US2378298A - Composite-modulation radio service system - Google Patents

Description

.lume 12, l1945. R. A. HILFERTY COMPOSTE-MODULATION RADIO SERVICE SYSTEM FiledvMarCh 12, 1943 3 Sheets-Sheet 1 June 12, 1945. l

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GOMPOSITE-MGDULATION EADIO SERVICE SYSTEM R. A. HILFERTY Filed March 12, 1943 s sheets-sheet 2 ATTORNEY 3 Sheets-Sheet 3 R. A. HILFERTY COMPOSITE-MODULATION RADIO SERVICE SYSTEM Filed March l2, 1945.

June. l2, 1945.

INVENTOR /P/cf/A/PO A #uff/Pry ATTORNEY Patented June 12, 1945 coMPosITE-MODULATION RADIO SERVICE SYSTEM Richard A. Hi1ferty,'Baldwin, N. Y., assignor to Press Wireless, Inc., Chicago, Ill.;- a corporation 1 of Delaware `-Appliettin March 12, 1943, serial No. 478,854 2 claims. (c1. 25o-9) This invention relates to radio transmission systems and more particularly-to systems wherein two different signal conditions or forms of intelligencecan betransmitted simultaneously over l the same radio channel. 1

A principal object is to provide an improved multiplex radio transmission systemwherein, simultaneously, two separate signals can be trans- 'mitted by 'frequency modulation and amplitude modulation over the same radio channel.

Another object is to provide an improved arrangement whereby existing radio frequency channels ...of the amplitudemodulation type can be simultaneously used to transmit and receive signals by frequency modulationwithout unde` sirable mutual interference.

A feature of the invention relates to Va radioV transmission system wherein a given'channel can be used to transmit such signalsas voice frequencies by amplitude modulation ofa carrier, and

other signals such as telegraph or facsimilesignals by frequency modulation without introducing key clicks or the like from the telegraph trans-` mission into the voice frequency transmission.

VAnother feature relates to an improved form of diversity transmitting and receiving arrangement using the simultaneous frequency and amplitude modulation features of the invention.

A further feature relates to asystemof simultaneous reception of frequency modulated and amplitude modulated emissionsfrom asingle radio transmitter on the same mean carrier frequency without mutual interference, while .at the same time providing signalling circuits of good I tions'and frequency modulations of the same mean y carrier frequency, and since certain parts'of the equipment are well-known in` the art, for purposes of simplicity yin explanation, block diagrams are used throughout the drawings. It will be unn derstood of course, that the various parts incorporated within the block portions can be of any well-known construction sc'long as they are modied or adjusted as disclosed herein for the purpose of achieving the inventive objects. Accordingly, in the drawings,

. Fig. 1 is a schematic block diagram of a radio system embodying features o f 'the invention.

Fig. 2 shows the invention as lembodied in a di; versity-type radio system. f j l Fig. 3 is a modificationy of Fig. 2 using Vsubcarrier transmission. V

Fig. 4 is aV further modification.

Referring to Fig. 1there is shown a radiating antenna I of vany known type, which'is adapted to be excited at a radiatingfrequency under control of the output ofthe R. F. oscillator 2. Oscillator 2 is controlled by any well-knownfrequency modulation arrangement 3 lwhich in turn is controlled by a signal generatorll which maybea facsimile machine, a telegraph machineor the like. By well-known principles, the signals from device 4 act through device 3 to vary the frequency of the oscillator `2. In accordance with the present invention, the device Lis Vadjusted so that the f telegraph or facsimile signals 4impressedmupon the modulator are free from square, orrsubstantially square wave fronts or square trailing edges. For example, if the device 4 isa telegraph keyer, it should be ,arranged` and provided with known wave shapingnetworks or other devices so that the keyed impulses are shaped to a quasisinusoidal form or to impulses havingcurved or smooth leadingfandtrailing edges. I have found that this is extremely important if mutual interference is Vto lbe avoided between the two rtypes of intelligence that are to be smultaneOuSly 15T-31,15-

vmitted from the antenna I. Preferably, modulator 3 is adjusted to vary the carrier from source 2 overa frequency range of 500 C. P. S. For eX- ampleif the mean carrier Vfrequency from source 2 is 5000 k. c., the modulator 3 will vary this frequency between 49.99.75 k. c. and 5000.25 k. c.

The frequency modulated carrier in device 2 is then passed through any well-known R. F. amplifier 5. Amplifier 5 is also connected so as to have the amplitude of its output waves modulated under control of a source of voice-frequency signals such as a telephone transmitter or the like, which controlsan amplitude-modulator? whereby the amplitude ofthe carrier waves impressed on antenna I is varied.` I have foundthat while a very high percentage of amplitude modulation can be employed, for example it should not closely approach amplitude modulation. Consequently, there is impressed upon antenna I, a .carrier wave whose mean frequency varies over a range of 500 C. P. S. i.e.-,

' 250 C. P. S. above and 250' C. P. S. below thesaid meanv carrier frequency. At the same time, this carrier is amplitude-'modulated in accordance with the voice frequency signals from source 6.

rThe receiving antenna 8 is connected to two separate radio receivers 9, I0. rReceivers yis a radio receiver of the amplitude demodulation type such as a tuned RMF. receiver, a superheterodyne receiver or the-like, butin no case shouldrthis receiver have a beating or heterodyne oscillator at thefnal detectorstage, inorder to'insure that the receiveryresponds fundamentally only to the amplitude modulations originating at the device 6, I have also found that it is necessary that the response of receiver 9 be. flat over a 500 C. P. S. range in variation of the carrier, so that no amplitude variations will be produced in the output of receiver 9 by the changing transmitter frequency.

The demodulated signals from receiver 9 can then be applied to any suitable form of voice frequency 'Y signal recorder I I.

The radio receiver I may be'of the frequencymodulation type which is tuned in the well-known manner to the assumed transmitter `mean, frequency of 5000 k. c. Receiver I0 may alsobe of the ceiver should be so adjusted that the local beatingy oscillator produces at least sixl times the current in the R. F. detector as is produced byincoming signal. This is so because of the requisite clear heterodyne beat note'which-must be relatively unaffected by amplitude modulations received on the same carrier frequency. Consequently, as the transmitter frequency is varied over its 500 C. P. S. range as above noted, the audible heterodyne frequency at the audio output receiver I0 will rise and fall in unison with the transmitter frequency. This heterodyne beat of varying frequency is applied to a suitable band-pass lter I3 designed so that it will transmit faithfully the range of tone to be studied or recorded, namely 1000-1500 C. P. S. Preferably, the receiver I 0 with its oscillator is so adjusted that the signals impressed upon iilter I3 are in such a frequency range that the frequency of the uppermost useful limit is less than the second harmonic of the lowermost useful frequency limit, and thus the band of subsequent demodulation frequencies contains only the original intelligence and excludes undesirable harmonics and noises.

The varying frequency signals from filter I3 areithen impressed on a level limiter I4 whereby al1 the input components are reduced to a common amplitude level, thereby removing any amplitude modulations which may be accidentally present in the signalsas they emerge from filter I 3. The remainder of ythe system `associated with receiver I0 is adjusted and designed so that it will not respond to amplitude variations having a modulation factor less than unity. The .demodulation is effected by means which is responsive to frequency variations such as network I5 which acts as a frequency discriminator to produce a variable output in proportion to the frequency of the impressed waves. Preferably, the discriminator is of the wide-band type such as disclosed in detail in application Serial No. 457,709, filed September 9, 1942. Preferably also, the discriminator4 `I5 is of the -uni-lateral type such that frequencies on one side of the midpoint of the input frequency range produce an increased `output while those on the otherside of the said midpoint produce a Vdecreased output. The discriminator I5 therefore acts to convert the frequencymodulations into amplitude variations. These amplitude variations are then impressed upon a detector I6, such as a rectifier, 'so as to produce `direct current signals which vary in the same manner as the original variations from device 4. These rectified signals may then 'be used to operate such devices as undulators, relays,

lamps, or other signalling devices for recording the intelligence embodied in the original frequency-modulated wave transmitted from antenna I,

Referring to Fig. 2, the invention is shown as applied to a -diversity-type radio reception sys- `tem. In general, Fig. 2 comprises vtwo complete composite-modulation systems at the receiver suchas the receiver shown in Fig. 1 and connected in parallel Vto the common driving load represented bythe receiving antennae I8, I9. The diversity receiving antenna I8 is connected in the usual way for diversity reception to two radio receivers 20, 2 I, of the amplitude demodulation type similar to receiver 9 (Fig. 1). The receivers 20, 2 I, are provided with A. V. C. circuits which are connectedtogether as indicated and the outputs of the receivers are fed to a common detector 22 Whose output is responsive to the particular antenna which delivers the greater driving load, as is well-known in diversity-type receiving systems. The detected output is therefore a counterpart of the original signals from device 6 (Fg. 1) and is used to control a suitable signal reproducer 23.

The two diversity .antennae I8 and. 19 are also connected in Vparallel to two radio receivers 24, 25, of the frequency-modulation type, each similar to receiver I (Fig; 1), and are heterodyned from a common local heterodyne oscillator 26. The A. V. C. leads of both receivers are'tied Itogether in the well-known manner. The receiver outputs are connected through respective band- pass vfilters 21, 28, similar to filter I3, and through respective limiters similar to limiter I4.` By means of the limiter selector 3|, the stronger ofthe two signals from devices 29, `30, is selected as described forexample in application of Robert M. Sprague and Richard A. Hilferty, Serial No. Z169,501, filed December 18, 1942. The frequency modulations from the selected channel 29 or 30 therefore control the respective frequency Adiscriminator 32 or 33, which in turn controls vthe respective rectifier detector 34, 35. The particular device 34 or 35, ythat is effective at any given instant, depending upon which of the two channels I8 or 'I9 delivers the stronger signal, operates a keyer device 36 which transmits the Vdemodulated F. M. signals over a local circuit to a central oiiice wherein a suitable recorder such as a facsimile or telegraph recorder 3'I is located.

Referring to Fig. 3, there is shown a further modification of* Fig. 2, wherein the voice frequency signals from source 46 instead of amplitude-modulating the amplified carrier from device 42 directly through theamplitude-modulator 41, are first used to frequency-modulate a subcarrier 60 by means of a subcarrier frequency modulator 6I. Thus `the voice frequency signals fromsource 46 are converted through devices 60, 5I, into a frequency-modulated subcarrier in the audio frequency handhaving a frequency-modulation range of 1800 C. P. S. to 3400 C. P. S. In other words the intelligence signals from source 46 are converted into a frequency spectrum e. g. 1800 C. P. S. to 3400 C. P. S., wherein the uppermost useful frequency is less than the second harmonic of the lowermost useful frequency. This varying frequency subcarrier is then used through the amplitude modulator 41 to amplitude-modulate the mean radio carrier from source 42. At the same time the facsimile or telegraph signals from source 44 are 'used through frequency-modulator 43 to frequency- 26, (Fig. 2).

modulate the mean radio carrier frequency from source 42, so that the latter has a frequency swing approximately of 250 C. P. S. above and 250 C. P. S. below said mean ycarrier frequency.

At the receiving end, the two` diversity anten- `nae 18, 19,V are connected to separate radio lreceivers 10, 1|, of the amplitude demodulation type, such as receiver 9 (Fig. 1), having their A..V. C. leads tied together in the well-known manner for diversity reception. The detector 12 therefore responds to the channel delivering the greater signal strength. If, in the example assumed, the devices 60 and 6| produce 1800-3400 C. P.4 S. signals, this frequency spectrum is detected by detector 1.2 and is applied to a band- At the same time, the frequency modulations n of the radio carrier Wave produced under control of devices 42, 43, 44, are received by the two respective diversity receivers 84, 85, of the frequency modulation type with their local heterodyne oscillator 86 similar to elements 24, 25,

'Ihe A. V. C. leads of the two receivers 84, 85, are tied together for diversity control. The device 86 is adjusted so that the outputs of receivers 84 and 85 are vin the desired frequency range e. g. 1000-1500 C. P S., which frequencies are then applied through respective band-pass filters 81, 88, so as to attenuate very greatly any frequencies below 1000 and above 1500 C. P. S. The signals passed by the respective band-pass filters are then applied to the respective level limiters 89, 90, which are connected to a common limiter selector 9| which may be of a type such as disclosed in copending, application Robert M. Sprague and Richard A. Hilferty, Serial No. 469,501, filed December 18, 1942, whereby one or the other of the limiters takes control f in accordance with which one of the diversity channels 18, 19, delivers the greater signal strength. Each limiter is connected in parallel to apair of frequency discriminators 92--93,

94-95, and each pair of discriminators 92-93 is nected in opposition.` The purpose of the double discriminators is for obtaining polar current from the output of the discriminators. Each mark discriminator (92 or 94) puts out a voltage which is, let us say, positive with respect to ground or some similar reference point. Each space discriminator (93 or 95) then produces a negative voltage. Since the mates are in series, the effective output of a pair is the differential between them. There are two pairs because of the two diversity receiving channels. While the mates are in series, the pairs are in parallel so that, due to fortuitous diversity action, a polar output is obtained so long as one discriminator of each sign receives a signal. At the center-frequency all discriminators would give the same output, but, due to opposing relationshin'the net output voltage would be zero. The detected output of the particular diversity channel which is effective at any given instant is then applied to control the facsimile or telegraph reproducer unit |00. It will be understood of course that in the foregoing description, while special forms of frequency de- .modulators employing level limiters and discriminators are disclosed, any other well-known form of frequency demodulation receiving system may be employed. n

Various changes and modifications may be made hereinwithout departing from the spirit and scope of the invention. For example, While in the foregoing embodiments, Where a level limiter is disclosed, it is incorporated in an audio frequency part of the system, in certain cases, it is desirable to effect the limiting action at a radio frequency stage. Thus, there is shown in Fig. 4, a receiving system similar to that of Fig. 1, wherein corresponding parts bear the same designation numerals. In this case, the radio receiver for the frequency modulation signals comprises a radio frequency tuner and amplifier 90,

a radio frequency level limiter 9|, and a detector v 92. The detector is also fed with a local heterodyne oscillator I2 so as to convert the detected oscillations into the desired frequency band e. g. 1000-1500 C. P. S. This band is then used to control a facsimile or telegraph reproducer |1.

In Fig. 4, the radio frequency limiter precedes the final detector in the radio receiver. This is a radio frequency limiter as distinguished from the audio limiters of the preceding embodiments described. The object in using a radio frequency limiter is to operate on the signal wave Where the frequency is highest in order to be able to use small components of very small time constant.

`Thus, instead of limiting at several thousand cycles, the limiting can be done at several hundred kilocycles, as, for example, the intermediate frequency of the receiver. The limited radio frequency is'then passed on to a heterodyne detector. The beat frequency due to the difference between signal and local heterodyne oscillator, is then passed through a disci'iminator and de tector before entering the reproducer.

What I claim is:

l. The method of simultaneous transmission of keyed telegraph signals of substantially square- Wave shape and of voice frequency signals over the same radio transmitter which comprises,

. modifying the wave shape of the keyed telegraph signals to convert them into substantially sinusoidal form, generating a radio frequency carrier, shifting the frequency of said carrier between certain limits under control of said modified keyed Waves, amplifying the` frequencyshifted carrier, amplitude-modulating the amplified carrier under control of the voice frequency signals, and radiating the doubly modulated carrier. Y'

2. The method of simultaneous transmission.

of keyed telegraph signals having a normal substantially square-wave shape and of voice frequency signals over the same radio transmitter which comprises modifying the wave shape of the keyed telegraph signals to render them quasisinusoidal, generating a radio frequency carrier, shifting the frequency of said carrier between certain limits under lcontrol of said amplified keyed waves, amplifying the frequency-shifted carrier, amplitude-modulating the frequencyshifted carrier under control of the voice frequency signals l'and maintaining the said amplitude modulation below 100%.

' RICHARD A. HILFERTY.

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