US2253450A - Automatic volume control system - Google Patents

Description

Aug. 19, 1941. C, TRAVlS 2,253,450

AUTOMATIC VOLUME CONTROL SYSTEM Filed June 1'7, 1939 figa. 7

antan-a a0 rPz/r Patented Aug. 19, 1941 AUTOMATIC VOLUME CONTROL SYSTEM Charles Travis, Philadelphia, Pa., assignor, by

mesne assignments, to PhilcoRadio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application June 17, 1939, serial No. 279,739

(c1. 25o-2o) 11 Claims.

This invention relates to an improved detector and automatic volume control system for useV in conjunction with modulated carrier wave signal systems, such as are employed in radio-telephony and in television. More particularly, the invention relates to a method of deriving an automatic volume control signal from a detector of the so-called degenerative plate rectification or infinite impedance type. By the presentinvention, means are provided whereby automatic volume control may be obtained with a minimum .of additional equipment and by employing circuits considerably more simple than those heretofore used. v

Accordingly one object of the invention is to provide an improved method of deriving automatic volume control voltage from a detector of the degenerative plate rectification type.

Another object of the invention is to provide a, modulated carrier wave signal system in which asingle space discharge deviceA is employed both as a degenerative plate rectification detector and as asource of automatic volume control voltage to control the gain of a signal transfer means prior to the detector stage, and which serves to amplify the signal supplied thereto.

Y Still another object of the invention is to `providev an improved method of obtaining a relativelylarge variation in automatic volume controlvoltage from a degenerative plate rectification detector for a relatively small variation in the magnitude of the signal applied to the input thereof.`

A further object of the invention is to provide animproved method of obtaining a delayed automatic volume control voltage from a degenerative plate rectification detector. .Y A still further object of the invention is to provide an improvod method of obtainingy directly from a sp-ace discharge' device employedV as a degenerative plate rectification detector a control voltage whichV varies in therequired sense for use in controlling the gain of, signal transfer means preceding the detectorand serving to amplify the signal supplied thereto. The invention may be clearly understood by reference to the accompanying drawing in which; Fig. 1 is a diagrammatic illustration of one .form of the invention; y

a load impedance in its cathode circuit. This impedance serves not only as a load impedance for the detected output but also as a biasing means, by reason of the presence of which the grid is prevented from going positive when the input signal thereto increases. Preferably, the impedancemaycomprise a parallel combination consisting of a resistor, the magnitude of` which is relatively large by vcomparison with the reciprocal transconductance of the tube, and a condenser of suii'icient capacitance adequately to by-pass the carrier frequency component of the signal to be detected. Preferably, no load impedance is used in the anode circuit of the space discharge device, since this Would Cause the anode potential to vary with the plate current which would impair the operation of the tube as a detector. The potential of any point on the cathode load impedance increases positively With respect to ground as the input signal to the detector increases, and hence the voltage across said impedance is not suitable for use directly to control the gain of a vacuum tube amplifier supplying signal to the detector input.

Various devices have been devised heretofore to obtain automatic volume or gain control when a degenerative plate rectification detector is employed, but such devices have been objectionably complex and have required an unduly large complement of additional equipment. On the other hand, as will appear presently, this invention provides a system which is simple, effective and requires very little additional equipment. "Moreover, 'the invention lends itself to the incorporation of certain desirable features of Vother automatic volume control systems, as will be vseen later. K Referring rst toFig. 1 of the drawing, which shows a'speciiic embodiment of the invention, l is a source 'of'modulated carrier wave signal, for example the antenna coil of a radio or television receiver. VV2 represents the various apparatus which may be located between the signal Vsource and Athe detector" tube.' In an ordinary superheterodyne radio receiver, for example, this might comprise radiofrequency amplifier stages, a lirstV detector, and intermediate frequency ampliiier stages, all familiar to those skilled in the art and which, for convenience, will hereinafter be referred to as signal transfer means.

According to the present invention, the triode which Would commonly be used in a degenerative plate rectification detector is replaced by a pentode 3. In the cathode circuit, is a load comprising the resistor 4 and the condenser 5 which are selected in accordance with the usual principles employed in designing degenerative plate rectification detectors. Anode circuit power is supplied from the source 6, and an impedance 1 is provided connecting the anode to ground. By this means, the automatic volume control voltage which is obtained from the plate of the tube 3 is always negative with respect to ground and such as to reduce the gain of the stages to which automatic volume control is applied. Of course, it would also be possible to connect the high voltage terminal of the impedance 1 to a source of negative potential, for example -3 volts, Whereby the control voltage would never become more positive than this value. The input signal from the signal transfer means may be supplied between the control grid of the tube 3 and the low potential terminal of the resistor 4. The detected signal may then be derived across the resistor A, and the automatic volume control voltage obtained from the plate of tube 3 may be supplied back to the grids of the tubes in radio frequency or intermediate frequency amplifier stages or both, the control voltage being subject to the smoothing action of the filter resistor 8 and by-pass condenser 9.

Considering noW the operation of the system as described, it may firstbe noted that although the detector output is from the load in the cathode circuit, 4automatic volume control voltage is obtained from the anode circuit by the insertion of asuitable load wherebyv the voltage of the plate decreases as the space current thereto increases, thus giving the desired change in control voltage. In a triode, this would be undesirable ysince lthe reduction in `plate voltage would tend further to reduce the plate cuurrent, thus modifying the current in the cathode load and affecting the behavior of the detector. By the method of the invention, advantage is taken of the fact that in a space discharge device of the pentode type the total cathode current is substantially independent of the plate potential; yet the major part of the cathode current is derived from the plate provided the latter does not fall too far beloW thepotential of the screen grid. Hence a load may be included in the anode circuit, thus permitting automatic volume control voltage to be derived which varies in the proper sense without disturbing the operation of the detector. The latter functions just as if a triode were being used. It Will appear that one of the so-called beam power tubes might be used in place of the pentode; in fact a tetrode might even be used provided that its plate potential is not permitted to fall to too loW a value. The requirements to be met in the selection of a tube for the purpose will be apparent to those skilled in the art from the preceding discussion.

As observed heretofore, the present invention A a large change in automatic gain control voltage results. This is achieved by means .of a diode connected in the plate circuit of the gain control tube. By suitably adjusting the b-ias applied to the diode, the action of the system may be delayed by any desired amount. 'Ihe use of a diode according to Applegarths teaching appears in the embodiment of Fig. 2 in which elements corresponding to those 0f Fig. 1 are designated by like reference numerals and need no further explanation. An additional source of positive voltage may be included in the anode circuit at i0. The diode Il may have its `anode connected to the plate of the tube 3 and its cathode may be l maintained at a point of predetermined, preferably negative, potential by means of the source I2. It will appear that no automatic volume control voltage can develop until the plate currient of the tube 3 has increased suliciently to permit the plate to assume a potential corresponding substantially to that of the cathode of the Vdiode when the latter is non-conducting. Hence the delay will depend upon the potential at which the cathode of the diode is maintained, the magnitude of the plate impedance 1, and the magnitude of the voltage provided by the source I0.

Under certain circumstances, it may be preferable to use a series arrangement of the diode of Fig. 2. Such an arrangement is shown in Eig. 3, Wherethe cathode of the diode I3 is shown connected to the plate of the pentode, the plate of the diode being connected through a resistor I4 to a source Iof predetermined preferably negative potential I2. Here, as before, no automatic v01- ume control voltage can be developed until the plate of the tube 3 assumes a potential suubstantially as negative as that of the diode anode when the latter will begin to conduct and the automatic volume control Voltage Will develop across the resistor I4. The particular practical advantage o-f this arrangement over the one shown in Fig.V 2 is that it is impossible for a positive control voltage to be applied to the grids of the'controlled tubes. This might happen in the system as shown in Fig. 2 due to the failure of the diode cathode to heat up soon enough when the apparatus is put into operation. This might be objectionable when the device is employed, for example, in a home radio set but may be completely avoided by using the circuit of Fig. 3.

It Will of course be understood that the invention is subject to modcation, as will appear to those skilled in the art upon reading the foregoing description. Therefore the invention is not to be regarded as limited to the specific embodiments shown but only by the scope of the appended claims. A

Iclaim:V

l. In an electrical signal modifying system; a source of a -modulated carrier Wave signal; a space dischargedevice having a cathode, at least one input electrode, at least one output electrode, and being so constructed and operated that the current from said cathode is substantially independent ofthe potential of said output electrode; an impedance associated with said cathode so as to form in conjunction therewith a degenerative plate rectification detector having an input circuit and an output circuit; variable gain signal transfer meansfor supplying signal from said source to the input of Isaid detector;` means associated with said output electrode and adapted to vary the potential thereof in accordance with the magnitude of the electron current ovving thereto; means adapted to derive a control voltage substantially dependent upon said output electrode potential whenever said potential falls below a predetermined value, said means comprising a diode space discharge device having its anode connected to said output electrode and having its cathode connected to a point of predetermined potential; and means for supplying said derived signal to said signal transfer means to control the gain thereof.

2. In an electrical signal modifying system; a source of -a modulated carrier wave signal; a space discharge `device having a cathode, at least one input electrode, at least one output electrode, `and being so constructed and oper-ated that the current from said cathode is substantially independent of the potential of said output electrode; an impedance associated with said cathode so as to form in conjunction therewith ya degenerative plate rectification detector having an input circuit and an output circuit; variable gain signal transfer means for supplying signal from said source to the input of said detector; means associated with said output electrode and adapted to vary the potential thereof in accordance with the magnitude of the electron current flowing thereto; means adapted to derive a control voltage substantially dependent upon said output electrode potential whenever said potential falls below a predetermined value, said means comprising :a 4diode space discharge device having its cathode connected to said output electrode and having its anode connected through an impedance to a point of predetermined potential; vand means for supplying said derived signal to said signal transfer means to control the gain thereof.

3. In an electrical signal modifying system.; a source of a modulated carrier wave signal; a pentode space discharge device having an anode, a cathode, and control, screen'and suppressor grids, and being so operated that cathode current is substantially independent of variations in plate Voltage; an impedance associated with the Icathode of said space discharge device so as to form in conjunction therewith Ia, degenerative plate rectification detector having an input circuit and an output circuit; variable gain signal transfer means for supplying signal from said source to the input of said detector; a source of anode supply voltage; a resistance connecting the said supply to the said anode; means for bypassing said resistance for currents of carrier frequency; a connection to the said anode for deriving a control voltage; and means for utilizing said control voltage to control the gain of said signal transfer means.

4. In an electrical signalling system; a source of an electrical signal; a space discharge device having at least a cathode, an anode, and a control grid, means for rendering the space current in said device substantially independent of th-e potential of said anode; variable gain signal transfer means for supplying signal from said source to said control grid; a resistor 'connecting said anode to a point of fixed potential; an impedance connecting said cathode to a point of relatively lower potential; means for deriving an output signal -across said last-named impedance; and a connection between said anode and said signal transfer means whereby the potential of said anode is applied to control the gain of said signal transfer means.

5. In an electrical signalling system; a source of an electrical signal; a space discharge device having at least an anode, a cathode, and a lco-ntrol grid; variable gain signal transfer means for supplying signal from said source to said control grid; a resistor connecting said anode to apoint of predetermined-potential; an impedance connecting said cathode to a point of relatively lower potential; means for deriving an output signal from said last-named impedance; and a connection between said anode and said signal transfer means whereby the potential of said anode is applied to control the gain of said signal transfer means.

6. In an electrical signalling system; a source of a modulated carrier wave signal; a space discharge device having at least a cathode, an

anode and a control grid; variable gain sig-nal transfer means for supplying signal from said sourceto said control grid; an impedance connected to said cathode for forming in cooperation with the said elements of said space'discharge device a degenerative plate rectification -detector; means for deriving an output signal from said impedance; a second impedance connected to the anode of said space discharge device for developing a control voltage from the recticationv action thereof; and means for utilizing said voltage to control the gain of said signal transfer means.

'7. In an electrical signalling system; a source of an electrical signal; a space discharge device having at least an anode, a cathode, and a control grid; variable gain signal transfer means connecting said source to the control grid of said space discharge device; an impedance connecting said anode to a point of predetermined potential; a second impedance connecting said cathode to point of predetermined relatively lower potential to form a degenerative rectifying circuit including the anode-cathode path of said space discharge device; means for rendering the current in said second impedance substantially independent of variations in the potential of said anode; means for deriving a signal proportional to the rectified current in said circuit; means for deriving a gain controlling voltage from said first impedance, and means for utilizing said last-named voltage to control the gain of said signal transfer means.

8. In an electrical signalling system; a source of an electrical signal; a space discharge device having at least an anode, a cathode, and a control grid; variable gain signal transfer means connecting said source to the control grid of said space discharge device; an impedance connecting said anode to a point of predetermined potential; a second impedance connecting said cathode to point of predetermined relatively lower potential to form a degenerative rectifying circuit including the anode-cathode path of said space discharge device; means comprising auxiliary electrodes in said space discharge device and supplied with suitable potentials for rendering the current in said second impedance substantially independent of variations in the potential of said anode; means for deriving a signal proportional to the rectified current in said circuit; means for deriving a gain controlling voltage from said rst impedance, and means for utilizing said last-named voltage to control the gain of said signal transfer means.

9. In an electrical signalling system; a source of an electrical signal; a space discharge device having at least an anode, a cathode, and a control grid; variable gain signal transfer means connecting said source to the control grid of said space discharge device; an impedance connecting said anode to a point of predetermined potential; a second impedance connecting said cathode to point of predetermined relatively lower potential to form a degenerative rectifying circuit including the anode-cathode path of said space discharge device; means for rendering the current in said second impedance substantially independent of variations in the potential of said anode; means for deriving an output signal across said second impedance; means for deriving a gain controlling voltage from said first impedance, and means for utilizing said last-named voltage to control the gain of said signal transfer means.

10. In an electrical signalling system; a source of a modulated carrier Wave signal; a space discharge device including triode elements constituting a plate rectication detector; variable gain signal transfer means connecting said source to said detector; means for rendering said detector degenerative; means for rendering the rectified current of said detector substantially independent of variations in the potential of its anode;

means for deriving a detected signal from the rectied current of said detector; means for deriving a gain controlling voltage from the anode of said detector; and means for applying said voltage to said signal transfer means to control the gain thereof.

11. In an electrical signalling system; a source of a modulated carrier Wave signal; a space discharge device including triode elements constituting a plate rectification detector; variable gain signal transfer means connecting said source to said detector; means for rendering said detector degenerative; means comprising at least one additional element in said space discharge device for rendering the rectified current of said detector substantially independent of variations in the potential of its anode; means for deriving a detected signal from the rectified current of said detector; means for deriving a gain controlling voltage from the anode of said detector; and means for applying said voltage to said signal transfer means to control the gain thereof.

CHARLES TRAVIS.

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