Oct. 29, 1940.
`2,219,749
`A. A. OSWALD
`SINGLE SIDE BAND DIVERSITY RADIO RECEIVING SYSTEM
`Filed Aug. 31,
`1939
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`IPR2018-01476
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`Patented Oct. 29, 1940
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`PATENT OFFICE
`UNITED STATES
`2,219,749
`SNGLE SDE BAND DIVERSITY RADIO RE
`CEVNING SYSTEM
`Arthur A. Oswald, Maplewood, N. J., assignor to
`Bell Telephone Laboratories, Incorporated, New
`York, N. Y., a corporation of New York
`Application August 31,
`1939, Serial No. 292,777
`(C. 250-20)
`6 Claims.
`receiving station. When carrier is transmitted
`This invention relates to radio communication
`at a reduced energy level, it is customary either
`systems, and more particularly to methods and
`to utilize it at the receiving station to control the
`means for reducing fading effects and maintain
`frequency of a local oscillator that supplies the
`ing a favorable signal-to-noise ratio in a diversity
`reception system. The present application re
`frequency for final demodulation, or, after re
`conditioning and amplifying at the receiving sta
`lates to a specific modification of the im Wention
`tion, to utilize it at the proper frequency directly
`disclosed and claimed in the copending applica
`to effect the demodulation of the signal side
`tion Serial No. 291,481 filed August 23, 1939, of
`bands. In either case the locally generated de
`F. A. Polkinghorn, assigned to the same assignee
`modulating frequency, or the demodulating fre
`to which the present application is assigned.
`quency controlled by the original carrier, or the
`In a diversity system the same transmitted sig -
`reconditioned and amplified original carrier itself
`nal is simultaneously received over a plurality of
`at the proper frequency for demodulation has in
`paths through space from the signal source at the
`Some cases been supplied to the final detectors at
`transmitting station. The amplitude of received
`a fixed amplitude that does not vary with the
`15 signal over the various paths usually varies as
`variation of carrier amplitude that may take
`between the different paths with changes in the
`place as a result of the presence or absence of
`Condition of the Space transmission medium, So
`fading over the space path.
`?*
`that as the amplitude of the signal received over
`" Such practice is not favorable to the best op
`one path increases, that over another or other
`20
`eration of a, diversity System in which the plu
`of the paths may diminish. These amplitude
`rality of paths over which the signal is received
`Variations are generally at random and are con
`are at all times connected with the signal receiv
`tinually taking place.
`ing apparatus at the receiving station. This is
`It is customary in certain types of diversity
`because the fading over any path which dimin
`Systems either to provide means for automatical
`ishes the amplitude of the signal does not dimin
`ly SWitching the Signal receiving apparatus at the
`ish the amplitude of the noise that enters the
`receiving Station. So as to keep it connected With
`receiving. System by way of the receiver input of
`the path over which the signal of the greatest
`that path. The constant amplitude of the locally
`amplitude at any particular time is arriving, or,
`supplied carrier, or of the reconditioned received
`alternatively, to permit the signal receiving appa
`carrier, causes the relative noise output of that
`ratus at all times to be connected with all of the
`30
`path to remain constant even though the signal
`paths and to automatically control the energy
`Contributed by the path has faded. Therefore
`contributions to the signal receiver by the various
`all of the paths contribute noise to the common
`paths in proportion to the varying amplitudes of
`Signal receiving apparatus, while only those of
`the signals being received over them. It is
`them in which fading has not occurred contribute
`toward diversity operation of the latter sort that
`35
`Signal at a favorable signal-to-noise ratio. The
`the present invention is particularly directed.
`result is that, in the absence of some corrective
`In the double side band type of radio transmis
`measure, the Over-all signal-to-noise ratio of such
`sion the carrier upon which the side bands are
`a system is adversely affected.
`based is usually transmitted with the side bands
`In the diversity System of the present invention
`at sufficient amplitude to provide for the proper
`this effect is overcome by the employment of a
`demodulation of the side bands in the final de
`method and means for controlling the amplitudes
`tector at the receiving Station. When general
`of the signal and accompanying noise passing by
`fading takes place, the carrier fades with the side
`Way of each of the diversity branches to the com
`bands; and as the noise introduced at the receiv
`mon signal receiver in accordance With the am
`ing station input of the fading path is trans
`45
`plitude of a selected portion of the energy re
`mitted to the receiving apparatus as a modulation
`ceived from the transmitting station over the cor
`of the carrier frequency, the same fading of the
`responding Space path. Specifically, in the pre
`carrier and its side bands which reduces the sig
`ferred embodiment, when the signal and selected
`nal amplitude also reduces the noise amplitude.
`controlling portion, which may for example be
`In the single side band type of radio transmis
`the accompanying carrier or an accompanying
`sion it has been the practice either to eliminate
`pilot frequency, of any path fade, the gain in the
`the carrier entirely, or to transmit it at a much
`signal and noise transmitting portions of the cor
`reduced energy level. When no carrier is trans
`responding diversity branch is caused likewise to
`mitted, a suitable demodulating carrier of the
`be diminished. Thus, the output of any one or
`proper frequency is generated and Supplied at the
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`more of the diversity branches upon which the
`branch 2, respectively, control the potential drop
`signal is fading is reduced in accordance with
`across the common resistance 30, and the variable
`the amount of fading, and a more favorable over
`potential thus developed is applied to the ampli
`all signal-to-noise ratio is maintained in the com
`fiers 2 and 2' of branch and branch 2, respec
`mon Signal receiver into which all of the branches
`tively, to give the usual diversity common auto
`feed.
`matic volume control to the System.
`The manner in which this result is accom
`In the operation of the System it is assumed
`plished Will be explained by reference to the ac
`that the distant station is transmitting a radio
`Companying drawing which represents Schemati
`frequency carrier of relatively small amplitude,
`cally a single side-band diversity receiving system
`and a single side band resulting from the modu
`consisting, as illustrated, of two radio reception
`lation of the carrier frequency with the signal
`branches, each arranged to receive radio signals
`frequency. The radio receiver elements
`and
`arriving from the same signal source over dif
`of the two radio reception branches and 2
`ferent Space paths, convert the signals to audio
`are tuned to select the carrier and its side band,
`frequencies and transmit them to a common Sig
`and the beat oscillator 20 is so adjusted as to
`15
`nal receiving device.
`produce, by modulation with the incoming wave,
`For the sake of simplicity, communication of
`an intermediate frequency Wave of the desired
`the radio receiving apparatus with the different
`frequency, for instance, 400 kilocycles. A por
`Space paths is represented as being by way of
`tion of this amplified intermediate frequency, as
`the antennas A and A2, which may be of any
`has been described, passes into the branch circuit
`20
`Suitable type. These antennas feed over radio
`including the band-pass crystal filter 5. This
`reception branches and 2, respectively, to the
`filter may be designed to pass a frequency band
`common signal receiver O. Only two antennas
`the width of which is sufficient to accommodate
`with their aSSociated radio receivers are shown,
`the carrier at its intermediate frequency value
`but it will be understood that any greater number,
`With Such frequency Variation as may occur on
`usually three in a space diversity system, may be
`25
`either side of this value as a result of imperfect
`used. The elements included in each radio re
`frequency stability in the carrier oscillator at
`ception branch are of identically the same type
`the transmitting station and in the beat oscil
`as those included in the other branch; and cor
`lator 20 at the receiving station. For a mod
`responding elements are designated by the same
`erate degree of frequency instability in these
`30
`reference numbers, excepting that a prime is
`OScillators the band passed by the filter 5 may
`affixed to the designating numerals of the second
`be 200 cycles wide, or narrower to the extent
`branch. The description of the arrangement and
`that the oscillator stability warrants. After
`operation of the elements in one branch may
`paSSing through filter 5 the narrow band in
`therefore be understood to apply to the operation
`cluding the carrier at its intermediate frequency
`35
`and arrangement of the elements in the other
`Value paSSes to the input circuits of detectors 6
`branch.
`and 7.
`The energy received by the antenna, passes to
`In the output of detector 6 there is produced
`high frequency amplifier and first detector elle
`a direct current the amplitude of which varies
`ments . In the first detector the received en
`With the variations in the amplitude of the car
`ergy is modulated by a high frequency wave sup
`40
`rier received over the corresponding branch.
`plied by the beat OScillator 20 and the desired
`Any fading of the carrier and its associated side
`products of modulation are amplified in a two
`band at the receiving antenna, of the correspond
`stage intermediate frequency amplifier 2. The
`45
`ing branch results in a reduction in current flow
`above-mentioned elements may be regarded as
`ing through the resistance 9 in the output cir
`, Constituting the radio receiver of the reception
`cuit of detector 6 and a corresponding potential
`branch. The intermediate, frequency amplifier
`reduction across the resistance.
`2 operates into a branched circuit, one branch
`The bias in the input circuit of the audio fre
`of Which includes an isolation amplifier 3 and
`quency amplifier tube 8 is determined by the
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`a second detector 4, and the other branch of
`potential of the biasing battery 22 and the po
`which selects a narrow frequency band includ
`tential drop across the resistance 9. This bias
`ing the carrier that passes through the band
`is so adjusted that with no carrier input to de
`pass crystal filter 5 to a rectifying device which
`tector 6, amplifier 8 substantially suppresses
`in the present instance consists of the two de
`transmission to the common signal circuit. With
`tectors 6 and 7.
`a normal carrier input to detector f 6 the in
`To the output of the second detector 4 there
`creased potential across resistance 19, in opposi
`is supplied a local carrier frequency from the
`tion to the potential of biasing battery 22, de
`local carrier Oscillator 30, and the audio fre
`creases the negative bias in the input circuit of
`quency products of modulation of the local car
`audio frequency amplifier 8 to produce normal
`rier and the signal intermediate frequencies feed
`into the input of audio frequency amplifier 8
`gain in the transmission of the signal of this
`branch to the signal receiver. Thus, when the
`by way of transformer 2. The degree of ampli
`fading Over any path is such as to produce an
`fication of the audio frequencies in amplifier 8
`unfavorable signal-to-noise ratio, the gain in the
`is controlled by the potential across the resistance
`corresponding branch of the System is automati
`9 included in the Output circuit of the detector
`6, this potential being applied in opposition to
`cally reduced in proportion to the reduction of
`the normal biasing potential in the input circuit
`the radio frequency input to the corresponding
`of the amplifier 8. The amplified audio fre
`antenna, and the contribution of that branch to
`the common signal receiver circuit is propor
`quencies of branch f, together with those of
`branch 2, are then inductively communicated by
`tionately diminished or Substantially suppressed.
`70
`way of transformers 23 and 23, respectively, to
`An indicated in the drawing, the amplifier 3
`a circuit in which they are combined, further am
`in the signal Sub-branch of each of the branches
`plified if desired, and passed to the Common Sig
`and 2 is of the screen grid type for preventing
`nal receiver 0, ..
`the local carrier oscillations which are fed into
`The detector tubes 7 and 7 of branch and
`the plate circuit of the second detector 4 from
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`3
`2,219,749
`feeding back through the channel and appearing
`signal Sub-branch that includes the second de
`tector and audio frequency amplifier is controlled
`in the other detector circuits.
`in amplitude in accordance with the amplitude of
`The diversity common automatic Volume con
`the rectified energy received from the transmit
`trol which is secured by employing the Voltage
`drop across the resistance 40 common to the
`ting station.
`*
`If it should be desired to effect a greater de
`plate circuits of the detectors 7 and 7' to con
`gree of frequency stability, this may be done in
`trol the grid bias of the amplifiers of Several
`branches simultaneously and equally, is well
`the manner disclosed in Patent 2,041,855 to R. S.
`Ohl, issued May 26, 1936.
`That is, the filtered
`known in the art and needs no specific descrip
`carrier received from the transmitting station
`tion. It ensures that the radio frequency Out
`may be reconditioned and then used for obtain
`puts of the respective receivers of the Several
`ing automatic frequency control of the beating
`branches shall have the same relative signal
`oscillator 20 and synchronization of the local
`strengths as the radio frequency. inputs to the
`carrier oscillator 30. Or, if desired, in accordance
`receivers.
`.???
`with the disclosure of the Ohl patent, the filtered
`If it be assumed that, from a condition of
`carrier after being reconditioned and amplified
`equality, the radio frequency energy received
`may consitute the source of locally applied car
`upon antenna. A and transmitted through radio
`rier Oscillations in place of the local carrier Oscil
`reception branch increases, while that received
`lator 3). As disclosed in the Ohl patent, the
`upon antenna, A2 and transmitted through radio
`reconditioned carrier is Supplied for demodula
`reception branch 2 diminishes,
`the Common
`tion at substantially constant amplitude regard
`automatic volume control effected by detectors
`less of fading in transmission, and therefore, if
`and 7' and the individual transmission gain
`applied in the present system in substitution of
`control effected by detectors 6 and 6' will
`operate in such a way as to tend to magnify the
`the carrier Oscillations generated locally by Oscil
`lator 30, the System is susceptible of and requires
`contribution of branch f which has the more
`the same automatic gain control in accordance
`favorable signal-to-noise ratio and to reduce the
`with the amplitude of the rectified received car
`contribution of branch 2 Which has the less
`rier energy of the corresponding branch that is
`favorable signal-to-noise ratio. The increasing
`utilized in the present invention.
`negative bias on the amplifier tubes of both
`The Specific disclosure herein is that of a di
`branches resulting from the increasing radio fre
`30
`versity system in which the same signal is simul
`quency input from antenna. A stabilizes the
`taneously received over a plurality of paths by
`volume contribution of branch f at a prede
`Way of Separate antennas. It will be understood
`termined value and reduces the relative Con
`that the invention is also applicable to an angle
`tribution of branch 2 in which the signal is
`diversity system in which the same signal is si
`fading. At the same time detector 6 of branch
`multaneously received at different vertical angles
`operates to reduce the negative bias in the in
`by way of a single array of spaced antenna units,
`put circuit of audio frequency amplifier 8 to
`and is demodulated at each angle by recondi
`permit a proportionately greater Signal trans
`tioned carrier arriving at that angle, or by locally
`mission through this branch to the common
`generated carrier Synchronized by the received
`signal receiver; while detector 6' of branch 2,
`carrier of the corresponding path.
`responding to the diminishing energy input re
`ceived from antenna A2, reduces the opposing
`It Will also be understood that the transmis
`Sion gain control of each reception branch in
`bias developed in resistance 9', thus increasing
`accordance With the amplitude of the rectified
`the negative bias in the input circuit of amplifier
`energy received from the transmitting station
`8 to diminish the audio frequency output in ac
`Over that branch may, within the scope of the
`cordance with the reduction in radio frequency
`invention, be alternatively applied at other points
`energy received over the corresponding branch.
`in the signal conveying portion of the sub-branch,
`The result is that the signal received in the com
`as at the input of the isolation amplifier 3.
`mon signal receiver O is principally that re
`In the foregoing the energy received over each
`ceived over the path of the diversity system which
`has the most favorable signal-to-noise ratio,
`branch and selected and rectified to control the
`signal transmission gain of that branch, has
`while the noise that Would otherwise enter the
`been specifically described as that of the trans
`common receiver over the path or paths of re
`duced signal energy is diminished or completely
`mitted carrier frequency. It will be understood
`that in systems where the carrier is entirely sup
`Suppressed by the device which reduces the gain
`55
`pressed and a pilot frequency is transmitted in
`of the transmission path in accordance with the
`its place, as a side-band of the suppressed carrier,
`reduction in the received radio frequency energy.
`this pilot frequency may be selected and rectified
`In a properly proportioned system in accordance
`and used to control the signal gain of the cor
`with the present invention the signal volume de
`responding branch in the same manner as has
`livered to signal receiver 0 is held substantially
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`been described for the carrier.
`constant.
`v. The interposition of a rectified-carrier-oper
`What is claimed is:
`1. In a diversity radio receiving system, a, plu
`ated gain controlling device, such as the audio
`rality of radio receivers arranged to receive radio
`frequency amplifier 8, in the path which the
`signals from the same signal source by different
`signals and noise must traverse to reach the
`65
`paths, means for selecting and rectifying a por
`common signal receiver 10, further serves the
`tion of the energy received from the transmitting
`useful function of excluding from the common
`station by each receiver, means for combining the
`signal circuit modulation products due to the
`rectified currents, means for controlling the am
`intermodulation of noise components With each
`other, which intermodulation may take place
`plification of all the receivers in accordance with
`even in the absence of the locally applied modu
`the combined rectified currents so that the radio
`frequency Outputs of the respective receivers have
`lating carrier frequency. Whatever the nature
`the same relative signal strengths as the inputs
`of the noise and signal demodulation in the Second
`detector may be, the transmission of the signal
`thereto, a signal detector for each receiver, said
`75
`and noise components through that part of the
`detectors having their inputs connected to the
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`4.
`2,219,749
`radio frequency outputs of the respective receiv
`ing system comprising a plurality of radio recep
`ers, means for Supplying carrier Oscillations to
`tion branches each connected With a common sig
`said detectors, an amplifier for each receiver con
`nal receiver, means Operatively coupled to each
`nected in the signal conveying circuit of the cor
`branch for Selecting and rectifying a portion of
`responding detector, an audio frequency circuit
`the energy received from the transmitting station
`common to Said amplifiers and signal detectors
`by Way of the corresponding branch, a signal de
`of all of Said receivers, and means responsive to
`tector for each branch, a substantially constant
`said individual rectified currents for so control
`amplitude Source of carrier oscillations connected
`ling the amplification of the respective amplifiers
`to the detectors of the various branches, an audio
`and detectors as to maintain the audio signal con
`frequency amplifier for each branch included be
`tribution of each receiver to said common circuit
`tWeen the detector output of the corresponding
`proportional to the respective received radio sig
`branch and Said common signal receiver, and
`nal strengths.
`means responsive to the amplitude variations of
`2. In a reduced carrier diversity radio receiving
`the rectified energy of each branch for controlling
`System comprising a plurality of radio reception
`the gain of Said audio frequency amplifier of the
`corresponding branch.
`branches each including a radio receiver and each
`connected with a common signal receiver, means
`5. In a diversity radio receiving system com
`operatively coupled to each branch for selecting
`prising a plurality of radio reception branches
`and rectifying a portion of the signal carrier,
`connected With a common signal receiver and
`each including a radio receiver, means operatively
`means under the control of the combined rectified
`20
`carrier currents of all the branches for similarly
`coupled to each branch for selecting and rectify
`controlling the amplification of the radio receivers
`ing a portion of the energy received from the
`of all of the branches so that the radio frequency
`transmitting station by Way of the corresponding
`Outputs of the respective receivers have the same
`branch, means Operated by the combined recti
`relative Signal strengths as the inputs thereto, a
`fied energy derived from all of the branches for
`25
`signal detector for each branch, a substantially
`similarly controlling the amplification of the radio
`constant amplitude Source of carrier oscillations
`receivers of all of the branches so that the radio
`at the receiving station connected to the detector
`frequency outputs of the respective receivers have
`inputs of the various branches, an amplifier for
`the same relative signal strengths as the inputs
`each branch included in the signal conveying cir
`thereto, a signal detector for each branch, a sub
`30
`cuit of the corresponding detector, and means
`stantially constant amplitude source of carrier
`responsive to the amplitude variations of the rec
`OScillations at the receiving station connected to
`tified carrier current of each branch for control- .
`the detectors of the various branches, an audio
`ling the gain of said amplifier of the correspond
`frequency amplifier for each branch included in
`i ing branch.
`the signal detector output of the corresponding
`35
`3. In a diversity radio receiving system com
`branch, and means responsive to the amplitude
`prising a plurality of radio reception branches
`variations of the rectified current of each branch
`each including a radio receiver and each Con
`for controlling the gain of said audio frequency
`nected With a common signal receiver, means Op
`amplifier of the corresponding branch.
`eratively coupled to each branch for selecting and
`6. The method of maintaining a favorable sig
`rectifying a portion of the energy received from
`nal-to-noise ratio in a diversity radio receiving
`System utilizing means for receiving in different
`the transmitting station by each receiver, means
`under the control of the combined rectified cur
`radio reception branches the radio signals arriv
`rents of all the branches for similarly controlling
`ing from the Same signal source over different
`the amplification of the radio receivers of all of
`Space paths, applying equal amplification to the
`the branches so that the radio frequency outputs
`energy received in each branch, separately select
`of the respective receivers having the same rela
`ind and rectifying a portion of the energy re
`tive signal strengths as the inputs thereto, a Sig
`ceived over each branch, and separately demodul
`nal detector for each branch, a substantially con
`lating the signal energy component of each
`stant amplitude source of carrier oscillations at
`branch by carrier oscillations supplied from a
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`the receiving station connected to the detector in
`Substantially constant amplitude local source,
`puts of the various branches, an amplifier for
`which consists in Separately amplifying the de
`each branch included in the signal conveying cir
`modulated signal energy of each branch, utilizing
`cuit of the corresponding detector, and means re
`the rectified portion of the energy of each branch
`sponsive to the amplitude variations of the recti
`to control in accordance With its own amplitude
`fied carrier current of each branch for Controlling
`the degree of amplification of the demodulated
`the gain of that portion of the corresponding
`signal component of the corresponding branch,
`branch which includes said signal detector and
`and combining and utilizing the demodulated sig
`said amplifier.
`nal Components of all the branches.
`4. In a single side band diversity radio receiv
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