US2072365A - Amplification control system - Google Patents

Description

March 2, 1937. D. GRIMES AMPLIFICATION CONTROL SYSTEM Filed April 26, 1932 2 Sheets-Sheet 1 INVENTOR DAV@ @mms W BY g /Wvh/ ATroRNEY March 2, 1937.

D. GRIMEs AMPLIFICATION CONTROL SYSTEM Filed April 26, 1952 2 Sheets-'Sheet 2 A Y RM M R m6 W.. N n ED V.I A mw D Bf we. gm

Patented Mar. 2, 1937 PATENT ori-ICE AMPLIFICATION CONTROL SYSTEM navidcrimes, pongan Hills, staten Island, N. Y., assignor to Radio Corporation of America., a. corporation of Delaware i Application Anna 26, 1932, serial Nn. 607,535

y l5 Claims.

My present invention relates to amplification control, and more particularly to a novel method of, and means for, controlling the gain of a radio receiver in such a manner that it is maintained substantially uniformover a predetermined frequency range. Y

It is well known, especially in superheterodyne reception, that a tunable radio frequency circuit inherently tends to increase in transmission efciency as the frequency of the range being amplified increases. Stated in other words, it is a well known fact that the gain of a radio frequency amplifier using constant grid bias varies directly with frequency of the amplified energy. In superheterodyne receivers, however, there exists an effect which can be taken advantage of to overcome the aforementioned inherent characteristic of tunable radio circuits. It is understood, of course, that in a radio receiver it is extremely desirable to maintain the maximum gain substantially uniform over the operating frequency range of the receiver.

The oscillator plate current increases with frequency in the local oscillator circuit of a superheterodyne receiver, and, hence, there is provided a device which can be readily employed for overcoming the inherent tendencies of the radio frequency circuits to increase their transmission efficiency with frequency. Now, I have discovered a method of, and means for, taking advantage of the aforementioned phenomenon, and it may be briefly stated that the present invention involves utilizing oscillator plate current to vary the bias of the amplifier grids in that direction which will tend to overcome the inherent tendency of the radio frequency circuits to increase in transmission eiiiciency-With frequency.

Accordingly, it may be stated that it is one of the main objects of my present inventicn to provide in a radio receiver employing radio frequency circuits normally tending to increase in transmission efficiency with frequency, a circuit adapted to supply a` biasing potential to the amplifier which varies with frequency variation in such a manner that the aforementioned tendency is constantly balanced.

Another important object of the present invcntion is to provide a superheterodyne receiver with a radio frequency amplification circuit having substantially uniform gain over the operating frequency range, such substantial uniform gain being produced by employing the local oscillator plate current, which increases with frequency, to effect biasing of the radio frequency amplifier.

Another object of the present invention is to provide a superheterodyne receiver arranged to maintain a constant intermediate frequency difference throughout a predetermined frequency range, and, additionally, to arrange the local oscillator circuit in such a manner with respect to the radio frequency amplifier and the frequency changer circuits that substantially uniform radio frequency amplification is secured over the said frequency range.

And still other objects of the present invention are to improve generally the simplicity and ampliiicationk efficiency of radio receivers, and to particularly provide a superheterodyne radio receiver .which is not only reliable in operation, but substantially uniform in its radio frequency transmission and economically manufactured and assembled. v

Thenovel features which I believe to be Ycharacteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization andA method of operation will best be understood by reference to the following description taken in i connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into. effect.

' In the drawings, Fig. l diagrammatically shows a receiver em bodying the present invention, n

Fig. 2 diagrammatically shows a receiver embodying a modified form of the invention.

Referring now to the accompanying drawings in which like characters of reference indicate the same parts in the different figures, there is shown, in Fig. 1, a superheterodyne receiver consisting of the usual grounded antenna circuit A, G, which is coupled, as at M, to the tunable input circuit of a screen grid tube I. The said input circuit includes a variable tuning condenser 2 which has its rotor grounded, as at 3, a radio frequency bypass condenser 4 being connected between the grounded side of the rotor and the low potential terminal of the secondary of the coupling means M. l The anode of the tube I is arranged to have suitable positive potential supplied to it through the primary of a coupling transformer M1. The screen grid of the tube I is arranged for application thereto of a suitable positive potential from a source (not shown) S. The cathode heating source, the screen grid source, and the anode potential source are not shown, but it is to be clearly understood that these may comprise battery,

or commercial power line, sources, and since such devices are well known to those skilled in the art, they need not be referred to in any further detail.

` The output circuit of the radio frequency amplier is coupled, as at M1, to the tunable input circuit of a screen grid tube 5. A variable tuning condenser 2', having its rotor grounded, as at 3', is arranged in the input circuit of tube 5. The radio frequency by-pass condenser 4 is connected in the same manner as shown in the input circuit of tube I. Tube 5 is included in the frequency changer, or first detector, stage. A radio frequency by-pass condenser 6 is shunted across the primary of the coupling device M2, the latter coupling the output of the first detector to the input of the intermediate frequency amplifier 1.

The amplifier 'l is shown in conventional form, and since the construction of the amplifier is very well known to those skilled in the art, it need not be described in any further detail. The amplified intermediate frequency energy is, of course, detected in a second detector; the output of the second detector is, then, amplified in an audio frequency amplifier, and subsequently utilized in any desired manner, as by headphones, loud speaker, or any other type of reproducer. Since the present invention applies particularly to the radio frequency amplifier, first detector and local oscillator circuits, it is not believed necessary to describe the portion of the receiver following the first detector any further.

The local oscillator circuit comprises a screen grid tube 8 having an inductance coil 9 connected between its control electrode, or grid, and its cathode. In series with the cathode and the coil 9 there is arranged a resistor I9 which is used for a purpose to be shortly described. The variable tuning condenser VI I is connected in shunt with the coil 9, the rotor of the condenser being grounded, as at I2. The anode of the tube 8 is inductively coupled, as at M3, to its input circuit, the potential source I3 having its positive terminal connected to the anode, and its negative terminal connected to the low potential side of the resistor I0. By means of the leads I9 and I9 the low potential terminals of the inductances included respectively in the tuned input circuits of the amplifier and detector are connected to the low potential side of the resistor I0.

A radio frequency by-pass condenser I4 has one of its terminals connected to the positive side of the source I3, and its other terminal grounded. The screen grid of the tube 8 is shown connected by an adjustable lead I5 to the source I3 so that appro-priate positive potential may be applied to the screen grid.

The rotors of the condensers 2, 2 and II are shown connected by dotted lines, to a common, or uni-control, tuning device I6. It is to be understood that the mechanical coupling of the rotors of these various tuning condensers may be arranged in any desired manner well known to those skilled in the art. For example, the three condensers may be arranged in a common housing, in the well known bath-tub construction manner, and in such case the three condensers will be similar. In order to tune the local oscillator through a frequency range differing from the frequency range of the radio frequency amplifier and first detector by a constant amount, which amount equals the desired intermediate frequency, resort may be had to the well known series-shunt fixed condenser construction disclosed by W. L. Carlson in his U. S. Patent 1,740,331 of December 1'7, 1929.

Briefly, this arrangement comprises a fixed condenser I l connected in series between the condenser II and the inductance coil 9, and a fixed condenser IS connected in shunt with the tuning condenser II. As explained in the Carlson patent, appropriate design of these condensers results in securing a constant intermediate frequency difference throughout the operating range of the uni-control device i6. Of course, more than one radio frequency amplification stage may be employed preceding the first detector, but in order to preserve simplicity of description only a single stage is shown.

The operation of the receiver shown in Fig. l will now be described with particular attention directed towards the functioning of the present invention. Those skilled in the art Well know that signal energy collected by the antenna circuit A, G is impressed upon the input of the radio frequency amplifier, the amplified energy then being impressed upon the input of the first detector which is tuned to the signal frequency. The local oscillator circuit is coupled, as at M4, to the input of the first detector, and, hence, impresses upon the input of the first detector, energy of a frequency differing from the signal frequency by the desired intermediate frequency.

The input of the intermediate frequency amplier is maintained fixedly tuned to the desired intermediate frequency, and there will be transmitted intermediate frequency energy from the first detector output to the input of the intermediate frequency amplier. As stated, heretofore, the oscillator plate current increases with frequency, and therefore the drop of potential across the resistor iii will vary directly with frequency. 'This latter relation follows in virtue of the fact that the resistor I is disposed in 'the plate to cathode current path of tube 8. It is not believed necessary to go into the theoretical, or physical, explanation of how the plate current increases with frequency, since it is believed necessary to merely point out that those skilled in the art are well aware of such a phenomenon.

By connecting the grids of tubes i and to the low potential side of resistor Il?, it is possible to regulate the bias of these grids in such a manner that the gain in the radio frequency ainplifier stage and the frequency changer stage will be substantially uniform over the operating range of the receiver. This is accomplished in virtue of the fact that, as the three tuned circuits are varied in resonance to increasing frequencies the bias on the grid of each of tubes I and 5 becomes increasingly more negative since the drop across the resistor I@ becomes increasingly greater. Thus, the inherent tendency of tubes l and 5 and associated circuits to increase their gain with increasing frequencies is overcome, for the increasing negative bias functions to reduce the gain.

It should, also, be noted that the increasing bias on the frequency changer tube insures sufficient bias at the frequencies where the strongest heterodyne voltage is impressed upon tre frequency changer tube. Thus, it will be seen that the local oscillator resistor I8 is connected to the radio frequency amplifier circuit for regulating the latter in such a manner that the amplication of the radio frequencies does not increase at the higher frequencies to the extent that it would with constant bias, and cooperates with the frequency changer circuit to insure sufficient bias at the frequencies Where the strongest heterodyne voltage is impressed upon thefrequency changer tube. l

It will now be appreciated that there is disclosed in the present application a general method for utilizing a biasing potential whichv is a function of frequency, especially a bias potential v'derived from an oscillator circuit. This general method is shown specificallyutilized in a superheterodyne receiver which necessarily includes an oscillator circuit, the lbias potential derived from the superheterodyne oscillator being `employed for remedying undesirableeffects inherent inthe operation of the radiofrequency amplifier and the first detector. L u l Hence, it should be recognized that the present invention is not limited to the specific `receiver shown in Fig. 1 inasmuch asother uses for the basic principle disclosed herein will be readily thought of by those skilled in the art. For example, the receiver need not necessarily be of the superheterodyne type, but may incorporate a local oscillator for the sole purpose of producing a variable bias potential which is a function of frequency for the purpose of controlling the gain of the radio frequency amplifier. Again, an independent local oscillator need not necessarily be employed, since the present inventionis equally applicable to the construction wherein a combined detector oscillator circuit is employed between the intermediate frequency amplifier and the radio frequency amplifier.V i

Of course, it is apparent that the tunable radio frequency circuits may be utilized between the antenna and the frequency changer tube without employing any radio lfrequency amplification. In such cases, the oscillator bias would be applied to the frequency changer, the intermediate frequency amplifier, or even the audio amplifier to is connected to ground through the biasing re- Y sistor R3. Suitable variable taps 23 and 25 on said resistor are vconnected respectively to the cathodes of radio frequency tube I and frequency changer tube A5 through lead 22 and self-biasing resistor R1 and lead 24 and self-biasing resistor R2. The grids of `tubes l and 5 being grounded, the potential of eachwith respect to its associated cathode will depend on the voltage drop through the resistors between cathode and ground. For example, the grid bias of radio frequency tube l will be the sum of the voltage drop across selfbiasing resistor R1 and that across the portion of R3 between tap 23 and ground. The grid bias of Yfrequency changer 5 will be the sum of the voltage drop across self-biasing resistor Rzand that across the portion of R3 between tap 25 and ground.

Appropriate radio frequency by-pass condensers 20, 2i are connected between the cathodes and grids of tubes I and 5, vwhile a tap on the oscillator circuit for the grid connection is desirable to reduce the effect of the tube capacity on tuning. This is likely to cause parasitic oscillations of high frequency. These may be avoided by adding a condenser C of sufficient magnitude. The more tightly coupled the two portions of the grid coil, the less the value of C required to avoid parasitic oscillations. The effect of the condenser C is to make thetendency of the oscillator to oscillate more strongly at high frequencies than at low frequencies even more pronounced. This may be desirable, in which case C may be made even larger than the value required to stop parasitics. i

VThe operation of the modification shown in Fig. 2 is :similar to that described in connection with the arrangement shown in Fig. 1. As in the latter` arrangement it is pointed out that since the translation gain of the radio frequency amplier stage is av function of the product of the signal energy and theoscillation energy, as the amplification of the signal energy inherently tends to increase with increasing frequencies, it is merely necessary to utilize the'increase of plate current of the oscillator to decrease the amplication of the signal energy.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing rfrom the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a superheterodyne receiver, a radio frequency amplifier including an electron discharge tube, a frequency changer including an electron discharge tube, a local oscillator including an electron discharge tube, a common means for tuning said amplier and frequency changer to a desired signal frequency, additional means, simultaneously operative with said common means, for tuning said local oscillator to a frequency differing from said desired signal frequency by a predetermined intermediate frequency, and connections between said local oscillator and said amplifier whereby the direct-current bais for the grid of said amplifier tube increases with increasing signal frequency so that the amplification of the radio frequency tube does not increase at the higher frequencies to the extent that it would with constant grid bias, said connections comprising a resistor and a shunt capacity disposed between the grid and cathode vof the oscillator tube and a conductor connected .between a point 'on said resistor and one of the electrodes of said amplier tube.

2. In a wave repeating system, a plurality of stages connected in cascade, a vacuum tube in each of said stages, a local oscillator, a resistor and a shunt capacity connected in a circuit of said local oscillator, so that the direct-current potential drop through the resistor changes with changes in the frequency of oscillation of the oscillator, means for tuning the input of one of said cascade connected stages, means for varying the resonant frequency of said local oscillator, uni-control means for both of the last meritioned means, and a direct current connection from said resistor to a grid electrode in the tube in one of said cascade connected stages for biasing said grid electrode in accordance with the Aresonant frequency of said local oscillator.

3. In a Wave repeating system, a plurality of stages connected in cascade, a vacuum tube in each of said stages, a local oscillator, a resistor and a shunt capacity connected in a circuit of said local oscillator, so that the direct-current potential drop through the resistor changes with changes in the frequency of oscillation of the oscillator, a variable condenser for adjusting the resonant frequency of said local oscillator, variable condensers for adjusting the resonant frequency in a plurality of said cascade connectedy stages, uni-control means for said variable condensers; and connections from said resistor to the input circuits of a plurality of said cascade connected stages forapplying the potential drop across said resistor to the grid electrodes of the tubes in said stages as a biasing potential.

4. A superheterodyne radio receiver comprising a plurality of cascade connected stages, one of said stages constituting a first detector, tuned input circuits in a plurality of said cascade connected stages, at least one of said tuned input circuits being variable, a vacuum tube in each of said stages, a loca-l oscillator, a tuned circuit for adjusting the resonant frequency of said local oscillator, means coupling said local oscillator to said i'lrst detector, a resistor and a shunt capacity connected in the circuit of said local oscillator so that the direct-current potential drop across said resistance varies with the resonant 'frequency of said oscillator, and connections for biasing the grid of the tube in one of said stages from the potential drop across said resistor.

5. A superheterodyne radio receiver comprising a plurality of cascade connected stages, one of said stages constituting a rst detector, tuned input circuits in a plurality of said cascade connected stages, at least one of said tuned input circuits being variable, a vacuum tube in each of said stages, a local oscillator, a tuned circuit for adjusting the resonant frequency of said local oscillator, means coupling said local oscillator to said first detector, a resistor and a shunt capacity connected in the circuit of said local oscillator so that the direct-current potential drop across said resistance varies with the resonant frequency of said oscillator, and connections from said resistor to the cathode and grid electrodes of the tubes in a plurality of said stages for applying biasing potentials to the grids of those stages.

6. A superheterodyne radio receiver comprising a plurality' of cascade connected stages, one of said stages constituting a rst detector, tuned input circuits in a plurality of said cascade connected stages, at least one of said tuned input circuits being variable, a vacuum tube in each of said stages, a local oscillator, a tuned circuit for adjusting the resonant frequency of saidY local oscillator, means coupling said ylocal oscillator to said first detector, a resistor anda shunt capacity connected in the circuit of said local oscillator so that the direct-current potential drop across said resistance varies with the resonant frequency of said oscillator, and connections from said resistor to the input circuit of said first detector for biasing the grid of said rst detector from the potential drop across the resistor.

'7. A superheterodyne radio receiver comprising a plurality of cascade connected stages, one of said stages constituting a first detector, tuned input circuits in a plurality of said cascade connected stages, at least one of said tuned input circuits being variable, a vacuum tube in each of said stages, a local oscillator, a tuned circuit for adjusting the resonant frequency of said local oscillator, means coupling said local oscillator to said iirst detector, a resistor and a shunt capacity connected in the circuit of said local oscillator so that the direct-current potential drop across said resistance varies with the resonant frequency of said oscillator, uni-control means connectingv the tuning means of said local oscillator and the tuning means in said stage having a variable tuned input circuit, and connections for biasing the grid of the tube in the last mentioned stage according to the potential drop across said resistor.

8. A superheterodyne radio receiver comprising a plurality of cascade connected stages, one of said stages constituting a first detector, tuned input circuits in a plurality of. said cascade connected stages, at least one of said tuned input circuits being variable, a vacuum tube in each of said stages, a local oscillator, a tuned circuit for'adjusting the resonant frequency of said local oscillator, means coupling said local oscillator to said rst detector, a resistor and a shunt capacity connected in the circuit of said local oscillator so that the direct-current potential drop across said resistance varies With the resonant frequency of said oscillator, and connections from said resistor to the input circuit of the stage preceding said first detector for biasing the grid of the tube in that stage.

9. A superheterodyne radio receiver according to claim 4, in which said resistor is connected in the circuit of said local oscillator so as to be common to the input and output circuits thereof.

10. In a superheterodyne radio receiving system including a plurality of cascade connected radio frequency stages and a local oscillator coupled to one of said stages for producing an intermediate frequency, said local oscillator having a resistor and a. shunt capacity connected in its plate to cathode circuit, the method of biasing the grids of the tubes in a plurality of said radio frequency stages of said receiver which consists of utilizing the direct-current potential drop across said resistor as a source of biasing potential.

11. In a superheterodyne receiver provided with several stages and a local oscillator, the method of controlling uniformly the over-all amplification of the receiver over its entire frequency range, which comprises deriving from the local oscillator a direct current voltage proportional to the oscillator frequency, and applying said direct current voltage to one or more of the stages in a sense to .compensate for the inherent tendency thereof to increase the amplification with increase in frequency.

l2. In a superheterodyne receiver provided at least with a detector and a local oscillator which are coupled together to produce a difference frequency, the method of controlling uniformly the over-all amplication of the receiver over its entire frequency range, which comprises deriving from the local oscillator a direct current voltage which. is proportional to the oscillator frequency, and applying said VvoltageA to the detector in a sense to' compensate for the inherent tendency thereof to increase the amplification with increase in frequency.

13. A radio frequency circuit of the superheterodyne type having at least a radio frequency amplifier, a detector and a local oscillation generator, means for coupling said local oscillation generator to' said detector for producing a diierence frequency, and means independent of said coupling means for deriving a direct-current potential from said oscillation generator and applying said potential to the control electrodes of both the amplierand detector, said last-mentioned means comprising a resistance and a shunt capacity included in the circuit of the oscillation generator and direct-current connections from said resistance to the control electrodes of said ampliiier and detector. y

14. A radio receiving circuit of the superheterodyne type having at least a radio frequency amplier, a detecto-r and a local oscillation generator, means for coupling said local oscillation generator tosaid detector for producing a difference frequency, and means independent of said coupling means for deriving a direct-current potential from said oscillation generator and applying said potential to the control electrode of the amplier, said last-mentioned means comprising a resistance and a shunt capacity included in the Vcircuit of the oscillation generator and a direct-current connection from said resistance to the control electrode of said amplifier. 15. A radio receiving circuit of the superheter odyne type having at least a detector and a local oscillation generator, means for coupling said oscillation generator to the detector for producing a difference frequency, and means independent of said coupling means for deriving a directcurrent potential from said oscillation generator and applying said potential to the control electrode of the detector, said last-mentioned means comprising a resistance and a shunt capacity included in the circuit of the oscillation generator and the direct-current connection from said resistance to the control electrode of said detector.

DAVID GRIMES.

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