US2482549A - Method of and apparatus for receiving radio signals - Google Patents

Description

Sept. 20, 1949.

R. T. KlLLMAN ET AL METHOD OF AND APPARATUS FOR RECEIVING RADIO SIGNALS Filed Dec. 3, 1945 12 AMP.

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Patented Sept. 20, 1949 UNITE o, PATENT.

METHOD: OF' AND APPARATUS FOR RECEIVING RADIO SIGNALS Robert '11.,Killmanand Frederick W..Beeslcy,.

Nashville, Tenn,

Application December 3, 1943,'ISerial=No.512,6941;

2 Claims- 1;

This" invention: relates to impmvementswin methods and apparatus for selectingand detecting radio waves;

In all; previous: methods ofyselecting a'desired signal from the many present in :the ether the signals, including; the one desired, .are usually picked up ;by an antenna and passed-ithrough one or more-resonant circuits, composedof suitable inductance a-nd'capacity which aretuned to the frequency-of the desired signal. These resonant circuits areusually parallelsresonant, and serve to increase the voltage appearin -across them at the frequency ofthe desired signal while the volt;- ageat another frequencies isnotincreased; or:at least not increased to such. extent as ,is the'voltage-at the resonant frequency; These resonant circuits are-usually coupled by electronic tube amplifiers; and due to the selective and: amplifying action of such a system the desiredisignalmay be picked out of the many signalsiimpinging on the-antenna andamplified -to the excl'usion of the unwanted signals.

Various, modifications have beenvmade' in the above described basic system, suchras for rexample, the; use of the. supereheterodyne principle wherein-the frequency of. the:.=-desi-red signal-is changed to :some arbitrary fixed intermediate frequency and then is passed through resonant circuits. and amplifiers as described abovel- However all "previously known-methods of selecting. the desired signal from the Welter of signals present in the ether rely on the resonant circuit.

This type of signal selecting system has several disadvantages. As is well, known bythose skilled in the art, in order to increase the selectivityof aresonant circuit it is necessary to decreasejhe resistance-of the circuit, or in other words, to in.- crease the Q. This has the. effect iofrdeoreasing the decrement of the circuit and if carried-sumciently-far the decrement of the circuit becomes so small that so called tails? are formed onthe ends of the dots and dashes of codelsignals and the, higher modulation frequencies ofa. modulated signal are distorted vand attenuated since. the deer rement. of the, signalis. less than that of thefIeSO-r nant circuit and therefore the currents ,-in rthe resonant circuit cannot vary in amplitude asrap; idly as the signal.

A further disadvantage of resonant circuits. as signal, selectors lies in the fact that they arecar pahle of being shock excited. A stray impulse oirsuch; short duration that it; couldnot beheard mayshock the;reso11am; circuit; so-that itgenens 1,-

ordinaryreceivers so; garbles; on,jams-.;

ates adamped wavertrainiofas fic nt dnrationitv cause-an-:.audible impulse.

Accordin ly it is; amobi'ect; of this inventlorsiio disclos s method ofnselectine. acles edsienaitfmm a number oi undesired-si nals: ofz diiferent flier quencies and to amplify and demodulate thre desired, signal, and to partially or totallyexclnde: the undesired signalswithout, makin useof tuned; .Or resonant circuits; or thejmeqnivalent:.:

A further object otthis inventiomis to provide means, w r ya sired-signal: e selected and. differentiated from an; ndesiredizsignalifor si n ls of: the ame: frequencit:provided-thereasiredrandundesired s enalsa enotrimphasei-the results beingmostsatisfactory when:thetwo sig; nals are of oppositephaser;

A further object of this invention is. toiprovlde means whereby the efiect; of- -atmosnheric 013218031 strays ;01 static upon gtheedesiredusignalt ma n-he materially reducede ifrleql ncy ut.- ofe pp site phase are transmitted simultaneously; one; of; said. waves being modue latedby the :desi-redsv intelligenceswhile-tthe rather wave is modula-ted by an unintelligible; ffj -ing? signal which when bothtswaves are receiyedaby themrdesired intelligence .modulation tha-tgi lS1j,-1l Iil ead;-;- able, but which whensreceived by; the receiving system of this invention canrabe separated: into its, twocomponents and the desiredintellieehce becomesreadablei The means; by which these andlothercbjects, as

will presently-appear; is accomnlished maywbe more. "readily; understoodi by"; efc rencemtoi the accompanying drawings; whereiniFig-gl is a single channel system for receiving radiolsignalsemalr i -use of,-the.i rincip esi tithis; inventiom-Eig. 2 s a two chann l-isystemzmakinguse ofmeutralizm tion-toeliminate a ls but :thedesired signal; and Fig'ure fi man-al ernate single channel system for receiving radi sis alsxmakin zusezof the:princivv ples, of this invention-.;;

Referrin naw; to-Fi 1n the. numeral ifl denotes theusual. rec iving :antennaspreferablyneriadic or untuned. The signals received by antenna-1E0 are passedthrough; and -,alnnlifi ed by thenntuned radiee reqne yzamp ifier:l [1.1.

The ,outnut; of amplifier l t isapfllied acrossthe grid-J 3-.andth cathode I 6 ofiitheel'ectronili$11138 I12. An. adiustablee square ii wave generator... l8

' GGGSF z- HMQHE hrou h @theaadiustable when:

shifter l9 to the grid l5 and cathode it of tube l2. The square wave output of the generator [8 is of such voltage that when its positive peak is applied to grid tube 12 operates as a class A amplifier but when the negative peak of the square wave voltage is applied to grid 15 tube I2 is completely blocked and is incapable of passing signals received upon grid [3.

A plate or anode I1 serves to receive the electron stream emanating from the cathode l6 and acted upon by the two grids l3 and I5 and also feeds the output of the tube l2 through an audio amplifier to a sound reproducer 2|.

The operation of the system just described is as follows: The desired radio frequency wave emanating from a transmitter (not shown) together with the many undesired waves impinge upon the antenna I0 and are amplified by th amplifier ll and passed on to grid l3. During the time interval that the desired radio frequency signal is passing from zero through its positive peak and back to zero on grid 13, the square wave generator l8, which is set to generate a square wave of a frequency identical to that of the desired signal, is adjusted to apply the positive peak of its square wave output to grid 15 so that, during this interval, the tube l2 acts as a class A amplifier and the plate current of the tube is a replica of the grid Voltage on grid 13, and the signal is passed on to the audio amplifier 20 and reproducer 2|. However during the time interval in which the desired radio-frequency wave passes from'zero through its negative peak and back to zero, the negative peak of the square wave output of generator I8 is applied to grid l5 so that tube I2 is blocked and no signal is passed on to the audio amplifier 20 and reproducer 2|.

Thus it will be seen that only the positive half cycles of the desired signal are passed on to the audio amplifier 20. The envelope of these positive half cycles constitutes the audio modulation and this audio wave or envelope is amplified by the audio amplifier 28 and reproduced into sound by the reproducer 2 I.

From the above it will be seen that the action is very similar to ordinary demodulation except that the only signal which is completely demodulated is the one of the frequency and phase of the square wave generator.

Any wave of a different frequency is not completely demodulated i. e. parts of both its negative and positive half cycles appear in the output and cancel each other.

It will be noted that, while in the above description it was said that the positive half cycles of the desired signal were passed but the negative half cycles were not, it might just as well be the other way round, i. e., the negative half cycles could be passed through and the positive half cycles blocked.

In order to make the system responsive only to positive half-cycles of the desired signal, a negative voltage may be introduced by the battery 22, as shown in Figure 3 of such magnitude as to just cause plate current cut-off. In this condition grid I3 will be responsive only to positive half -cycles, while the negative half-cycles applied thereto serve only to drive grid l3 further negative.

By adjusting the phase shifter l9 so that during the interval that positive half cycles of the signal are applied to grid I 3 (the tube I2 being biased to cut-off by the voltage above referred to inserted in its grid circuit by battery 22, as shown in Figure 3) so that the negative peaks of the square wave voltage are applied to grid l5, thus blocking the tube during this interval, the signal may be eliminated, while any other signal having positive half-cycles which appear between the positive half-cycles of the eliminated signal will appear in the output.

This phenomena allows a second circuit arrangement to be set up as shown in Fig. 2. In this figure the numeral l0 denotes the antenna, the untuned radio-frequency amplifier H, tube l2, grid [3, grid l5, cathode I6, plate IT, square wave generator l8, phase shifter [9, audio amplifier 20 and. reproducer 2| being the same as previously described in reference to Fig. 1. The circuit through tube I2 is called channel A.

However a second channel, marked channel 13, between the amplifier II and the amplifier 20, comprises the detector 24 and the phase inverter 25. The detector 24 is biased to cut-off by the battery 23 and the tube I2 is likewise biased to cut-ofi by battery 22. The connections, polarities, and voltages are so adjusted that the output of amplifier II is divided, half going through channel A and half going through channel B.

The phase inverter 25 inverts the phase of the currents in channel B so that they oppose those in channel A and any signal which appears both in channels A and B will be completely neutralized and will not appear in the combined output of channels A and B and will not be passed on to audio amplifier 28 nor reproduced by the reproducer 2 l, while any signal appearing in only one channel will not be neutralized and therefore will be passed on to audio amplifier 2c and will be reproduced by reproducer 2 i.

In operation of the circuit shown in Fig. 2, the desired signal together with many undesired signals, strays, static, etc., impinge upon antenna 10 and are amplified by amplifier H and passed on to channels A and B in equal proportions.

Since the tubes in both channels are biased to cut-off only positive voltages are effective in either channnel. In channel A the desired signal is eliminated by means of the action of the square wave generator in blocking tube [2 during the intervals that the positive half cycles of the desired signal appear on grid l3, but at all times between these intervals the tube is responsive to any other positive voltages from other signals or static and the envelope of these signals will be passed on as an audio signal to the amplifier 20. However the envelope of the desired signal will not be passed since each positive half cycle of the desired signal was eliminated as above described and all negative half cycles have been eliminated due to the high bias voltage 22.

However the positive half cycles of the desired signal together with the positive components of all the undesired signals and static etc. are passed by channel B and the envelope of these positive impulses constitute an audio wave which is passed through the phase inverter 25 which causes its phase to be opposite to the audio signal coming through channel A. These two audio voltages are combined and as pointed out above all portions which are similar cancel out and only the desired signal which appears in only channel B is amplified by amplifier 2B and reproduced by reproducer 2i.

It is of course understood that the devices shown in Figs. 1 and 2 include all the necessary and usual accessories for proper operation such as power and voltage supplies, shielding, filters,

networks, etc. some of which, such as the shield grid I4 and the filter condenser 20a, are shown,

.tive half cycles of the oscillator output,

but others are not shown, these details and auxiliaries forming no part of the basic concept of this invention. It is also understood that this invention is not limited to the actual devices shown, but equivalents for any of the devices shown for the purpose used herein may be substituted therefor. For example, for the square wave generator I8 shown, there may be substituted a simple radio frequency oscillator together with a half wave rectifier which removes the posi- The negative half cycles which remain, when applied to grid 15, if of sufficient amplitude, have the same effect thereon as did the square Wave output of generator l8, since the tube l2 will be blocked very early during each of these negative half cycles and will remain so until very late in said negative half' cycle and it will not matter that the negative half cycle has a much greater negative amplitude between these two points than is required to block the tube. 7

Also a properly wound neutralizing transformer or other means may be used to differentially couple channels A and B to amplifier of Fig. 2 instead of making use of the phase inverter to cause the signals in channel A to neutralize like signals in channel B.

While there has been described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications maybe made therein without departing from the spirit of the invention, and it is therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a radio receiving system, a signal wave collecting system'which is non-difierentiative as between desired and undesired signal waves, a two channel signal wave passing system, each of said channels being normally responsive only to signal wave half cycles of one polarity, one of said channels being capable of being intermittently blocked so that during the blocked interval said channel is not responsive to signal waves of either polarity, means for blocking said last named channel during the interval that the half cycles of the desired signal wave of the polarity to which said channel is normally responsive are applied thereto but allowing said channel to pass all signal waves of the polarity to which it is normally responsive which are applied thereto during the interval that half cycles of the desired signal of the opposite polarity to that to which the said channel is normally responsive are applied thereto, means for differentially combining the outputs of the two channels so that all signal waves appearing in both channels are cancelled cut, while signal waves apearing in only one channel are not cancelled, and means for I translating the uncancelled waves into sensible phenomena.

2. In a radio receiving system, a signal collecting antenna, an untuned radio-frequency amplifier coupled to the antenna, two signal passing channels A and B coupled to the amplifier, channel A comprising an electronic tube having at least an anode, a cathode and two grids, the first grid being connected to the amplifier, means for generating an oscillatory voltage of rectangular wave form, means for adjusting the frequency of said voltage, means for adjusting the phase of said voltage with respect to time, means for applying the voltage to the second grid of the tube in such manner that during the interval that the positive half-cycle of said voltage is applied thereto the tube Will operate as a clas B amplifier and will be responsive only to the positive half-cycles of signals applied to the first grid, but during the interval that the negative half-cycle of the locally generated oscillatory voltage is applied to the second grid the tube will be completely blocked and not responsive to signals of either polarity applied to the first grid, channel B comprising a detector or rectifier system responsive to signal half cycles of only one polarity and means for differentially combining the outputs of channels A and B so that signals appearing in both channels will be cancelled while a signal appearing in only one channel will not be cancelled, an audio amplifier for amplifying the uncancelled signal, and an audio reproducer for translating said signal into sound.

ROBERT T. KILLMAN.

FREDERICK W. BEESLEY.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 913,521 Latour Feb. 23, 1909 1,244,697 Carson Oct. 30, 1917 1,616,923 Ranger Feb. 8, 1927 1,658,672 Davis Feb. 7, 1928 1,723,451 Von Arco Aug. 6, 1929 1,802,745 Whitaker Apr. 28, 1931 1,874,910 Conrad Aug. 30, 1932 1,959,275 McCaa May 15, 1934 1,979,463 Goshaw Nov. 6, 1934 2,048,081 Riggs July 21, 1936 2,194,292 Bligh Mar. 19, 1940 2,240,500 Gabrilovitch May 6, 1941 2,266,401 Reeves Dec. 16, 1941 2,268,001 Farnholz Dec. 30, 1941 2,272,070 Reeves Feb. 3, 1942 2,295,207 Gabrilovitch Sept, 8, 1942 2,352,634 Hull July 4, 1944

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