US2083474A - High frequency receiving apparatus - Google Patents

Description

June 8, 1937.

E; Y. ROBINSON 2,083,474 HIGH FREQUENCY RECEIVING APPARATUS Filed July 25, 1934 3 Sheets-Sheet 1 Fig. I.

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b wA f meg June 8, 1937.

E. Y. ROBINSON y Y 2,083,474 HIGH FREQUENCY RECEIVING APPARATUS Filed July 25, 1934 3 Sheets-Sheet 2 Fig 4.

Inventor: Errwes't Y. Robinson His Attorney 1 June 8, 1937. E. Y. ROBINSON 2,083,474

HIGH FREQUENCY RECEIVING APPARATUS Filed July 25, 1934 3 Sheets-Sheet 3 Inventor: Ernest Y. Robinson,

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l atenteci June 8,

UNITED STATES 2,083,474 HIGH FREQUENCY RECEIVING APPARATUS" Ernest Yeoman Robinson, Eniield, England, as-

signor to General. Electric Company, a corpration of New York Application July 25, 1934, Serial No. 736,837

In Great Claims.

My invention relates to high frequency receiving apparatus and'more particularly to such apparatus incorporating automatic volume control means.

5 In high frequency receivers provided with automatic volume control means the gain of the receiver is varied inversely with the incoming signal and therefore the gain of the receiver is greater when the receiver is off tune with the inincoming carrier wave. For example, if the receiver be tuned to an incoming modulated carrier wave and the tuning of the receiver be varied, the sensitivity of the receiver first increases as the receiver is gradually detuned from the condition of resonance with the received carrier Wave, and continues to increase until the incoming signal after amplification is of such a low amplitude that the voltage after amplification does not exceed the delay voltage on the automatic volume control circuit. Subsequently the sensitivity of the receiver remains constant unless it is arranged for noise suppression, in which case at some state after the receiver has reached maximum sensitivity the output voltage from the re ceiver is reduced to zero.

When the receiver is not properly in tune with the carrier wave but is sufficiently in tune therewith to remove the noise suppression voltage applied, for example, to thelow frequency amplifier of the receiver, the sensitivity of the receiver is greatly above normal, thus leading to excessive amplification of the side bands due to the higher audio frequencies. This effect renders the operation of the receiver very unpleasant particularly 5 when stations of a strong field strength are being received.

Further owing to the fact that the amplification of the receiver is increased as the receiver is detuned from. resonance with the received carrier wave the aural output volume of such a receiver may be held constant, or even increased, over a certain range of the tuning control on either side of resonance. Aural tuning of the receiver is, therefore, rendered difficult and additional devices such as visual tuning meters are desirable.

One object of the present invention is to provide a new method of noise suppression, whereby the receiver has a very low inter-channel gain.

A second object of the invention is to provide means whereby excessive amplification of the receiver when it is tuned to the side band of the received signal is reduced, whereby the generation of unpleasant noises and side band shriek is eliminated or reduced. A third object of the Britain November 8, 19-33 invention is to provide means whereby the acoustic output of the receiver is greatest when the receiver is exactly in tune with the incoming signal, whereby the receiver may be accurately tuned by aural means and without recourse to additional devices such as visual tuning meters.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying draw ings in which Figsg'l, 3, and 4 to '7 represent different embodiments of my invention, and Fig. 2 represents certain characteristics of the operation ofthe form of the invention shown in Fig. 1.

Referring to Fig. 1 of the drawings I have shown therein a portion of the circuits of a radio receiver involving electron discharge devices I, 2 and 3. The electron discharge device I is a radio frequency amplifier of the conventional variable mu type, this tube being provided with the well known variable pitch control grid 4, a screen 5, and suppressor grid 6, the latter being connected directly to the cathode I. The control grid 4 is connected to a tuned input circuit 8 to which radio frequency electromotive forces may be supplied either from the antenna circuit, or from preceding stages, not shown, of the receiver. The anode of the discharge device I is likewise connected to a tuned output circuit 9 which in turn is closely coupled to a tuned circuit IB comprising the input circuit of the elec- 35 tron discharge device 2. The discharge device 2, as shown, is one'of the type having a cathode i! which cooperates with an anode I2 in the manner of the electrodes of a diode. It also cooperates with a variable pitch control grid 4, screen grid l3, suppressor grid l4 and anode 15 in the same manner as in discharge device I. The anode l5 of the discharge device 2 is connected to a tuned circuit I! which is loosely coupled to a tuned circuit I8, 4 these circuits being loosely coupled forhigh selectivity, the circuit I8 comprising the input circuit to the valve 3.

The electron discharge device 3 is provided with 50 a cathode and two anodes 59 between the latter of which the tuned circuit I8 is connected, this. portion of the discharge device 3 cooperating in. the manner of a full-wave detector or rectifier.

The discharge device 3 is also provided with five 55 cooperate in a manner later to be described;

I oscillations supplied to the input circuit 8 are" amplified in discharge device I and supplied 7 negative, this ,negative unidirectional voltage be- Itwill now be observed that high frequency through circuits 9 and I to the diode rectifier 'of the discharge device 2, and also to grid 4 of device 2 in which they are amplified and supplied to circuit l1. Rectifier H, I 2 is the auto-- matic volume control rectifierof the radio receiver, the anode being connected to the cathode through a load resistance 2| and-a cathode'bias resistor 22, this latter resistor being connected 1 Y received carrier wave has anintensity less than aicertain predetermined value. When the received high frequency electromotive force is greater than said predetermined intensity a unidirectional voltage appears upon resistance 2| causing the upper terminal thereof to become ing applied through the filter comprising resistance 23 and condenser 23, and input circuit 8, to the grid of discharge device I, and through inputcircuit ID to grid 4 of discharge device 2. In this way the grids ofldischarge' devices I and 2 are driven negatively by an amount sufficient to maintain the signal wave electromotive forces in the circuit l1 substantially constant irrespec-' tiveof any variations in intensity ofthe received carrier wave produced, for example, as by fading. High frequency oscillations in the circuit I! are supplied to'the rectifier section l8, IQ of the discharge device 3 whereby'they are rectified. When this, occurs a unidirectional voltage, and also the audio frequency voltage resulting from demodulation of the'carrier wave, are produced upon'resistance. which is connected between the cathode and an intermediate point on the inductance of the tuned circuit l8. The unidirectional voltage produced on this resistance issupplied to the control grid G1 of device'3 through a resistance 25. The audio frequency voltage is likewise applied 7 to thisgrid through a condenser 26 which may be variably connected to the resistance 24 for manual volume control purposes. Grid G1 is a variable pitch grid similar to grids 4 and 4 of devices l and 2.

The action of discharge device 3 will now be described. Itjwill be ,observed'that the anode 20 of the dischargeldevice 3 is connected to the positive side of the power source through aresistance 21 and that grids G2 and G4 areconnected directly to the positive side of this supply source. The cathode is connected to the negative side of the supply source'through a resistance 28, and the grid G3 is connected through a resistance 29 team intermediatepoint on 'a bleeder resistance.

30 -across the supply source.

In theabsence of a signal in the circuit l8 having intensity greater than a predetermined amplitude, due to currentfiowing in resistance 28 and the tap connection ofresistance 29 to resistance 30, the grid G3 is sufiiciently negative with respect to the cathode to preclude the flow of any 2,083,474 grids G1, G2, G3, G4, G and an anode which current to anode 20. All of the current flowing through resistance 28 flows from the grid G2 to the cathode. This prevents any audio frequency output from appearing on resistance 21 where it might be tapped off through an output condenser 3| to any desired load circuit such as a loudspeaker or subsequent audio amplifier. This predetermined intensity may be so chosen as to preclude the production of noise currents in the output when the carrier is of less than a desired value.

As the signal electromotive force in circuits l1 and It! increases, however, the unidirectional voltage on resistance 24 increases thereby driving the grid G1 increasingly negative with respect to the cathode. This in turn reduces the amount 01' current flowing in resistance 28 and thereby causes the grid G3 to become gradually lessnegative with respect to the cathode. When the received signal exceeds the said predetermined amplitude current starts to flow in the resistance 21 across which the audio voltage appears which is supplied through the condenser 3| to the load circuit. This output thus'increases in accordance with the intensity of the received signal wave due to the increased amplification of the amplifier section of the valve 3 in response to the increased carrier wave.

In this way'noise suppression in the receiver iseffected. Electromotive forces supplied to the grid Gi, due to atmospheric electricity, static, etc. are not ordinarily of suflicient mean valueto affect appreciably the voltage on grid Ga. By means of resistance 29 and bypass condenser 29', the circuit of grid Ga is given a time constant such that large atmospheric and static electromotive forces do not appreciably affect this grid. In this way and by proper adjustment of the tap point of resistance 29 to resistance 30 atmos pherics and other interference may be completely suppressed.

The tuning elementsof the tuned circuits 8, 9, l0, l1 and. I8 may if desired be provided with unicontrol means whereby the receiver be tuned to any desired' frequency. Preferably, however, these circuits are comprised in the intermediate frequency portion of .a superheterodyne receiver whereby. theyImay be tuned to a single fixed frequency. In such a receiver the intermediate frequency is supplied to the circuit 8 from the first detector, not shown, which may be tuned by unicontrol with the tuning of a local oscillator and antenna circuit, which may include radio amplifiers. These portions of the equipment being entirely conventional are omitted for simplicity of the drawings. Thus whenthe receiver is in tune with any desired. carrier wave a constant intermediate frequency is suppliedto the circuit 8. Upon variation of the unicontrol mem-' ber in either directionfrom the resonance position, however, the intermediate frequency varies and is, therefore, rapidly attenuat'edby reason of the selectivity of the radio, and intermediate frequency, circuits. i

The operation of my inventionlwith respect to aural tuning may now be described. Let us assume that tuned circuits 8, 9,10, I1 and I 8 are all adjusted for resonance with a desired intermediate frequency, which varies in amplitude, as due to fading; 'Due to the automatic volume control action of diodes II and I2 the amplification of amplifiers l and 2, and if desired, earlier radio frequency stages, isvaried inversely with the intensity of the received carrier wavewith the re suit that a substantially constant carrier electro- I motive force is maintained in circuits [1 and I8.

maximum in the audio This result may be obtained to an extremely high degree particularly in view of the fact that the automatic volume control results from rectification of the high frequency voltage at a point in the circuit prior to the amplification produced by the amplifier section of discharge device 2. The amplification of discharge device 3,.therefore, remains constant irrespective of the intensity of received signals, and the output audio volume likewise remains constant.

Let us assume now that the input voltage applied to the receiver is constant but that'the in.- termediate frequency varies by reason of variation of the tuning of the local oscillator from the point where the exact desired intermediate frequency is produced. As this variation in frequency occurs the signal intensity applied to the rectifier ll, 12, of course, reduces by reason oi the selectivity of the circuits thereby causing in-- creasedamplification of the discharge devices l and 2 and a tendency to cause either a constant or an increased carrier voltage in the circuit l'l over a certain range of operation of the tuning control. However, owing to the greater selectivity of the circuits between the input to the receiver and the anodes I9, over that between the input to the receiver and the anode l2 a reduction in the signal voltage appearing in circuit 18 immediately occurs. this reduction in carrier wave voltage greatly reducing the amplification of the discharge device 3 until a point is reached where no output occurs. It will thus be seen that in the tuning operation a relatively sharp output of the receiver is exactly in tune This characterisproduced when the receiver is with the received: carrier wave.

tic of the apparatus greatly facilitates the tuning of the radio receiver by the aid of the aural response notwithstanding the use of the automatic volume control circuits.

This latter characteristic of the apparatus also has the desired advantage that it reduces the unpleasant sounds produced in the radio receiver commonly known as side band shriek. These sounds are produced when the radio receiver is adjusted for resonance with a side band of a received carrier wave owing to the increased amplification of the amplifiers as the receiver is detuned for resonance with the carrier wave. Since in accordance with my invention the amplification of the discharge device 3 is reduced at this time these side band shrieks are removed.

To increase the effectiveness of the operation of my apparatus both for aural tuning purposes and for reduction of side band shriek, and interchannel gain, it is desirable that the selectivity of the portion of radio receiver prior to the anodes l9 be as much greater than that of the portion of the receiver prior to anode l2 as possible. For this reason circuits 9 and I desirably are relatively closely coupled whereas circuits l1 and I8 are relatively loosely coupled. Of course this difference in selectivity may be increased by any of the well known expedients, such as the inclusion of additional stages of amplification or tuning between the circuits i1 and i9.

In Fig. 2 I have shown certain characteristics of the operation of the discharge device 3 of Fig. 1. The curves a, b, c, d, and e of this figure express the relation between voltage E applied to the control grid plotted as abscissa and current Ia flowing in the anode of the discharge device for different values of voltages E between the oathode and the grid G3. The curve a expresses the I relation between the current in the anode and the control grid voltage when the voltage between the grid G3 and the cathode is relativelylarge,

whereas the curve e expresses the same relation when the voltage betweenv the cathode and the voltage G3 is relatively small, curves 1), c, and d expressing the same relation for intermediate values of the voltage between the cathode and the grid G3.

In the circuit illustrated the variation of the unidirectional voltage applied to control grid G1 causes a variation in the unidirectional voltage E appearing between the cathode and the grid G3. The dotted line i represents the locus of the latter voltage occurring as the control grid voltage increases. That is, the curve 1 is drawn through points on the curves at to e, each of which points corresponds to the value of Is. which may occur when the voltage EG is such as to produce a voltage E corresponding to the different curves. Of course the pitch of this curve may be varied by Variation of the different resistances and connections affecting the potentials on grids E6 and E Fig. 3 shows a different embodiment of the invention in which the noise suppression feature of the apparatus shown in Fig. 1 is not employed. In this figure high frequency oscillations from the output 9 of amplifier I are supplied directly through condenser 33 to an automatic volume control diode l2, II. This diode although represented as in a separate envelope from the discharge device 2 has its cathode connected with the cathode of the discharge device 2 and functions in the same way as the diode ll, [2 of Fig. 1. The bias resistance 22 in the cathode circuit of discharge device 2 serves'to produce a delay bias between the anode and cathode of the automatic volume control rectifier as previously described. The voltage on the load resistance 2| of the automatic volume control rectifier is supplied through the filter comprising resistance 23 and condenser 23' to the grid of the radio frequency amplifier I and also through resistances 34 and 35 to the grid of amplifier 2, which amplifier in this case serves as an audio amplifier, the

output of which is coupled thereto through a condenser 36. Resistance 34 is the load resistance of the aural tuning rectifier 31 of the apparatus, this rectifier being connected across a tuned circuit 38 which is loosely coupled to the circuit 9.

Thus in this figure if we assume that the receiver be tuned to resonance with a received carrier wave the amplification of tube I will be varied by action of the automatic volume control detector 1 I, I2 by an amount sufiicient to maintain a substantially constant intermediate frequency electromotive force in the circuit 9 with the result that the amplification of discharge device 2 and the output signal volume remain substantially constant. The signal electromotive force in the circuit 9, however, varies to a certain extent. Owing to the fact that the voltage upon the circuit 38 is considerably smaller than that upon the circuit 9 and also owing to the fact that the voltage produced on resistance 34 is opposed to that produced on resistance 2| in the direct current circuit between the cathode and grid of .tube 2, the output from tube 2 is less subject to variation than that from tube 1 and may be relatively constant. volume of the radio receiver, when it is adjusted for resonance with the received carrier wave, is

maintained; constant irrespective of fading.

Thus the output .f-Duringvariation ofthe-tuning of the receiver from resonance with": the carrierwave, the automatic volumelcontrol circuit tends ,to maintain a constant signal electromotiveforce in the circuit 3, but, owing to the greater. selectivity of that portion'of the receiver'prior to the aural tuning rectifier 31 over. that prior to the automatic volume'oontrol rectifier It, |2-,,the carrierwave voltage-in circuit 38 immediately reduces thereby reducing the voltage on resistance 34 and callsing the negative voltage produced by rectifier H; l2 to become efiective on the control grid of amplifier 2. 'This,'of course, decreases the amplification of the amplifier 2 and thus immediately reduces the output volume. Thus as. the tuning control is operated through resonance to acarrier wave a distinct maximum of output volume occurs at the position of the tuning control where the receiver is in resonance with the carrier wave whereby tuning of the receiver in accordance with the aural response is facilitated. Fig.4 difiers fromFig. 3 only inthat discharge device 2 in this case operates as a radio amplifier, the condenser 38 being connected between the grid of. discharge device 2 and a point on the .tuned' circuit 38. The element 35 inthis case maycomprise either a resistance or a'choke coil,

' as desired. It is believed that the operation of When the tuning of the receiver is varied, howthe explanation already The equipment illustrated in Fig. 5 differs from this form of my invention will beapparent from given.

the forms of the invention already described in the'aural tuning voltage is applied, not to an amplifier subsequent in the circuit to the aural tuning rectifier, but to one-prior thereto. In this case the automatic volume control voltage which appears on'resistance 2| is'silpplied to the grid of discharge device I' through filter resistance 23, resistances 34 and 35,'and tuned input circuit 8 ,of'the amplifier I. The aural tuning rectifier 31 produces a undirectional voltage upon resistance 34 which is opposed to that on resistance 2| in the above-described circuit. However,- the automatic volume control voltage is sufiicient to maintain constant volume when the radio receiver is in afixed tuned condition.

ever, owing to the difference in selectivity of the receiver prior to volume control rectifier H, l2 and that prior to'aural tuning rectifier 31, the reduction in voltage across circuit 38 due to variation in the intermediate frequency is suflicient to cause a rapid drop in voltage on resistance 34 thereby causing the grid of amplifier 'l to become increasingly negative and the amplification of this device to be reduced. This in turn causes the output volume to vary in'such a way as greatly to facilitate the tuning of the receiver by aid of the aural response. As thus. arranged the degree of aural tuning,.however; and noisesuppression arising out'ofaural tuning is limited by the fact that the minimum gain of the amplifier l mustbe suflicient to produce an appreciable output voltage across the load 34 of the aural tuning control rectifier 31.

In Fig. 6 Ihave shown a further modification of my invention inwhich the low frequency amplifier 3.is one having an amplifier section similar tothat shown in Fig. 1, the operation of which is controlled bya diode 39. The diode ll, I2 operates for automatic volume controlpurposes in the same way as described in connection.

3,1 4,, mid s. Thediode awn cooperation with the discharge device '3operates to eflfect. noise suppression.

Itwill .be observed thatthe cathode of the discharge device 3isjconnected to the negative side of the supply sourcenot only. through the bias resistor 28 but also through a dropping resist-.

ance 40. The negative end of the dropping resistance is connected to the grid Ga through a resistance 4|,connected across the diode 38, and

a filter comprising-resistance. 42 andycondenser I 43. The cathode of the'diode 39 is connected through. coupling condenser 44 to one side 01 the aural tuning circuit 38 whereby .this discharge device is effectively energized with high frequency oscillations. The grid G3, in the absence of a signal of greater than a predetermined intensity, is sufliciently negative with respect to the cathode to prevent the flow of anode current and thus audio output is precluded from this device.- Resistance 4| is so poled, however, that when'signals of greater than a predetermined intensity are received the negativevoltage of the grid-G3 is reduced to such an extent that current starts to flow in the anode of the discharge device. The voltageappearlng on resistance 4|, however, is sufliciently greatto cause the tube 3 to produce a switch-like action whereby the output signal when sufliciently reduced from normalsuddenlydrops to zero. In this way noise suppression during periods when. the receiver is oif tune with any carrier wave or when the received signalsare-of less thana predetermined intensityis effective;

.In other respects the operation of the equipment of Fig. 6 will be readily understood from the explanation already given it being noted furtheronly that the additional tuned circuits 45 and 4,6'are interposed between circuit .9 and circuit 38: to increase the diflerence in selectivity between the portion of the receiver prior to the automatic volume control diode ll, l2 and that portion prior to the aural tuning control diode 31. These circuits areconnectedv together across the. power supply'mains by capacity "and may, of

' course, be employed in connection with the equipment of any of the figures ifincreased selectivity between circuits 9 and 38bedesired.

In all of the figures previously described the selectivity of the system prior to demodulation of the carrier wave for reproduction of the desired signalis the same as that for aural tuning control.

In Fig. 7 I have shown the aural tuning control rectifier'3l coupled to the tuned circuit 8 through the additional tuned circuit 48, electron discharge amplifier 49, and tuned circuits 50. A

diode'5l in series with load resistance 52 is connected across the circuit 48, this diode having its load resistance connected through the audio coupling condenser 53 to the grid of audio amplifier 2. Thus the selectivity of that portion of the receiver prior to aural tuning control diode 31 is much greater than that of that portion 0! the receiver prior to thedemodulator. 5|. In this While I have shown particular embodiments of my invention it will of course be understood that I do not wish to be limited thereto since many modifications both in the circuit arrangement and in the instrumentalities employed may be made, and that I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1. In combination, a high frequency amplifier having an amplification control electrode, a circuit connected to the output of said amplifier, means to supply two opposed unidirectional electromotive forces to said control electrode, one of said electromotive forces varying directly with the intensity of the signal currents in said output and poled to reduce the gain of said amplifier, and the other of said electromotive forces varying directly as the intensity of signals at a certain point in said circuit, and frequency selective means between said output of said amplifier and said certain point.

2. In combination, an amplifier having a gain control electrode, means to supply two opposed unidirectional electromotive forces to said electrode, one of said electromotive forces varying directly as the intensity of signals supplied to said amplifier and poled to reduce the gain of said amplifier, frequency selective means, means to supply oscillations having the frequency of oscillations amplified by said amplifier to said fre" quency selective means, and means to rectify said oscillations after traversing said frequency selective means to produce the other of said two opposed unidirectional electromotive forces.

3. The combination, in a high frequency receiver, of an amplifier having a control electrode circuit having opposed unidirectional electromotive forces therein, means to produce one of said electromotive forces by rectification of the received carrier waves at one point in said receiver, means to produce the other of said electromotive forces by rectification of the received carrier wave at a different point in said receiver, and a frequency selective circuit connected between said points.

4. The combination, in a high frequency receiver, of a pair of electron discharge amplifiers, a frequency sensitive circuit between said amplifiers, means to transmit received oscillations through said amplifiers and circuit, means to produce two unidirectional electromotive forces, one of said electromotive forces varying directly as the intensity of said oscillations on one side of said circuit and the other of said electromotive forces varying directly as the intensiy of oscillations on the other side of said circuit, means to control the amplification of one of said amplifiers in accordance with the magnitude of one of said electromotive' forces, and means to control the amplification of the other of said amplifiers in accordance with the difference in said electromotive forces.

5. The combination, in a radio receiver, of an amplifier including an electron discharge device, means to control the amplification of said electron discharge device in opposite senses in response to the intensity of high frequency oscillations at different respective points in said receiver, said means being operative to effect said control during desired reproduction of signals by said receiver, and means to transmit said oscillations frozn'one of said points to the other through a frequency selective circuit.

ERNEST YEOMAN ROBINSON.

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