US2111765A - Automatic volume control - Google Patents

Description

AUTOMAT I C VOLUME CONTROL k x Y mm v vINVENTOR 2 CHRISTOPHER J. FRANKS QL BY f Y a Q l I l M ATTORNEY c. J. FRANKS4 AUTOMATIC VOLUME CONTROL March 22, 1938.

Filed May 14, 1955 2 Sheets-Sheet 2- iam/ww 0n/77090 W907- CH RISTOPHEB J. FRANKS BY /fl Ww ATTORNEY Patented Mar. 22, 1938 UNITEDA STATES AUTOMATIC' VOLUME CONTROL Christopher J Franks,

Boonton, N. J., assigner to Radio Corporation of America, a corporation of Delaware A Application May 14,

8 Claims.

My present invention relates to superheterodyne receivers, and more particularly to novel and improved methods of, and devices for, automacally regulating the volume of superheterodyne 5 receivers.

It has heretofore been proposed to provide automatic volume control arrangements for superheterodyne receivers. In such volume control arrangements the gain of the converter netl0 Work has been varied, as well as the gain of the various super-audible frequency amplifiers, in response to received signal carrier amplitude variations. 'For example, and as shown in my Patent No. 2,078,072, dated April 20, 1937, it has been proposed to vary the gain of a pentagrid converter tube of a superheterodyne receiver by varying the signal grid bias of the converter tube,1this being accomplished in addition to the variation of signal grid bias of the radio frequency 2O and intermediate frequencyamplifier stages.

However there are many situations wherein it is desired to have a more rapid automatic gain control action on the pentagrid converter tube of a superheterodyne receiver. By increasing the rapidity of the automatic volume control action on the converter tube it may be possible, for exjample, to dispense with automatic bias control of the super-audible frequency amplifier stages. One of the problems to be overcome in providing a more rapid automatic gain control of the converter network of a superheterodyne receiver involves the need for maintaining substantial local oscillator voltage in the converter network throughout the automatic gain control action.I y

Accordingly it may be stated Vthat it is one of the primary objects of my present invention Vto provide in a superheterodyne receiver employing a pentagrid converter network, an automatic volume control arrangement which functions 40 simultaneously to vary the converter tube gain and the local oscillator voltage amplitude in response to signal carrier amplitude'rvariations.

Another important object of theinventionvis to provide in conjunction with a pentagrid converter network of a `superheterodyne receiver, an automatic gain control arrangement which op- -erates'to vary the gain of the converter tubeby varying the negative grid biasr on the signal grid of the converter, 'and simultaneously varies the local oscillator Vvoltage amplitude in a sense suoli` that the oscillator voltageV amplitudeldecreases `appreciablywith signal amplitude in.- crease thereby providing amore rapid automatic `regulationof the converter network.'r z .Y

^Another:object of; the; ,inventionis to` provide 1935, serial No. 21,341 (C1. asoao'y an improved and rapid automatic volume control method for a superheterodyne receiver employing a pentagrid converter network, the con'- verter being particularly characterized by its inclusion of a double feed back arrangement from 5 the oscillator anodeelectrode and plate of the converter tube, andthe automatic volume control arrangement functioning toincrease the negative bias on the signal grid ofthe converter tube and thereby substantially render yinoperative the feed l0 back from the plate of the converter tube as the received signal carrier amplitude increases.

Still other objects ofthe present invention are to improve generallythe efficiency of superheterodynexreceivers employing pentagrid converters 15 and utilizing automatic volume control, and more especially toprovide such receivers which are not only. reliable andeiiicient in operation, but economicallymanufactured and assembled.

The'novel features which I .believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself,.however, as to both its organization and method of operation will best be understood by reference to thefoll'owing description taken in 25 connection with the drawings in which I have indicated diagrammatically several circuit organizationswhereby my invention may be carried into effect.

Y v.In the drawingsr- 1 Y 30 Fig. 1 diagrammatically shows a superheterodyne` receiver embodying the present invention,

Fig'. 2 showsl a converter network of Fig. 1 embodying a modified form of the invention, Fig; Bshows a furtherfmodification of the con- 35 verter network, :.L I f Y Fig. 4 graphically illustrates the operation of. athepresent invention.V Y

Referring now tothe accompanying drawings, -wherein like Areference characters in the different 40 lgures designatesimilar circuit elements, there Y isshownzin Fig. 1 thenetworks of a superheteroydyne receiver. of conventional and well known construction; The receiver embodies a source of signals l, and thismay comprise the usual signal collector lwhich may be a grounded antennacircuit; a loop antenna; Yan automobile signal pickup device and even aradio frequency distribution line such as usedinY hotels or apartmenthouses zatthepresent time. The source I may, also, be

considered :as including one or more, stages of tunable radio frequency amplification, and the number of stages to be employed will depend .upon ,the signal amplitude 4desired at the input circuit ofthe pentagrid converter tube. l

The signal source I is followed by a pentagrid converter tube 2 which is of the well known 6A'7 type. Since the electrode structure of such a tube is well known at the present time, and its circuits, and their functions, are also very well known, it is sufficient to point out that the converter functions to convert the signal input energy to a desired intermediate frequency which is produced in the intermediate frequency output circuit 3 of the converter tube.

The signal energy is impressed upon the tunable signal input circuit 4 of converter tube 2, and the tunable local oscillator network 5 functions to tune the local oscillator network to that frequency which will differ from the frequency of input circuit 4 by the frequency of the network 3. The converter tube is followed by an intermediate frequency amplierfnetwork 6; the latter may comprise one, or more, stages of intermediate frequency amplification. The network 3-3 is resonant tothe operating intermediate frequency, and this frequency may be chosen from a range of 75 to 450 kc. The amplified output of the intermediate frequency amplifier network (i is then impressed upon a second detector 1, and the latter may be of any desired type. The demodulated output of the second detector 'I is impressed upon an audio network, and the latter may comprise one, or more, stages of audio frequency amplification, followed by a reproducer.

In order to overcome the effects of variations of received signal strength there is employed an automatic volume control arrangement, and this latter arrangement functions to maintain the signal carrier amplitude at the input of the demodulator 'l substantially constant over a wide range of signal carrier amplitude variation at the signal collector of the receiver. The automatic volume control arrangement (hereinafter designated as AVC) may comprise a rectifier 8 of any desired and well known type, and the function of the rectifier is to produce a` Varying direct current voltage across the output load resistor 9. The varying direct current voltage is impressed as a gain control bias upon the signal grid of the converter tube 2, and this is accomplished through the lead I which includes the filter network II.

The function of the network Il is to suppress pulsating components of the rectified signal energy, and prevent them from being impressed on the signal grid of the converter tube 2. Of course, there may be additional variable bias leads from lead I0 to the intermediateV frequency amplifier grid circuits, and also to the grid circuits of the radiofrequency amplifier. However, such additional leads are .not shown since such control circuits are well known to those skilled in the art. While the demodulator 1. and rectifier 3 are shown as conventional in nature, it will be understood that any desired type of specific circuits may be used for accomplishing their functions. For example, the multi-function tubes disclosed in my aforesaid copending application may be utilized for these demodulation and rectification functions. In general, it is to be understood that any of the automatic volume control circuits shown in my aforesaid copending application may be utilized in conjunction with the tunable converter tube 2 disclosed herein.

The pentagrid convertertube 2 Vincludes the usual cathode and plate, and the intermediate five grids. The grid Grfunctions as the local oscillator grid, while the grid G2 functions as the oscillator anode. Grid G4 has the signal energy impressed thereon, and the signal grid is disposed between a pair of screen grids which are connected to a point of positive direct current potential, the screening grids functioning as electrostatic screens because they are at ground alternating current potential, The cathode of converter tube 2 is connected to ground through the usual signal grid bias resistor I2, the latter being suitably bypassed by condenser I3. The variable tuning condenser E4 in the signal input circuit 4 has the grounded side thereof connected to the low alternating potential side of the signal input coil through a blocking condenser I 5.

The variable tuning condenser I of the local oscillator network 5 has one side thereof grounded, while its high potential side is connected to the oscillator grid G1 through a condenser Il. The grid side of condenser I'.' is connected through the resistor I8 to the cathode. The resistor I8 and condenserl'l function as a leaky grid condenser network for the local oscillator section of the converter network. The dotted line representation used in conjunction with condensers I4 and I6 designates that these two condensers have their rotors mechanically uni-controlled, and it is to be understood that the local oscillator circuit 5 also is provided with proper padding condensers to keep the local oscillator network 5 properly tracked with the tuning of the signal input circuit 4.

Whereas in prior pentagrid converter arrangements it has been the practice to provide feedback from the oscillator anode G2 to the oscillator grid G1 in order to create the local oscillations, in the present converter network the plate of the converter tube 2 is also utilized to provide feedback. This double feedback arrangement is provided in order to have a strong local oscillator Voltage amplitude when weak signals are received, and a substantially weak oscillator voltage amplitude when relatively strong signals are received. This is accomplished by connecting the plate of converter tube 2 to an appropriate source of positive potential B through a path which includes the coil of the intermediate frequency output network 3 and the oscillator feedback coil 20.

The coil 2U is magnetically coupled to the coil 2l of the local oscillator network 5. denser 22 connects the B side of feedback coll 2U to ground for proper bypassing action. The oscillator anode G2 is connected to a source of positive potential L, which potential has a mag- 'nitude substantially less than that of source B,

through a path which includes the radio frequency choke 23, the low potential side of coil 23 being connected to ground through a proper bypassing condenser 24. The plate side of feedback coil 20 is connected to the grid side of choke 23 through a blockingcondenser 25, and the latter is given a magnitude of about 250 micromicrofarads.

It will, therefore, be seen that the grid G2 feeds back radio frequency energy to the oscillator grid G1 through a path including condenser 25, the feedback coil 2U and the oscillator circuit 5. The plate of converter tube 2, also, feeds back radio frequency energy to the oscillator grid G1 through a path including feedback coil. 20 and oscillator circuit 5. By means of this arrangement the grid G2 does not furnish the entire oscillating anode conductance. 'I'he Vplate of the converter tube furnishes some of this oscillating The conanode conductance, andthe latter is controlled substantially vto cut-off by the increasing negative bias on the signal grid G4. Ultimately, and after the point of cut-off of the plate feedback through coil 20, there will only be left the feedback action due tothe grid G2, and this feedback action is designed to be sufficient to keep the oscillator voltage of sufficiently high andi adequate amplitude no matter what value of AVC bias is applied to G4.

The arrangement shown in Fig. 1 is the preferred embodiment of the invention, Whereas Figs. 2 and 3 show alternative embodiments which fromV practical considerations are not as preferable as the arrangement shown in Fig. 1. In Fig. 2, for example, the oscillator Yanode G2 is connected by a lead to a point on the feedback coil 20, this point being atV a lower radio frequency potential than the point of coil 20 to which the plate of tube2 is connected. In other words, in Fig. 2 there is shown an alternative embodiment for securing the result secured with the arrangementlshown in Fig. 1 by tapping the grid G2 down upon the feedback coil 20.

It will be observed that the arrangement in Fig. l differs from that shown in Fig. 2 in that while the grid G2 is connected to the same point of feedback coil 20 as the plate of tube 2 inso far as radio frequency potentials are concerned, yet it is provided with direct current voltage through a parallel path and through the choke 23. The energizing direct current voltage L applied to grid G2 in Fig. l is substantially lower than that applied by source B to the plate so that the initial feedback due to the grid G2 is much less than that from the plate.v In Fig. 2

this diminished initial feedback bythe grid G2IV is secured by tapping Vdown the grid upon the feedback coil 20, and supplying the plate and grid G2 from a common source of voltage B.

The alternative embodiment in Fig. 3 resembles that shown in Fig. l in that thevoltageL is applied to the coil G2, but an auxiliary feedback coil 20' is utilized. The auxiliary feedbackcoil 20', connected inI the lead to the gridV G2, is Wound on the plate feedback coil 20. The coils 20 and 20 are poled so that the voltages which they induce into circuit 5 are in phase. They are Wound in the same direction,` and like ends are grounded. 'Ihis'modif'lcatiom of course, differs from that shown in Fig. 2 in that the energizingdirect current voltage ofthe grid'G2 is much lower than that used for the plate of the converter tube. As stated heretofore, the arrangement shown in Fig. 1 is preferable to those shown in Figs. 2 and 3. From practical considerations, it will be observed that in the arrangement of Fig. 1 there is avoided the necessity of cutting down the plate turns on the oscillator feedback coil, and there is, furthermore, avoided the need' for an auxiliary feedback coil. However, it is to be clearly understoodthat the operation and functioning of the circuits'are substantially the same, the circuit arrangement in Fig. 1 being more efficient insofar as the rapidity of volume control action is concerned.

Considering now the operation of the invention, and with particular reference to the circuit arrangement of Fig. 1, it is pointed out that when relatively Weak signals are received the AVC bias on the signal grid G4 is substantially zero, and the converter tube operates at its maximum gain. Both feedback paths are then functioning at `their maximum efficiency. As pointed out heretofore theV energizing voltage L on the grid G2 is such thatthe initial feedback from this grid is much less than that from the plate of the converter tube. In Fig. 4 there is graphically shown these relationships. CurvevA denotes the variation of oscillation voltage amplitude with increasing negative bias on grid G4 with feedback from rthe plate of converter tube 2 alone. The curve B shows the same relationship for the grid G2 furnishing feedback alone and with substantially |32 Volts on the gridGz.

By way of contrast the curve C shows the same relationship for the grid G2 feeding back by itself with +15 volts on this grid. The curves D and D show the resultant oscillator amplitude variation curves.- In other words these curves show the change in oscillator amplitude, considering the effects of the plate feedback and the grid G2 feedback. It will be observed from the curves of Fig. 4 that with zero AVC bias on grid G4 there is a maximum resultant oscillator voltage amplitude produced in the converter tube 2, and that the component of oscillator voltage due to the grid G2 is substantially less than that dueto the plate acting by itself. The curve D shows the resultant oscillator curve due to curves A and C; curve D shows the resultant of the curves A and B.

As the signal carrier amplitude increases, the negative bias on signal grid G4 increases; and eventually cuts oif the feedback from the plate of the converter tube. Considering Fig. 4 again, it Will be seen from curves D and D' that When the signals have reached the point where substantial AVC bias is produced the effective oscillator voltage amplitude has been substantially reduced to the point where curves B and C correspond With the resultant oscillator voltage amplitude. f

Thus there is a substantial decrease of the effective oscillator amplitude from zero AVC bias to large values of such bias. As a result of the present invention there is not only secured a reduction in gain of the converter tube due to the increasing bias of signal grid G4, but the local oscillator voltage amplitude is substantially decreased, and both these effects combine to secure a very rapid automatic volume control action.

It is to be particularly noted that the decrease of the effective oscillator voltage: amplitude, even at large AVC bias values, is not suicient to interfere with the converter action of tube 2. The gridA G2, as shown by curves B and C in Fig. 4, feeds back sufficient radioV frequency energy to produce a satisfactory and adequate oscillator voltage amplitude.V

By way of example, and in no sense limiting, it is pointed out that in general the magnitude of the B voltage is about three times as great as Vthe voltage L. Thus, Where L is about 32 volts, the value of B is some 100 volts. If 250 volts were used on, the. plate,.as is normal for 6A7 operation, then the value of L should .be in the range of 75 to 150 volts.

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 from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. In a converter network using a pentagrid tube provided with oscillator and mixer electrodes, the method of automatically controlling the output of the converter which includes feeding back radio frequency energy from the oscillator anode, additionally feeding back energy from the mixer plate of the converter tube to an extent substantially greater than the oscillator feedback, and increasing the negative bias on the signal grid of the converter tube as received signal carrier ampltiude increases whereby the feedback from the aforesaid plate is substantially eliminated at a predetermined value of negative bias on the signal grid.

2. In a converter network using a pentagrid tube provided with oscillator and mixer electrodes, the method of automatically controlling the output of the converter which includes feeding back radio frequency energy from the oscillator anode, additionally feeding back energy from the mixer plate of the converter tube, increasing the negative -bias on the signal grid of the converter tube as received signal carrier amplitude increases whereby the feedback from the aforesaid plate is substantially eliminated at a predetermined value of negative bias on the signal grid, and maintaining the initial feedback from the oscillator anode much less than that from the plate.

3. In a detector-oscillator system using a tube having oscillator electrodes and detector electrodes, the method of automatically regulating the gain of the tube in response to received signal variations consisting in feeding back radio frequency energy from the detector output electrode to the oscillator grid electrode, proportioning the last feedback and the oscillator normal feedback so that the last feedback initially predominates, and substantially eliminating the last feedback when signals above a desired amplitude are received.

4. In a detector-oscillator system using a tube having oscillator electrodes and detector electrodes, the method of automatically regulating the gain of the tube in response to received signal variations consisting in feeding back radio frequency energy from the detector output electrode to the oscillator grid electrode, proportioning the last feedback and the oscillator normal feedback so that the last feedback initially predominates, substantially eliminating the last feedback when signals above a desired amplitude are received, and maintaining the normal oscillator feedback of adequate amplitude regardless of the amplitude of received signals.

5. A converter network including a pentagrid tube provided with oscillator and mixer electrode sections, means for providing a double feedback from the oscillator anode electrode and the mixer output electrode, the mixer output electrode feedback initially exceeding the oscillator anode feedback to a substantial extent, and automatic gain control means for increasing the negative bias on the tube signal grid, as signals increase, to thereby eliminate the mixer feedback.

6. In combination, a tube having at least a cathode, an oscillator grid, an oscillator anode electrode, a signal grid and an output electrode, a signal input circuit connected between the cathode and signal grid, a network including a resonant circuit tuned to a predetermined local oscillation frequency, coupling the oscillator anode and grid, a beat frequency circuit connected to said output electrode, means reactively coupling the output electrode and the oscillator grid to provide an energy feedback and thereby produce oscillations of said local frequency which are of an amplitude substantially exceeding the amplitude of oscillations produced by said first coupling, said signal grid being disposed between the oscillator anode and output electrode, and means for varying the direct current potential relations between the signal grid and cathode thereby to regulate the magnitude of energy feedback through said reactive coupling means.

'7. In combination, a tube having at least a cathode, an oscillator grid, an oscillator anode electrode, a signal grid and an output electrode, a signal input circuit connected between the cathode and signal grid, a network including a resonant circuit tuned to a predetermined local oscillation frequency, coupling the oscillator anthereby to control the energy feedback through "l said reactive coupling means, said output electrode being maintained at a substantially greater positive direct current potential than said oscillator anode whereby the feedback from said oscillator anode is a minimum when said potential difference is a minimum.

8. In combination, a tube having at least a cathode, an oscillator grid, an oscillator anode electrode, a signal grid and an output electrode, a signal input circuit connected between the cathode and signal grid, a network including a resonant circuit tuned to a predetermined local oscillation frequency, coupling the oscillator anode and grid, a beat frequency circuit connected to said output electrode, means reactively coupling the output electrode and the oscillator grid to produce a feedback of energy which substantially exceeds the energy feedback due to said first coupling, said signal grid being disposed between the oscillator anode and output electrode, and means for varying the direct current potential relations between the signal grid and cathode thereby to control the energy feedback through said reactive coupling means, said last varying means comprising a beat frequency energy rectifier, and connections for impressing between the cathode and signal grid the direct current output of the rectifier.

CHRISTOPHER J. FRANKS.

Images