US2459485A - Multiplex radio system - Google Patents

Description

'F. A. BARTLET'T MULTIPLEX RADIO SYSTEM Filed Aug. 30, 194.4

4 I Y mm H .MJ

Jan. 18, 1949.A

Patented Jan. 18, 1949 Forrest A. Bartlett, Gardena, Calif., assignor to Press Wireless, Inc., Chicago, Ill., a corporation of Delaware Application August 30, 1944, Serial No. 551,848

(i Claims.

This .invention relates to multiplex radio systems and more especially to methods and means for radiating tWo or more messages simultaneously froma common transmitter.

A principal object of the invention relates to a .multiplex yradio transmitter and receiving system, wherein an existing radio service transmitter having a relatively restricted. assigned or licensed frequency tolerance, can be used to transmit simultaneously a plurality of separate message signals Without mutual interference at the receiving terminals.

Another principal object of the invention relates to an improved radio transmitter and receiver system of the carrier shift type.

`il feature of the invention relates to a novel receiving system for receiving and detecting a plurality of separate messages transmitted by the carrier shift method.

A further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to produce a more efcient and flexible 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.

Inasmuch as the present invention relates to i improved methods of plural message transmission. from an existing transmitter, only those parts of a radio transmitter and receiver system will be schematically illustrated as are required for an understanding of the invention concept and of certain particular modes of practicing the invention. Accordingly in the drawing, there is shown in block diagram form a radio trans-- mitter and receiver system embodying features of the invention. The invention is in the nature of an improvement on the carrier shift system disclosed in copending application Serial No. 498,278, filed August 12, 1943, now U. S. Patent No. 2,387,098, issued October 16, 1945, wherein a radio service transmitter, which is licensed to operate between certain well defined frequency limits, is provided With special means for effecting a carrier shift to represent two .separate signals, e. g., mark and space (in the case of telegraphy), and black and White (in the case of facsimile). The invention is also .in

the nature of an improvement 'on the carrier Ail) 2 shift method of application Serial No. 502,871, 1'filed September 18, 1943, now U. S. Patent No. 2,378,299, issued June 12, 1945.

Referring to the drawing, there is shown in composite schematic-block diagram form a transmitter and 'receiver wherein there are shown two separate signalling devices l, 2, which may, for example, be telegraph keying sets, facsimile machines or a combination of both. The audio frequency signals from each of the sources I and `2., are applied to respective modulators 3, 4, which are supplied with high. frequency oscillations from respective oscillators 5, 6. The oscillators 5, 6, are of the same normal frequency F; and the respective modulators are arranged so that the carrier frequency F from source 5, is lowered to (F-500) c. p. s., when a 4mark signal is received from source l; and when a mark signal is received from source 2, the carrier frequency F from source 6 is raised to (F-l-500) c. p. s. Therefore, the carrier frequency difference between the two channels when both are in mark condition, is 1000 c. p. s.

A Instead of using two separate oscillators 5 and 6, and two separate modulators, a single oscillator 'and modulator maybe employed and connected to the two sources l and 2, so that mark signals from one source raises the carrier frequency 4500 c. p. s., while mark signals from the other *source lower the carrier frequency by 500 c. p. s. A typical arrangementl for this purpose is disclosed in said application Serial No. 498,278, filed August`12, 1943, now U. S. Patent No. 2,387,098, issued October 16, 1945.

` The (F4-500) c. p. s. carrier from the rst channel, and the (1F-500) c. p. s. carrier from the second channel are applied to any well-known radio transmitter whose 'final or output stages are represented by the block The transmitter is tuned by means 'of the tuning network 8, 9 so as `to cover the band (Fi500) c. p, s.

At 'the receiving station, the two carriers o-r the single frequency-shifted carrier is applied 'to a converting system such as is used in hetero- 'dyne receiver comprising for example a radio frequenc'y ampliiier I0 and a mixer-converter stage VH, which is supplied with local oscillations from crystal-controlled source i2. The output of the .mixer-converter stage H in the well-known manner consists of intermediate frequency carriers representing respectively the tWo original signals and with a maximum frequency difference therebetween of 1000 c. p. s. The intermediate frequency carriers from device il are applied to a frequency multiplier i3 which, in accordance with the invention, is in the form of a harmonic generator. Preferably, this harmonic generator is of the grid-distortion electron-tube type comprising a grid-controlled electron-tube i4, whose control grid i5 operates at a positive potential for a small part of each cycle of impressed signal. For a detailed description of such a harmonic generator, vreference may be ha-d to Radio Engineering, by F. E. Terman, first edition, page 223, Ipublished by McGraw-Hill Book Company, Inc., New York. Preferably, a parallel resonant network IB, l1, is connected in series with the grid input circuit and tuned to the desired harmonic in order to increase the output in the desired frequency range. I have found that by tuning to the tenth harmonic of the input frequency, the desired results are obtained.

Thus with the above-described frequency separation of 1000 c. p. s. between the two carriers or between the extremes of frequency-shift of the single carrier, there exists in the output of the harmonic generator i3, two carriers havinga frequency separation of 10,000 c. p. s. The desired tenth harmonic must be amplied to a degree` where it can be used for keying purposes, and at the same time the selectivity must be sufficient to give the necessary interchannel separation. For this purpose, the tenth harmonic signals are applied to a niiXer-coverter i5', which is supplied from a local oscillator i0, to reconvert the harmonic into an intermediate frequency signal which is amplified in the intermediate frequency amplifier il. The result is that the output of device il consists of an intermediate frequency carrier wherein the signal modulations representing the original signals from sources i and 2, have a frequency separation of 10,000 c. p. s., as distinguished from the original frequency separation of i000 c. p. s. between the two channels at the transmitter,

The output of device Il is applied in parallel to two second detectors i8, i9, of the beat frequency detection type, each of which has its own beat frequency oscillator 2d, 2|. The second detectors and their respective oscillators are arranged so that each detects by well-known beat frequency methods the respective signal modulations in the intermediate frequency carriers from amplifier il', it being understood of course that each second detector is supplied with suit- 4 interchannel frequency spacing at the receiver is expanded as described.

Various changes and modifications may be made in the disclosure without departing from the spirit and scope of the invention.

What I claim is:

1. In a signalling system of the carrier frequency-shift type, means to transmit from a common transmitter a plurality of carrier frequencies each representing a different signal and with the frequency difference between the carriers confined to the audio-frequency band, receiving means for said carriers comprising means to convert said carriers into corresponding intermediate frequency carriers, means to subject said intermediate frequency carriers to selective harmonic amplication to produce carriers having a frequency spacing which is a multiple of said frequency difference, means to reconvert said amplied harmonics to corresponding intermediate frequency carriers while preserving said multiplied frequency spacing, and detector means to detect in separate channels the signal modulations in the said reconverted intermediate frequency carriers.

2. In a signalling system of the carrier frequency-shift type, means to receive a carrier whose frequency is shifted a predetermined amount in one direction for one signal and a predetermined amount in the opposite direction for another signal, the maximum frequency difference of the shift being in the audio-frequency range, said means comprising in sequence a first mixer-converter for producing intermediate frequency carrier signals, a harmonic generator on which said intermediate frequency signals are impressed, means to select a harmonic from the output of said generator, a second mixer-converter for reconverting the selected harmonic into intermediate frequency signals, and a plurality of beat detectors for detecting and separating the respective modulations in the reconverted intermediate frequency signals.

3. In a carrier frequency-shift signalling system, means tc receive a plurality of separate signals by a frequency-shifted carrier whose freable filter means or other suppression means so that in. the output of one detector the upper beat is passed to its respective audio-frequency amplifier 22, while the lower beat only from the other detector I9, is applied to its audio-frequency amplifier 23. The respective amplified signals may then be rectified in suitable rectiers 24, 25, and vapplied. to suitable telegraph keying or recording devices 26, 2T, of Well-known type. While in the foregoing specic frequencies are mentioned, it will be understood that the invention is not necessarily limited thereto but that these frequency gures arey supplied merely for explanatory purposes. It will be understood of course that the invention is not limited to the transmission of two messages since more than ice frequency band of the transmitter and the quency excursion is in the audio-frequency range, said receiving means comprising a mixer-converter of the superheterodyne type, a harmonic generator fed from said converter means to select a harmonic `of the output from said generator, and means to amplify said harmonic and to detect in separate channels the signal modulations corresponding to the upper and lower frequency limits of said shifted carrier.

4. A receiving arrangement for receiving a plurality of messages transmitted by radio carrier` ,frequency shift and including a first converter for converting the received radio frequency carriers into corresponding intermediate frequency carriers with a fequency difference F cycles per second, a harmonic generator to which said intermediate frequency carriers are applied and including means to select harmonics of said intermediate frequency carriers so that the harmonic carriers have a frequency difference of nF, wherein n is greater than unity, a reconverter for changing said harmonic carriers back into intermediate frequency carriers while preserving said frequency difference 12F, and means to detect said reconverted intermediate frequency carriers to derive corresponding signals therefrom.

5. A system according to claim 4 in whichsaid harmonic generator is of the grid-distortion `electron tube type.

6. A receiving arrangement for receiving two radio carrier frequencies each representing' a corresponding intelligence signal, means to convet said carriers into corresponding carriers of lower carrier frequency with a frequency difference therebetween of F cycles per second, means to multiply the frequency of said lower frequency carriers to produce correspondingly higher frequency carriers with a frequency difference therebetween of 11F, where n is greater than unity, means to recouvert the last-mentioned carriers into corresponding but lower frequency carriers while preserving said frequency difference nF, and means to detect intelligence signals from the lastmentioned lowered frequency carriers.

FORREST A. BARTLETT.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,541,608 Abraham June 9, 1925 1,607,157 Hammond, Jr Nov. 16, 1926 1,690,299 Horton Nov. 6, 1928 1,740,859 Hammond, Jr Dec. 24, 1929 1,861,370 Taylor May 31, 1932 1,930,924 Curtis Oct. 17, 1933 2,265,033 Peterson Dec. 2, 1941 2,364,756 Van Roberts Dec. 12, 1944 2,378,299 Hilferty June 12, 1945 2,395,478 Hansell Feb. 26, 1946

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