US2378013A - Synthetic modulator system - Google Patents

Description

R. A. HILFERTY SYNTHETIC MODULATOR .SYSTEM June l2, 1945.

Filed Sept. 18, 1943 Sheets-Sheet 1 l Qu zal.. Qns

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AT1-ORNE duzz (am June 12, 1945 R. A. HILFERTY SYNTHETIC MQPULATOR SYSTEM 2 Sheets-Sheet 2 Filed Sept. 18, 1945 ATTOR Patented ilune 1?,.1-945 l UNITED STATES Richard A. Hilferuf, BaldwinyN'; y.; assieme-0 Press Wireless, Inc., Chicago, Ill., avcorporation of Delaware Application september'is, 1943,serial No; 502,870 y 6 claims.` (ci. 25o-frz),

This invention relates to radio transmitter and receiver systems and more especially to systems where intelligence signals are represented byk tone or audio frequencies. 'i f A principal object'of the invention is' to'provide aynovel system of transmitting facsimile signals and the like without using' expensive audio frequency modulating equipmentat the transmit-l ter. i One very wellknown method of transmitting facsimile or similar signals bycrne'ans of`a 'radio transmission link, is to convert the' shade'values of the original subject matter into 'audio frequency tones which either directly or indirectlyjare used to amplitude-modulate ar radio frequency carrier. Such a method requires expensive ampli-tude-modulating equipment at the transmitter for amplitude modulation of the carrier. Not only are such modulators expensive as regards initial costland installation, .but they are seldom used' at their full frequencyutilizing capacity. be-

cause the audio frequency modula-ting'band to be controlled shiftabl frequencywhichcan be transmitted is usually very narrow, e, .g., 1500- 3000 C. P. S., ascompared with thefaudio modu-l lating equipment which'isusually designed for operationbetween 50 and 10,000 C.`P. S.

Accordingly, another object is to provide' a system of transmitting different audio frequency tone signals by means'o'f radio carriers without using conventional audio modulator equipmentat the transmitter, while 'allowing reception to take place with conventionalradio receivers of the am'- plitude-demodulation kind. A feature of the invention relates to a flexible service radio system employing a pair of nonamplitude-modulated transmitters whose relative frequency difference is varied in a desired audio frequency manner; these transmitters can alternatively be used in the conventional manner as independent telegraph-keyed transmitters.

Another feature relates to the novel organization, arrangement and relative interconnection of partswhich cooperate to produce a novel and exible radio service system.

Other features and advantages not' specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims. In the drawings,

Fig. 1 represents in schematic block diagram form, a radio transmitter and receiver system according to the invention.

Fig. 2 is a modification of Fig. 1.

Fig. 3 is another modification. y

Referring to Fig. 1, theradio frequency excitation portion of the transmitter comprises two separatesradio frequency oscill'ators'l and 2,' which may be of any Well-'kri'ovvn-type, and if desired, providedfwith suitable" amplifiers *and 'buffers as Well-known "in the radio trjansmitterl art. *The frequency 'of oscillator l ymay befiixed for example at 15,639 kilocycles, While-oscillatoris `of a ysignal shifted for examplefrena 153640'te 15,640.5 kilocycles'.`A` The oscillators aredcsigned's that their actual frequency-difference is equal to the range of audio t'onesyvhiclritis desired to transmit and reproduce at the receiver.' Forfexampl'e, inthe case' of facsimile transmission; i ar suitable y facsimile scanning arrangement 3`is' provided'whereby the shades ofthe original subject matter'are convertedvinto-currents?which,` attire receiver, will appear' asv audidr'equenCytones Afor example" 1000 to 15000. P, S.` Thus the lowest"`cur rent may "represent f whit'e while the `highest currertmay represent "black'for` Avice V:versati If ldesir-d, :although it is notriec'essary,` the converter ymay be adjusted As o'that for the rv'angefof, shadestobe.

covered, the uppermost audio frequencyv tone, `or the uppermost portion of the-1000 to 1500 CIP. S; spectrum, is always less thanmthe second harmonic of the lowermost torre-and therefore vless thanwthe lsecondharrrn'n'nic 'of anyy Itoneoi'` frequency inthe said,s1:' e'ctrum.V `Fora 'detailed dey scripti'on of 'such an"arranjgement, reference may beliid' to 'Pat-elfltr No.'2,299,937,"thedisclosure 0f which is herein incorpora-ted byvrefe'renc'e.

'The two'radio frequencies from"`oscillators l and2 rare fed to a common rradiofrequencyltransmittin'g4 amplifier fl whose 'outputiis applied to a radiating antenna 5. Amplifier ishould be' of the distortionless type such as a class B ampli-` fier." The two radio frequencies will therefore be radiated as two discrete frequencies. At the receiver terminal the two radiated frequencies arel picked up by the receiving antenna y'6 to which is connected an ordinary radio receiver of the amplitude-demodulationv type. For illustrative purposes, a superheterodyne receiver is shown comprising the radio frequency amplifier section l'1; the first detector or intermediate frequency converter section `8; the intermediate frequency amplifier section 9; the detector section I0 such as usedin detecting amplitude modulation; and

suitable audio frequency amplifier section (notshown). Assuming the original signals tobe in the G-1500 C. P. S. band, the receiver is tuned to the mean frequency between the two radiated frequencies of 15,640 and 15,640.5 kilocycles. Un-

der those conditions, when the variable oscillator rests at 15,640 kilocycles, the audio frequency output 0f the receiver is 1000 C. P. S. When the variable oscillator is shifted to 15,640.5 kilocycles, the receiver output is 1500 C. P. S. This 1000 to 1500 C. P. S. signal can then be used to control the reproduction of the original facsimile shades in the Well-knownk manner and as explained in said Patent No. 2,299,937.

Preferably, and as described in said patent, the output of the radio receiver may be passed through a band-pass lter having an eifective band pass of 1000 to 1500 C. P. S., and it is thenv subjected to a level uniforming or limiting device to bring all the frequencies to the same am-` As indicated in Fig. 3, instead of using -a single transmitter to radiate the two radiofrequencies from a single antenna, two entirely separate transmitters and antennae may be employed. With such an arrangement, class C amplifiers may be used to amplify the radio frequency carriers before application to the antenna. Whereas, in the case of Figs. 1 and 2, it may be necessary to use class B linear ampliers which are more critical than class C amplifiers.

It will be understood that each transmitter in the system of Fig. 3, can be independently keyed for separate telegraph message transmission or the like when the simultaneous use for facsimile as above described, is not required.

quency oscillations, generating other primary radio frequency oscillations, shifting the frequency of said other oscillations without audiofrequency amplitude modulation so that the actual frequency difference with the rst radio frequency oscillations equals the frequency of the tone to be transmitted, and radiating both said radio frequency oscillations from -a common antenna.

4. The method of radio carrier transmission which comprises, generating a rst primary radio frequency carrier, generating a second primary radio frenquency carrier, varying the relative frequencies of said carriers without audio-frequency amplitude modulation of the carriers so that the actual frequency difference is in a predetermined audio frequencyband, and radiating said carriers from separate antennaefto' a common receiving point. 5. A primary radio frequency oscillator of xe frequency, another primary radio frequencyoscillator of shiftable frequency, means to shift the frequency `of said other oscillator Without audio frequency carrier amplitude modulation-and under control of signals to be transmitted so that the actual frequency difference between said oscillators equals the frequency of the signal-to'be transmitted, an amplifier for both of said oscillators, and a common radiating antenna fed from said amplifier. y

6. 1n a radio transmission system of the charr acter described, a pair of primary radio frequency oscillators, means to vary the frequencies of said oscillators Without audio-frequency amplitude modulation thereof but under control of4 audio frequency signals so that the frequency difference between the oscillators equals the frequency of the signals to be transmitted, a common amplifier for both of said oscillators, and a common radiating antenna fed from said amplifier.

RICHARD A. HILFERTY

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