuit of condenser 9 and tube 5 and high resistance t 'and condenser 8' is thus in parallel with half the input coil it o! oscillator l.

The output of oscillator t is udjustably oonnected to control grid il oi tube i2' which is surroundedby screen srid it. The voltage on this screen is preferably lower than that on the plate of the tube. The output oi oscillator l is ad v 'Justnbly connected to another control grid it oi tube it. t The output oi tube l2 is passed through ltcr it to lter out all frequenciw fue beat i This varying beat frequency which is the sub-carrier frequency previously 1: to, is. amplied at 2@ und modulates'e. frmuenw carrier 2l at il in om way. tter it rsdiates the signal by means oifant's tt. The modulator 2l' man be of any type desired.

in Fig. u the receiving antenna a is connected w an oph'm'iste receiving apparatus it which may consist oisudio frequency amplifiers, e. detector and auo frequency ampliers und enr other desired devices for converting the signals into audio frequencies oi the desired intensity. The output o' the simplifier to coutume two primortes it, il, providing two paths for the signal, one lor the :fundamental sub-cerner. und the other for the second hsrmonic of the sub-cerner.'

'the poth for the demented consists oi u hemd poss titer it, voltage limiter 2t, low pass lter tt, and frequency doublerl iii. The output lines 32 of the frequency doubler are connected to electron switcht. Filter du is also connected to theswitch to by lines tt' for n purpose explained later herein. y

e path for the second harmonic consists oi e. d s iilter itt, the output lines $3 0f which accuses grounded. It is preferable to use this resistance but it can be omitted when desired.

The anode of rectiiier Eli is connected to the cathode of rectifier 53 and the anode of rectiiier 52 is connected to the cathode of rectiiier Il. The an oi rectiiers Il and 53 are connected to the grids of tubes 58 and 59 and resistance 6B is connected across these grids, the center of the resistance being grounded. The cathodes .oi tubes 58 and 59 are connected by adjustable tap 5l to potentiometer resistance $2, one end of which is connected to the positive terminal 58 and the other one oi which is connected to nosative rminal tl through mound. This resistance is connected directlyl to the cathod 88 and Il are connected to the electron switch t. e

output lines tt ofthe electron switch are connected to the roltefe limiter 3l und the circuit is continued from conductors 3l' through freuuency detector 38, rcctier St to recorder t which records o. picture of the object on the surface i.. 'lhis recording may be carried out by the photographic process, visual process, or be om? other methodes our-invention is independent oi the color type oi the recorder. the voltage limiter, frequency doubler, electron switch and frequency detector are shown in Fiss. 3, d, 5 and 7, respectively. The details oi the other devices m Fics. i and 2 indicated in block diagrams cre old und well known in the srt an are not illustrated.

In Fig. 3 which illustrates the voltage limiter. input coil d2 feeds into the input oi amplier. 'Ihe output lil feeds into push pull ampliher @it sists Voi tour rectiners 59, ti, l2 and 53, so ar- Y ranged that the rectified current for each hal! wave passes through two rectiiers in series and both hall' waves are produced bythe same coil 53'. 4The cathodes of rectiners It, Si, are connected by adjustable tap 56 to potentiometer resistance 5 5, one endv of this resistance being connected to the positive terminal It of a sourceof supply, the other terminal Il oi this source The details ci p tubes lili and |02.' The other end of resistance of tubes t@ and t9 and to the negative suppl?. The srlds it and li or these tubes are connected to opposite ends of resistance iid through resistences l i It will be recognized that the input or tubes te ond t9 are connected in the well known push pull lion. The anodes 'l2 and li; however, ere 'connected in parallel to the output primary lt. is e. well known form oiireouencr doubler having e. double frequency output voltage thatle substantially a sine wave.

ln the electron switch shown in detail in Fis. 5 the control lines t3. (see also Fig. 2) introduce the fundamental oi the signal into the input 15 of the empliuer tube lil. The circuit of this ampiier is oi the'usual kind and will be understood from the drawings Without further detailed description. The output circuit ll introduces the sional into double rectiiier le which also may be two separate tubes.- 'The positive terminal "i9 of ts recier is connected to one end of resistance tt und the negative terminal t! is connected to the other end of this resistance. A iilter condenser t2 is connected across these terminals and an adjustable tap on tt is grounded. The negative end oi resistance @il is connected through an appropriate resistance t3 to the grid 8d of a gas triade to. The positive enel of resistance Bil is connected am .a a negative grid bias 8S and appropriate resistance el grid t8 of a similar sus triode te. These gas triodes are variously known as hyratrons and Grid glow tubes and usually have an internal drop of. about 15 `volts across the anode and cathode. The anodes ti and t2 oi the ses triodes are joined together and connected to the positive terminal oi o source of which the negative terminal is indie cated at 9d. The terminal 93, may, for example. have 90 volts and e. still higher voltage terminal tt may have volts. This latter is connected to the plate of ampliiier tube 75 and to'other electrodes to be referred to later. 'The cathodes te and @l or the gas triodesr are connected together through commutating condenser !8.`

Cathodel S7 is also connected through. resistance 9% to one end of resistance i, which end is also connected to the cathodes of push pull amplifier itt is grounded and is connected to the center of y the secondary of transformer |03 which is'fed by input connection 35- (see also Fig. 2). The 0pposite, ends of thistransi'ormer secondary are connected to the srlds'of tubes lili and M2. 3

` The anodes of tubes |01, |02, are'connected to output coil |94 across which is connected resistance |05. The center of this resistance has an adiustableconnection extending from cathode 95 of the gas triode 95. Cathode 951s connected .through resistance |01 to resistance |09 and also to the cathodes of push pull amplifier tubes |59 and H0. The opposite end of resistance |55 is connected to ground and to the center of the secondary of transformer which is fed by the input lines 22. The opposite ends of the secondary of'transformer Ill are connected to the `grids of tubes |09 and |I'0. The anodes of these tubes are connected to the primary of transformer |I2 across which is tapped resistance lll. An adjustable connection ||4 is made between the central portion of this resistance.

and cathode 91.

The secondary of transformer' |04 is connected between ground and the grid of4 amplifier tube |I5. The secondary of transformer II2 is connected between ground and the,` grid of arnplifier tube IIS. The anodes of tubes ||5, IIE, are connected in parallel to output transformer ||1 which is connected to positive terminal 95. The cathode of tubes ||5, H9, are likewise connected in parallel through grid bias resistor ||1 The antenna thus radiates a radio carrier frequency modulated by the variable sub-carrier frequency, the variations being produced by the scanningapparatus.

The sub-carrier frequency-modulated radio Afrequency carrier will be received by antenna 24 land receiving apparatus 25 of Fig. 2. .It will be amplified, detected and amplified at audio frequency in the usual way and the output will consist of the varying frequencies of the sub-carrier. These frequencies may not, and in most cases will not, be ofconstantamplitude because of the fading effects previously referred to. When the 4 selective fading is such as to cause the fading of the radio frequency carrier the two side bands of the sub-carrier beat together and form even harmonics, chiefly the second harmonic frequency.

It will be noted from Fig. 2 that the audio output of the radio receiver 25 is fed to the recording apparatus through two paths, one F for the varying fundamental of the sub-carrier, and an` other H, for its correspondingly varying harmonic. If the fundamental of the sub-carrier has not been materially affected by fading of the lradio frequency carrier the main signal energy to ground. Across this resistor may be placed a filter condenser H8. The secondary of transformer |i1 is connected to lines 35 which feed the limiter 31 'of Fig. 2.

In the frequency` detector 32 of Fig. 2 shown in ldetail in Fig. 7, the input lines 31 introduce the signal into a low pass filter through transformer H9. The low pass illter consists of series inductances ||9, |20, `and shunt capacities |2|, |22 and |23, arrangedin the usual way.

These inductances and capacities are of such number and value that the characte'ristic curve A `shown in Fig. 6 will have a substantially straight portion a, b, between the frequencies fi and fz. The frequencies fi and fz may have various values but in practice we have found it satisfactory to usea sub-carrier frequency varying between 800 and 1000 cycles per second l which when doubled by the frequency doubler would have a value of 1600 to 2000 cycles per second. 'Ihe lower frequency f1 would therefore have a value of substantially 1600 cycles per second land the higher frequency fz a value of substantially 2000 cycles per second. As an alternative form the illter may have series condensers and shunt inductances, if desired, and thenv the characteristic will be like curb B in Fig. 6.

The operatlonwill now be described'. The tone output of scanner is rectiiied by double rectiner 4 and varies the plate-cathode impedance of tube 5 and thus varies the Atuning effect of condenser 9 in oscillator 1. Oscillator 5 is set to oscillate at some suitable frequency, for example, 75 kilocycles and oscillator 1 will oscillate under control of the scanner at a frequency that varies above or below the frequency of oscillator 5 a s'ufilclent amount to produce a beat,`the latter varying from about 800 to 1000 cycles. This will pass through band pass filter 28 which has been designed to have suiilcient breadth to pass the sub-carrier fundamental 'and its variations produced by the scanning apparatus. The fundamental then passes through the limiter 29 which is so designed as to provide an output of substantially constant amplitude for wide variation in input amplitude. The output of the limiter is fed through low pass filter 30 to eliminate harmonics caused by the action of limiter 29.

The fundamental frequency is then doubled byV doubler 3| so that it is the same as the frequencies in the other path. The limiter 29 is inserted so as to give a constant input to th`e frequency doubler 3|. If it were not for this, the limiter 31, to be later described, would suflice.

The limiter 29 need not be of a particular type, but the limiter shown in Fig. 3 is-preferred. As the signals pass into limiter 29 through input conductors |24 the signals are amplified bythe 4single stage 46 (Fig. 3) and pass into the input of the push-push stage 48. lThe purpose of resistors 48 is to aid in the limiting of the modulations. i The output of the stage 49 is then rectied and the rectified signal fed through the push-pull limiter tubes 59, 59. When the signal is introduced into the input of rectiilers 49 with -slider 54 moved to the groundterminal of resistance 55 and with terminal |25 positive, current will passfrom the anode of tube 5l to the cathode, through adjustable contact 54 to ground and from ground to the lower part of resistance 5l to the anode of tube 5| and thence to the cathode and end |21 ofthe vresistance |29.

When terminal |21 is positive currentwill pass from the anode to the cathode of tube 52 and thence through contact 54 to ground and through the upper part of resistance 50 and from the anode to the cathode of tube 53 to opposite'end |25. In Fig. 8, |28 represents the drop in the lower half of resistance 5l and |29 the drop'in the upper half. That is, an oscillograph placed across the whole of resistance 50 would produce the graph |28, |29. By sliding the contact 454 towardsl the `positive side `oi' resistance 55, a

counter E. M. F. is introduced'and the part of the wave lextending from the Vzero axis to the lines 'c, d, will be eliminated. 'I'he portion 'beyond these lines will be all that is passed. The form of the wave with this adjustment of slider 5t will bev as indicated at |39, Iii, in Fig. 8. The amount of threshold action can be made any deaired value by adjusting the contact 54. The potential applied across the grid and cathode of tube 59 is like either |28 or |38, depending upon the adjustment of slider 54 for regulating counter E. M. F.' The shape of the waves appearing in the primary of the output of the push pull stage depends on the adjustment of contact Bi which places a negative bias on the grids.

Assuming for purpose of explanation that an appropriate negative bias has been placed on the grids of tubes 58 and 59 from resistance G2, the

, steady electron ow in tube 58 may be of the value indicated by line B, B, and that of tube 58 plied as a negative voltage to the grid of thatA tube and the last hair of the cycle is the electron now with no signal voltageapplied to the'grld.

During the'same complete cycle the current iiow in the plate circuit of tube 58 is indicated by graph |34, the slgnbeing reversed because of the cppong relation in the push pull plate connection. In both oi `the graphs its and -lsl the signal drives the grids to negative cut-ou', the parts oi the wave out off being indicated by the dotted lines. It will be apparent that the current, or voltage, in the output circuit |82 will be as indicated by graph |35 or i3d-depending upon the adjusting of slider 56 on resistancel 55. 'The output of the vlimiter ,23 passes through the low pass filter 38 (Fig. 2) to eliminate any harmonics existing therein and thence to the doubler 3|.

The frequency doubler ti produces in output lines 92, a signal of the same frequency as the second harmonic appearing in lines 35 of the other channel, this harmonic being produced by the 'beating together of the radio frequency side bands when the radio irequency carrier selectively fades. The second harmonic when thus formed passes through the band pass filter 3d n which excludes the fundamental.

' to control the operation of the electron switch,

a control voltage is introduced at 99. shown at `the left of Fig. 5 andl indicated generally in Fig.

2. This voltage is ampliiied by the vacuum tube il. The output of this amplifier is rectied by 'rent will flow from plate 92 through resistances' 9S and '|0 to ground. `When no signal voltage exists in resistance 88 thyratron tube 35 conducts current but when thyratron tube S2 strikes it raises the potential at the terminals oi the condenser 98 by the amountof the drop therein.

The potential of cathode 96 is therefore momentarily raised above the potential of the anode 9|. The current through tube 85 is thus interrupted. The time constant of the condenser is such that it cannot discharge fast enough tov When tube 92 struck and tube 85 was eatinl guished plate voltage was removed from amplier tubes itl, iti, and was applied to amplicr tubes its, im. Blocking bias was' simultaneously removed from tubes its, itil, mi, |02. Therefore the doubled fundamental at 32 is amplified by tubes its, it@ and tube H6 and is passed onto the limiter 3l, which is the same as limiter Et already explained. The output of the limiter is passed to the frequency detector @t and is changed from frequency modulation to aurlplitude modulation. Various frequency detectors may be used but a satisfactory oneis shown in Fig, 7. By referring to graph A of Fig. 6, it will be seen that the highest frequency f2 has the lowest amplitude V2, while the lowest frequency i1 has the highest amplitude Vi. current flowing from the frequency detector varies in amplitude oppositely to the varying freouencies of the modulation. If inductances Ils and i2@ in the frequency detector areremoved and capacities substituted, and inductances substltuted for capacities |2I, |22 and its, the curve would be as indicated in graph B. Either type of iilters will produce the desired result.

The fundamental circuits to the recorder wili remain closed and the harmonic' circuit open, as long `as the fundamental voltage in control lines 33 is above a predetermined valve which double rectifier 19 and the output o1' the rectier i is passed through resistance Il, an intermediate partof which is grounded. .As shown by the drawing, this places a positive voltage onfthe arid Il anda negative' .voltage on grid `81| of the 99,'inabsence of signal voltage, to block 'this tube,

but when thel positive signal voltage is placed on may be made anything desired.

y when the fundamental drops beiow the pre1 determined value the negative bias in resistI ance .8@ is insumcient to prevent thyratron tube' @E from striking and it immediately strikes. This at once stops thyratron tube @d from pass`== ing current by action ofthe commuting con denser 9S, as already explained. Blocking bias is simultaneously applied to tubes |69, I IG, and

is removed from tubes Ill, |02. Plate voltage4 also is applied to tubes IDI, |02, and removed from tubes |09, llt. 'I'he harmonic caused by selective fading now passes through ampliiier mi, |82 and H5, to the limiter 3l where the amplitude variations produced by fading are removed without affecting the frequency modulation `of the signal. 'Ihe output of theA limiter isv then passed to the frequency converter where the frequency modulations lare converted' into amplitude modulations and recorded as previthe grid 0! tube 90'thls'tube will strike and cul- 75 ously referred to.

As long as the fundamental remains below a predetermined point the thyratron 85 will cause the harmonic 'circuit 'to be closed to the recorder Consequently, the

circuits to exclude undesired frequencies. are well known in the art and to illustrate them or other well known devices in every conceivable and the fundamental to be open because thyra-j tron tube ll cannot restrike.due to insufficient positive signal voltage to overcomev the negative bias I8. VWhenever the fundamental voltage in control lines 3l reaches the predetermined point ltube 90 will restrike and commute tube I5. The

fundamental will then be switched in circuit and the harmonics switched out.

A limiter and filter could beplaced between the harmonic filter 34 and the switch 33, if desired, but we have found that limiter I1 is sumcient. This limiter 31 4would also suffice for the fundamental but as previously explained the limiter 29 is preferably used so as toprevent a constant input to the frequency doubler.

In Fig, 9 we have shown a modified form of sistance in the bridge |40 the input into -the vacuum tube stage |39 will have the phasedshifted any desir-ed angle in a way that will be apparent. This will take care of difference in phase between the two channels where the phase shift is of a slow character.

In Fig. 10 we have shown a modified form of transmitting system in which the frequency modulated sub-carrier is passed through two channels, in one of which a frequency doubler |39 is used to produce a frequency having the value of the second harmonic of the sub-carrier.

`The fundamental and the second harmonic'are combined at -I40f after being filtered and the two Vfrequencies are used to modulate the transmitter as in Fig. 1.` l

The receiver for this system of transmission is shown in Fig. ll. The receiving,v rectifying and amplifying apparatus will be of the usual type. The band pass filter |41 will be designed to pass the harmonic frequency sent out at the transmitter of Fig. l0. 'Ifhe other devices in this Fig. l1 will be similar toV those in. Fig. 2 having the same 'reference characters.

Whenv there is no fading, the harmonic frequency will be received and recorded in Fig. 11. When the radio frequency carrier selectively fades the transmitted harmonic frequency will be doubled and it will not pass through the filter III. The fundamental of the sub-carrier will simultaneously be doubled and it will be passed by the filter I4| and will pass to the `recorder iny a waythat will be understood from the previous description of Fig. 2.

The conversion of the frequency modulation into amplitude modulations can obviously be accomplished by means other than low or high pass filters as other devices are known in the art.

The foregoing description has been given as an example of the devices that will produce desired fired and the other is extinguished.

Fig. 3 but the latter is preferable because it utilizes the entire input transformer for each half of the wave and the input impedance of the' rectifiers is therefore identical. Various other modifications may be made without departing from the spirit of our invention.

` Having described our invention, what we claim is:

1. In reception of a radio carrier wave containing a modulating frequency, a receiver for said radio carrier wave, a frequency doubler, a

Vfilter for passing the fundamental of the modulation frequency to said frequency doubler, a second filter for passing a harmonic of the modulationfrequency formed by fading of the radio carrier wave. a gas tube having a cathode, an anode and a control electrode, a second gas tube having a cathode, an anode and a control electrode, means for firing one gas tube and for extinguishing the other when the fundamental frequency rises to a predetermined value, a signal indicator and means for-forming a `closed cir-` cuit between the frequency doubler and the indicator. and an open circuit between the second filter and the indicator when the one gastube is 2. In reception of a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, a frequency doubler, a filter for passing the fundamental of the modulation frequency to said frequency doubler, a second filter for passing a harmonic of the modulation fre- `quency formed by fading of the radio carrier wave, a gas tube having a cathode,an anode and a control electrode, a second gas tube having a cathode, an anode and a control electrode, means for firing the one gas tube and for extinguishing the other when the fundamental frequency falls results but the invention is not limited thereto.

Appropriate low and high pass or band pass fil-` ters may be connected in various parts of the These to a. predetermined Value, a signal indicator and means for forming a closed circuit between the second filter and the indicator, and an open circuit between the frequency doubler and the indicator when the one gas tube is fired and the other is extinguished.

3. In reception of a radio carrier wave containing a modulation frequency, a receiver for saidradio carrier wave, a filter for passing the fundamental of the modulation frequency, a frequency doubler connected to said filter, a second filter for passing the fundamental modulation frequency formed by fading of the radio carrier wave, an'amplifler tube connected to the frequency doubler having a cathode, an anode and a control electrode, a second amplifier tube connected to the second iilter having a cathode, an anode and a control electrode,a gas tube having a cathode, an anode and a control electrode, a second gas tube having a cathode, an anode and a control electrode, a source of voltage normally biasing the control electrode below :dring value4 tube, and means for raising the control electrode voltage above firing value in the second gas tube Y when the fundamental frequency passing the first filter rises above a predetermined value.

4. In reception of a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, a filter for passing the fundamental of the modulation frequency, a frequency doubler connected to said filter, a second filter for for the bridge rectifier shown in the limiter of passing the fundamental modulation frequency formed by fading ofthe radio carrier wave, an amplifier .tube connected to the frequency doubler having a cathode, an anode and a control electrode, a second amplifier tube connected to the second filter having a cathode, an :anode and c a control electrode, a gas tube having a cathode, an anode and a control'electrode; a second gas tube having a cathode, an anode and a control electrode, a source of voltage normally vbiasing the'control electrode below firing value in the second gas tube, means for applying anode voltage to the first amplifier tube when anode current flows in the second gas tube, means for applying anode voltage to the second amplifier tube when anode current flows in the -rst gas tube, and means for raising the control electrode voltage above firing value in the first gas tube when the fundamental frequency passing the firstV filter falls below a predetermined value.

anl anode and a control electrode, a second gas tube having a cathode, anv anode and a control electrode, a source of voltage normally biasing the control electrode below firing value in the second gas tube, means .for blocking the control electrode of the rst amplifier tube when anode current for passing the fundamental lmodulation frequency formed by fading of the radio carrier wave. an amplifier tube connected'to the frequency doubler having a cathode, an anode and a control electrode, a second amplier. tube connected to the second lter having a cathode, an

anode and a control electrodes gas tube having a cathode, an anode and a control electrode, 'a

. second gas tube having a cathode,`an anode and a control electrode, a source of voltage normally biasing the control electrode below firing value in the second gas tube, means for applying anode voltage to the rst amplifier tube and for blocking the control electrode of the second amplifier tube when anode current flows in the second gas tube, means for applying anode voltage in the second amplifier tube and for blocking the control electrode of the rst amplifier tube when anode current o'ws in the first gas tube, and means for raising the control electrode voltage above firing l value in the second gas tube when the fundamental frequency passing the first filter rises above a predetermined value.

A8. In reception of a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, a' filter for passing the fundamental of the modulation frequency, a fre- V80 a control electrode, a gas tube having a cathode,

flows in the first gas tube, means for blocking the control electrode of the second ampliiiertube when anode current flows in the 'second gas tube,

and means for raising the control electrode voltage above firing value in the second gas tube when the fundamental frequency passing the rst filter rises above a predetermined value.

6. In reception of a radio carrier wave contain..

i ing a modulation frequency, a receiver for said radio carrier wave, a filter for passing the fundamental of the modulation frequency, a frequency doubler connected to said filter, a second nlter for passing the fundamental modulation frequency electrode of the first amplifier tube when anode' current flows in the first gas tube, means for blocking the control electrode of the second ampliiler tube when anode current' flows in the second gas tube. and means for raising the control electrode. voltage above firing value in the rst sas tube `when the fundamental frequency passing the first nite: falls below a predetermined value. l, v.

7. In reception of a radio carrier wave con YYtaining aniodulated frequency, a receiver for said radio carrier wave, a nlter for the fundamental of the modulation frequency, a frequency doubler connected to 1 filter, a second filter quency doubler connected to said filter, a second filter for passing the fundamental modulation frequency formed by` fa of the radio carrier wave, an amplifier tube connected to the frequency doubler having a cathode, an anode and a control electrode, al second'amplier tube connected to the second nlter having a cathode, an

anode and a control electrode, a gas tube having a cathode, anv anode and a control electrode. a

second gas tube having a cathode,.an anode and a control electrode, a source of voltage normally biasing the control electrode below firing value Y in the second gas tube, 'means for applying anode voltage to the first amplifier tube and for blocking the control electrode of the second amplifier tube when anode current flows in the second gas tube, means for applying anode voltage to the secondamplifier tube and for blocking the control electrode of the first amplifier tube when anode current ows in the first gas tube, and means for raising the control electrode voltagev above firing value in the first gas tube when the fundamental frequency passing the first filter falls below a predetermined value.

-9. In reception of a radio carrier wave containing a' modulation frequency, a receiver for said radio carrier wave, a filter for passing the fundamental of the modulation frequency, a frev quency doubler connected to said filter, a second filter for .passing the fundamental modulation frequency formed by. fading of the radio carrier wave, an amplifier tube connected to the frequency doubler having a cathode, an anode and a control electrode, a second amplifier tube connected to the second filter having a cathode, an anode and a control electrode, a gas tube having a cathode, an anode and a control electrode, a

second gas tube having a. cathode, an anode and a control electrode, a source of voltage normally bi the control electrode below firing value in f the second gas tube, means for applying anode voltage to the nrst amplifier tube when anode current flows in the second gas tube, means for Iapplying anode voltage in the second ampliner tube when anode current flows in the first sal tube, means for raising thercontrol electrode voltage above ring value `in the second gas tube when the fundamental' frequencyl passing the taining a modulation frequency, a receiver for said radio carrierl wave, a filter for passing the fundamental of the modulation frequency, a frequency doubler connected to said `filter, a second filter for passing the fundamental modulation frequency formed by fading of the radio carrier "wave, an amplifier tube connected to the frequency doubler having a cathode, an anode and a control electrode, a second amplifier tube connected to the second filter having a cathode, an anode and a control electrode, a gas tube having a cathode, an anode and a control electrode, a second gas tube having a cathode, an anode and a control electrode, a source of voltage normally biasing the control electrode below firing value in the second gas tube, means for applying anode voltage to the first amplifier tube when anode current flows in the second gas tube, means for applying 4anode voltage to the second amplifier tube when anode current flows in the first gas tube, means for `raising the control electrode voltage above firing value in the first gas tube when the fundamental frequency passing the first filter falls below a predetermined value, anda commutating condenser connected from a point in the anode circuit of one gas tube to a? point in the anode circuit of the other gas tube for extinguishing one tube when the other onev fires.

11. In reception of a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, a filter for passing the '4 fundamental of the modulation frequency, a frequency doubler connected to said filter, a second filter for passing the fundamental modulation frequency formed by fading of the radio carrier wave, an amplifier tube connected to the frequency doubler having a cathode, an anode and a control electrode, a second amplifier tube connected to the second filter having a cathode, an

`anode and a control electrode, a gas tube having a cathode, an anode and a control electrode, a second gas tube having a cathode, an anode mally biasing the control electrode below firing value in the second gas tube, means for blocking the control electrode of the first amplifier tube when anode current flows in the first gas tube, means for blocking the control electrode of the second amplifier tube when anode current flows in the second gas tube, means for raising the control electrode voltage above firing value in the second gas tube when the'fundamental frequency passing the first filter rises above a preacsacsa and 'i anda control electrode, a source of'voltage nordetermined value, and a commutating condenser @n connected from a point in the anode circuit of one gas tube to a point in the anodecircuit of the other gas tube for extinguishing one tube when the other one lires. l

12. In reception of a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, a filter for pasting the, fundamental of the modulation frequency, a frequency doubler comiectecll to said a see-B ond'filter for passing the fundamental modulo tion frequency formeel by of radio carrier wave, an amplifier tube a' nected te the frequency doubler having a cathode, an anode and a control electrode, a second amplifier tube an anode and a control electrode. a gas tube having a cathode, an anode anda control electrode, a second gas tube having a cathode. an anode and a control electrode, a source of voltage normally biasing the control electrodev below firing value in the second gastube, means for blocking the control electrode of the first amplifier tube when anode current flows in the first gas tube, means for blocking the control electrode of the second amplifier tube when anode current flows in the second gas tube, means for raising the control electrode voltage above ring value in the rst gas tube when the fundamental frequency passing the first filter falls below a predetermined value, and a commutating condenser connected from a point in theranode circuit of one gas tube to a point in the anode circuit of the other gas tube for extinguishing one tube when the other one fires.

13. In reception of a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, a filter for passing the fundamental of the modulation frequency, a frequency second gas tube having a cathode. an. anode and' a control electrode, a source of voltage normally o biasing the control electrode below firing value in the second gas tube, means for applying anode -voltage to the first amplier tube and for blocking the control electrode of the second amplifier tube when anode current flows in the second gas tube, means for applying anode voltage lto the second amplifier tube and for blocking the control electrode of the first amplifier tube when anode current flows in the first gas tube, means for raisingtheicontrol electrode voltage above y rises above` a predetermined value, and a com-I mutating condenser connected from a point in the anode circuit of one gas tube to a point in the anode circuit of the othergas tube for extinguishing one tube when the other one fires.

14. In reception of a radio carrier wave containing a modulation frequencya receiver for said radio carrier Wave, a filter for passing the fundamental of the modulation frequency, a frequency doubler connected to said filter, a second filter for passing the 'fundamental modulation frequency formed by fading of the radio carrier wave, an amplifier tube connected to the frequency doubler having a cathode, an anode and a control electrode, a second amplifier tube connected to the second filter having a cathode, an 'anode and a control electrode, a gas tube .having a cathode, an anode and a control electrode, a second gas tube having a cathode, an anode and a control electrode, a source of voltage normally biasing the control electrode below firing value ln the' second gas tube, means for applying anode voltage to the first amplluer tube and for blocking the control electrode of the second amplifier tube when anode current flows in the second gas tube, means for applying anode voltage to the second amplifier tube and for blocking the control electrode of the 'first amplifier tube when anode current flows in the iirst gas tube, means for raisingv the control electrode voltage above firing value in the rst gas tube when the fundamental frequency passing the first filter falls below a, predetermined value, and a commutating condenser connected from a point in the anode circuit of one gas tube to a point in the anode circuit of the other gas tube for extinguishing one tube when the other one lires.

l5. In a signal system employing a radio carrier wave containing a modulation frequency, a

. receiver for said radio carrier wave, means forl 1 separating the fundamental component of the modulation frequencyfrom a harmonic component thereof, a signal indicator, and means for eifectively switching the indicator from said fundamental frequency to the harmonic frequencyl responsive to a reduction in the amplitude of the fundamental frequency to a predetermined value due to selective fading of the received carrier wave and -forV switching the indicator from' the harmonic frequency to the fundamental frequency .responsive to an increase in amplitude ofl the fundamental frequency to a predeterminedvalue due to reduction :in said selective fading.

17. In a signal system employing a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, means for vseparating the fundamental of the modulation frequency from a harmonic thereof, means for increasing said fundamental frequency to equal said harmonic frequency, a signal indicator, and means for effectively switching the indicator from said increased frequency' to said harmonic frequency responsive to -a reduction in the amplitude of the fundamental frequency to a prede- "separating the fundamental of the modulation frequency from a harmonic thereof, means for increasing said flmdamental frequency to equal said harmonic frequency, a signal indicator, and

means for effectively switching the indicator from said harmonic frequency to said increased frequency responsive to an increase in amplitude of the fundamental frequency to a predetermined value due to the decrease in selective fad ing of the received carrier wave. n

19. In a signal' system employing a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, means for separating the fundamental of themodulation frequency from a 4harmonic thereof, means for increasing s'aid fundamental frequency to equal said harmonic frequency, a limiter, a signal indicator connected to said limiter, and means for effectively switching the limiter from said increased frequency to said harmonic frequency" responsive to a reduction in the amplitude of the fundamental frequency to a predetermined value due to selective fading of the received carrier increasing said fundamental frequency to equalv said harmonic frequency, a signal indicator, and means for effectively switching the indicator from said increased frequency to the harmonic frequency responsive to a reduction in tneamf plitude ofr the fundamental frequency to a predetermined value due to selective fading of the received carrier wave and for switching the indicator from the harmonic frequency to the increased -frequency responsive to an increasel in amplitude of the fundamental frequency to a predetermined value due to the decrease in sei lective fading of the received carrier wave.

21. In a signal system employing a radio carrier wave containing a modulation frequency, a receiver for said radio carrier wave, means for separating the fundamental of the modulation frequency from a harmonic thereof, means for increasing said fundamental frequency to equal said harmonic frequency, a limiter. a signal indicator connected to said limiter, and means for effectively switching the limiter from said increased frequency to the harmonic frequency responsive to a reduction in the amplitude of the-fundamental frequency to a predetermined value due to selective fading of the received carrier wave and for switching the limiter from the harmonic frequency to the increased frequency responsive to an increase in amplitude of the fundamental frequency to a predetermined value due to the decrease in selective fading of the received carrier wave.'v

JAMES' E. SMITH. JAMES N. WHITAKER'. GILBERT R. CLARK.

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