US1616892A - Duplex radio system - Google Patents

Description

Fgb. s; 1927, 1,616,892

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I DUPLEX RADIO SYSTEM I Filed Aug. 18. 1922 Mb ATTORNEY Patented Feb. 8, 1927.

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LLOYD ESPENSOI-IIED, OF HOLLIS, NEXV YORK, AND DE LOSS K. ."M i'zrRCCIh'l', OF ORANGE,

NEW JERSEY, ASSIGNOR-S TO AMERICAN TELEPHONE AND A CORPORATION OF N YORK.

TELEGRAPH COMPANY,

DUPLEX RADIO SYSTEM.

This invention relates to an improvement in duplex radio signaling, and in particular has for its purpose that of enabling a better and more complete separation of the transmitted and the received waves.

In duplex radio signaling, especially in those cases where oneantenna is used for the transmitted and received signals, the enormous value of the ratio of the transmitted power to the received power makes the problem of separation an extremely difficult one. In this invention we propose to use one frequency for transmitting and a slightly different frequency for receiving. Heretofore, i such cases, reliance has been placed for separation mainly on the discrimination which is afforded by tuned circuits. However, if the tuning of such circuits is made too sharp and selective, the range of frequencies transmitted by the respective circuits is too narrow for practical signaling purposes. This is especially true in the case of radio telephony, for in that case the band of frequencies transmitted or received must be at least as wide as the speech range of frequencies which is to be transmitted.

Normally this speech band is about 2,500.

Wide, and if the carrier frequency is, for example, 100,000, then the transmitted or received oscillations will vary from about 97,500 to about 102,500. This means that the antenna and associated circuits should not be tuned too sharply; otherwise import tant parts of the signal will be substantially eliminated. The usual .method heretofore for obtaining suflicient width for these circuits is to refrain from too sharp tuning, either of the antenna or its associated circuits. But in flattening the tuning of the circuits, discrimination is lost so far as frequencies outside of the desired range is concerned. As a result, it is extremely difficult to obtain satisfactory separation of the transmitted and the received oscillations. Conditions, so far as this discrimination or separation is concerned, may, of course, be improved by separating the frequencies of transmission and reception, but such separation means that a larger portion of available frequencies is required for one signaling channel. One object of this invention is to bring the transmitted and the received frequencies closer together and still satisfactory separation.

In this invention we propose to makeuse of the peculiar characteristics of wave filters which have the property of transmitting or suppressing certain bands of" frequencies, and which at the same time enable 'a sharp cut-off at any desired point. Such wave filters have been used heretofore in radio and other circuits in various places, but in this invention we propose to use them in combinations to bring out their inherent selective properties to the best advantage.

The invention will be better understood by reference to the accompanying drawings, in which Figure 1 shows one form of circuit for duplex signalingyFig. 2 shows a modificationthereof; andFigsB to 3 show various forms of-Wave filters with their characteristics which may be used in one place or another as disclosed hereinafter.

Referring more particularly to Fig. 1, there is shown an antenna 5 which is used both for transmission and reception. The first of these may, for example, take .place on a'frequency of 525,000 and the second'on a frequency of, say, 500,000. This antenna may be tuned with sufficient flatness to be sensitive to both of these frequencies with their associated side bands, or it may be given two degrees of freedom, one of these corpermit of I responding to the transmission frequency f and the other to the reception frequency, for both of these frequencies the tuning being sufiici-ently broad for the passage of the necessary band of frequencies required for signaling purposes; Associated with this antenna is a circuit adapted to take 0& the received oscillations or impress the transmitted oscillations, this circuitfin turn being associated with some remote subscribers station,the message coming in and going out this filter the circuit leads to an amplifier I 10 which may be of any suitable form but is preferably of the thermionic vacuum tube type. A radiofrequency oscillation generator 12'is arranged to impress oscillations on a modulating device 14, which latter is preferably of the thermionic vacuum tube type. The amplified speech signals are also, by means of the transformer 15, impressed on the input circuit of the device 1a and in series with the oscillations from the genera tor 12. The device 141- is a modulator by virtue of its distorted or'curved characteristic and will contain in its output circuit, among other things, a modulated wave on a carrier of 525,000 cycles. If s, be taken to represent the variable frequency of the speech message to be transmitted, the modulated' wave will consist of three components Whose values will be h, f -l-s' and f,s where f, represents the carrier frequency which is here taken as 525,000. These modulated waves will now be impressed upon and radiated" from the antenna 5 by virtue of a transformer coupling including the coils 17 and 18: Thecondenser 19 may be used in I addition to the coil 17 for the purpose of tuningjthis circuit to any desired frequency. Received signals consisting of some carrier frequency 72-, with the corresponding modulated side band-s f +s and f s will be received by the antenna and impressed on the output circuit of the modulating device 1 1. By virtue of its curved characteristic, this device will also act as a demodulator to give certain" frequencies including, among others, sum and-difference frequencies of the received signals and the signals to be transmitted. If, forexample, the person at the remote'end' 0f the line L is silent, there will be present in the output of the device 1 L oscillations of frequency f,. These, combining with the incoming signals, will give the difference frequencies f f and 7" f is,. In the case assumed, f,;, has the value of 25,000 but this may be made an y value de sired, although we prefer to have it of fairly low ultra-audio frequency, for reasons which will appear later. For convenience, this intermediate frequency f,,f will be designated by f,. Associated with the circuit of the device 1 L by means of a transformer 20 is a circuit which leads to a detecting device. This circuit comprises first a band exclusion'filter 22 which has the property of excluding anv band of frequenciesfor which it may be adjusted, and: in this case it is adjusted to. exclude frequen cies in the neighborhood of f and 7", but in such amanner as to permit freely the passage-of the intermediate frequency f and its associated side bands. Following this, there is inserted a band filter 23 which passes the intermediate frequency and its side bands but is adjusted to cut off as sharply as possible rr-bo h sid s; The: cui ll W le d to an amplifier 24, then to a second band filter 25 of the same properties as the filter The output of the filter 25 is impressed on a detecting device 26 which may be of any desired form, although again we prefer the thermionic vacuum tube type. The output of this device 26 will contain a plurality of different oscillations among which will be the message 8 and to permit the ready passage of this message but the suppression of all other frequencies, there is now inserted in the line a filter27, preferably of the lowpass type, allowing all frequencies below, say, 2500 to pass. From here the circuit goes to the conjugate terminals of the coil 7, thus permittinga message to be transmitted to the remote speaker.

In the description thus far we have spoken of the use of different types of filters. Such filters are described fully in the patents to Campbell, No. 1,227,113, May 22,1917, and No. 1,493,600, dated May 13, 1924:. iVithout desiring to go into a detail-ed explanation of these, reference may be made to Figs. 3 to 3, which willillustrate the characteristics of these filters. If the filters for example, be constructed of a series of elements consisting of an inductance in series and a capacity C in shunt, the circuit will'have the properties of a low pass filter, that is, it will permit all frequencies below a certain value to pass but will suppress all frequencies above that value as indicated in the characteristic curve of Fig. 3 The sharpness of the cut-off will be increased by using inductances and capacities of very low resist ance values, andit is the intention in this in vention that filters with sharp cut-off characteristics shall be used. By increasing the complexity of the filter arrangement so that each element of the filter consistsfof an inductance and capacity in series with each other in the line, and of an inductance and a capacity in parallel to each other and in shunt to the line, a filter may be obtained which permits the passage of a band only as indicated in the curve of Fig. 3 Such a filter'has been termed a band-pass filter and with suitable precautions the cut-0H" may be made: such as to give a selectivity far surpassing that obtainable with tuned cir cuits. Fig. 3 indicates the structure for a l'iandexclusion filter. In this case, the series elements comprise an inductance and capacity in parallel to each other, and the shunt elements comprise an inductance and capacity in series with each other. The characteristic then obtain-ed is as indicated by the curve of Fig. 3, where frequency is again plotted as abscissae and attenuation as ordinates.

In view of the large voltages of the transmitting-frequency present in the circuit of the modulator 1 1, it is necessary to provide some protection for the amplifier 24 and it is for this reason that we insert the band p a band of width 25 andha-ving f, as. its

mid-value. If this f, is in the neighborhood of some tens of thousands, a filter can be constructed which is extremely selective, that is,

having extremely sharp cut-off points.

Obviously, the output from the filter 23 may pass directly to the detector 26. However, as in the figure, we prefer to use an amplifier and an additional wave filter. The filter 23 in front of the amplifier 24 serves the added function of protecting the amplifier from excessive power which might possibly come in, of frequencies either above or below the band which is passed. At the same time, the band filter 25, following the amplifier, performs the function of suppressing any additional frequencies which may be developedin the circuit of the tube 24 by virtue of the fact that it. is not a perfect amplifier. Whilet-he low-pass filter 27 is not absolutely essential in this circuit, we desire to use it tosuppress such 'h1gh frequencies as will be generated in thecircuit of the detector 26. If such oscillations were permitted to pass, they would, in general, find sufficient unbalance in the coil 7 to permit the passage of such frequencies into the circuit 8 with the possibility that the transmitting and receiving circuits will oscillate at some undesired frequency. With such a filter as 27, however, this is prevented'and the oscillations which do pass the filter will find a well balanced coil at 7, permitting the ready transmission of the signal 8 to the remote end of the line.

It is evident that in case the message 8, is coming forward at the same time as s, is received, modulated signals for s, will still be transmitted through the receiving circuit and be carried to the'speaker. It

will thus be evident to the speaker that the party at the other station is endeavoring to speak. I

In Fig. 2 we have shown amodification of the circuit arrangement. The message 8, is, as before, impressed on a low-pass filter 9 and an amplifier 10. An oscillator 32, preferably of the thermionic tube type, is supplied to generate oscillations of the transmitting frequency f,, which ma in this case be taken as 525,000. A modu ating device 34 is associated with the oscillator in any will be certain ones.

suitable manner, as, for example, inthe wellknown Heising shunt circuit which is shown in this figure. Theiamplified signal 8 from the amplifier 10 is then impressed on'thc modulator 34 and as aresult thereexists in the output circuit of the tube 32 modulated oscillations of thefrequencyf, and 348,, whichmay then be radiated as before.

Incoming modulated messages on the car rier 7, are received on the antenna and picked up by acircuit36 tuned to the ca rrier f by which these oscillations may be impressed on the grid of the oscillator 32. There will then been modulation in the oscillator circuit of the locallygenerated frequency f, and the received signal giving, among other things, the intermediate frequencies f f "andf -f is These oscilations-of'intermediate frequency are transmited by means of the transformer 37 to a receiving circuit which we have shown, as consisting of a low-pass filter 38, a band-pass filter 39, an amplifier 40, a band-pass filter 41,

a detector 42and alow-p'ass filter 43. These variouselements have the following proper ties and. functions. The low-pass filter 38 has a cut-off which may be in the neighbor hood of 30,000. The band-pass filter 39 has a mid-value of f f which in this case is 25,000, and the width of this filter is such as to just permit the passage of the band .f /"gis The filter 41 has the same properties as the filter 39. The detector42 may be of any suitable form, such as the thermionic type, and detects the modulated intermediate frequency; down to the speech signal. The low-pass filter 43 transmits all frequencies below 2500 and suppresses all others.

The low-pass filter'38 performs the same function as the band-exclusion filter of Fig. 1. In general, it may be stated that filters which cut off on one side only and which have fewer elements, as seen from Fig. 3*, can be constructed to withstand higher volt ages and also with sharper cut-off characteristics. It is desirable, however, to follow this filter 38 with a band-pass filter to eliminate circuit of the oscillator 32, of which there The arrangement of two band-pass filters with an amplifier between has the same desirable properties as indicated in connection with Fig. 1.

Having now described our invention, what is claimed is:

1. In a duplex high frequency signaling system. a transmitting and receiving circuit, a modulat-ing-demodulating circuit, associated therewithcomprising a vacuum tube, the plate circuit ofsaid tube'carrying the oscillations to be transmitted and the received oscillations and acting to clemodulate said received oscillations to an intermediate frequency, a band-excluding filter connected only.

with the said plate circuit to suppress the transmitting and receiving frequencies and to pass the demodulated oscillations, a bandpassing filter to pass the intermediate frequency, and a detector for demodulating said passed intermediate frequency.

2. In a duplex radio telephone system, an antenna, a transmitting circuit associated therewith, means for separating the transmitted and the received signals comprising a filter adjustedto exclude the transmitting and the received frequencies, and band filters in series therewith to pass an intermediate frequency produced by beating the re- I ceived frequencies with the transmitting carrier frequency.

3. In a duplex radio telephone system, a

transmitting and receiving antenna adaptedto transmit on one frequency and receive on a different frequency, means for generating and impressing modulated high frequency oscillations on said antenna, said means com prising a vacuum tube circuit, said vacuum tube circuit having means to receive and de- :modulate the incoming signals to an intera. In a luplex high frequency telephone system, a transmitting and receiving circuit,

a vacuum tube associated therewith for impressin'gon the circuit the oscillations to be transmitted and for demodulating the received oscillations, a receiving circuit associated with said tube, a filter for suppressing the high voltage transmitting oscillat-1ons,a nd abandpass, filter for passingthe demodulated frequencies. I

5. In a duplex radio telephone systemadapted to transmit on one frequency and re-v ceive on another frequency, a. transmitting and receiving circuit, a Vacuumtube having a source of oscillations associated therewith for impressing on the circuit the oscillations ceived and transmitted frequencies and a band-pass filter for passlng the intermediate frequency.

6. In a duplex radio telephone system adapted to transmit on, one frequency and receive on a different frequency, an antenna,

a transmitting and receiving circuit, a vacuum tube having a sourceof oscillations associated therewith for impressing on the antenna the oscillations to be transmitted and for demodulating the received oscillations to an intermediate frequencyequal to the difference between the transmitted and received frequencies, a receiving circuit associated with said. tube, a filter for suppressing the high voltage high frequency oscillations, and a band-pass filter in series therewith for passing the intermediate modulated frequency, and a detector for demodulating said lastfrequency to the message frequency.

In testimony whereof, we have signed our names to this specificationthis 14th day of August, 1922.

LLOYD ESPENSCI-IIED. DE LOSSK, MARTIN.

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