US2063814A - Demodulator - Google Patents

Description

8, 1936. A, A LEQNARD 2,063,814

DEMODULATOR Filed Feb. 18, 1955 I'. PatentedDec. 8, 19.36

PATENT oFl-lcE`Iv nEMoDULA'roR."

Anthon A. Leonard; Glenside, Pa., assignor to Transitone Automobile Radio Company, Philadelphia, Pa., a corporation of Delaware Application February 18, 1933, Serial No. 657,442

4 Claims.

This invention relates to carrier wave receiving systems and has particular reference to detectors,

especiallyv those for use with automatic control systems. This application is a continuation in part of my co-pending application, Serial N0. 598,245, filed March 1'1, 1932.

An object of the'invention'is to provide a more eilicient detector than has heretofore been available.

A more specific object of the invention is toprovide a detector having a continuously varying amplification constant over the normal range of incoming signal modulated carrier waves, particularly for the demodulation of high level signais when only a moderate plate supply voltage is available.

A further object of the invention is to provide means for limiting 'the output volume of a system to a certain predetermined level regardless of the amplitude of the carrier wave input to the system, while efficiently detecting and amplifying signals of'all input amplitudes without appreciable Another object of the invention is'to provide means for so controlling the output volume of a radio-receiving system that strong carrier Waves are reduced to a. predetermined level and detected without distortion or overload while ,carrier waves which are weaker than that level will be onlyl slightly affected, and will be very efficiently detected. *In other words, unless a carrier wave is present and is higher than thev predetermined level, the Whole receiving system will maintain its full normal sensitivity.

i A further and more specific object of the invention is to provide a system wherein a uni-directional control ,voltage corresponding in amplitude y to the amplitude of the carrier voltage is derived 'I'he invention may be clearly understood from from the input or grid-cathode circuit of a vaculum tube detector of the system, the detector meanwhile maintaining useful sensitivity and over-load characteristics. invention contemplates the use of the grid rectification current Aof a grid-leakgdetector to derive the uni-directional control voltage, such voltage being applied to the control grid or grids of one or more of the radio frequency amplifiers, the de'- tecting tube meanwhile maintaininguseful sensitivity and overload characteristics.

A further object of the invention is to provide a system wherein a detector circuit having high translation and overload characteristics is utilized as a source of control energy for controlling th radio' frequency amplifiers.

Otherwise stated, the

the following detailed description and they accompanying drawing, wherein three embodiments are illustrated. Inthe drawing: f

Figs. 1 to 3 are schematic circuit diagrams, il-

` lustrating various embodiments of the invention;

- matic volume control system of the type described hereinafter and in combination with certain other features also described hereinafter. 'I'he manner in which such a tube operates as a detector and the desirable features thereof are pointed out in detail in the description following relative to Figs. 4 'and 5. In order-to fully understand the present invention,v it might be well to define briefly what is meant by a variable mu tube. As isV now generally known, the Greek letter mu is used to' designate the amplification factor of. a vacuum tube. This factor is an expressionA of the ability of a vacuum tube to amplify voltages. When a tube has a variable mu characteristic, therefore, its amplification factor or ability to amplify voltages varies continuously over the normal operating range. For the present purpose, a variable mu tube may be defined as one whose a plification factor becomes continuously less as t grid voltage becomes` more negative without 'a sudden change at anyv point. I have discovered lthat a variable mu characteristic may be imparted to a vacuum tube detector in various Ways, some of which are illustrated herein for the purpose of disclosure. As the term variable mu is generally used in the art, it refers to a tube which is specifically designed to impart to it the desired characteristic. As used herein, however, the term is to be understood as referring to any detector tube having the desired characteristic regardless of how it is obtained. The invention, therefore, contemplates broadly the use of adetector having the desired characteristic whether such characteristic be imparted to the tube by virtue of the design thereof or by provisions made in the circuit of the tube.

Referring now to Fig. illustrated a` receiving i system having the usual antenna l connected through the primary wind- 1 of the drawing, there is ing of an input transformer T to ground as at 2. Transformer T serves to couple the antenna circuit to a tuned radio frequency amplifier stage comprising a radio frequency vacuum tube amplifier V. 'Ihe usual tuning condenser 3 is connected across the output of transformer T, and the usual energizing sources, herein illustrated as batteries l and 5, are provided for amplifier V.

In the present illustration, a single stage of radio frequency amplification is shown but this is merely for the purpose of illustration and clarity, it being understood that the signal may traverse any desired number of carrier frequency amplifying or detecting stages. The carrier frequency stage or stages may take any conventional form and may include as elements thereof any suitable type of potential-controlled device. In the present illustration, the single radio frequency amplifying stage is shown as including a screen grid tube which comprises, as is well known, a. shielding grid in addition to the usual cathode, control grid, and anode. Radio frequency by-pass condenser 6 and filter resistance i are included in the circuit of the shielding grid in the usual manner.

A second radio frequency transformer T1 serves to couple the amplifying stage just described to the input of a vacuum tube detector D. I have found that certain commercial pentode tubes by virtue of the fifth element, namely, the auxiliary grid connected to the cathode, have variable mu characteristics. Therefore, the desired detector may be had by using such a pentode tube, as illustrated at D in the embodiment of Fig. 1. A radio frequency by-pass condenser 3 and a choke coil 9 are preferably connected in the output circuit l resistance and condenser.

of the detector in the usual manner. The fila'- ment and anode energizing sources IIJ and II,

respectively, are provided for the usual purpose, and may be combined with sources Ii and 5 as is customary. A transformer T2 may be utilized to couple the output circuit of the detector to any desired load circuit. A

As'will be understood, the use of a screen grid tube as a detector is highly desirable because of the high signal amplification which is had with this type of tube. It is well known that linear grid-leak detection with a screen grid tube at moderate plate potentials has heretofore been impractical because such a tube overloads when thus used. This is particularly the case when a large automatic volume control voltage is present at the input terminals of the detector. Means is therefore provided in the form of resistor I2 which enables the use of a screen grid tube as a linear grid-leak detector at moderate plate potentials, thereby obtaining the advantages inherent in the use of this type of tube as well as in the particular manner of detection. I provide in the grid-cathode or input circuit of the de'- tector D a resistor R and a condenser C in shunt therewith. These elements comprise a time circuit, as is well understood, andthe inherent time lag` action thereof may be given any desired value by assigning proper predetermined values to the The value of the time constant of circuit RC should be of the order of .0001 second. One terminal of the parallel circuit including the elements just .mentioned is connected by conductor I3 through `filter resistor I4 to one terminal of the secondary winding of transformer T. A radio frequency by-pass con-` denser I5 is connected between the point of connection of conductor I3 and the ground connection of the input circuit of tube. Filter resistor I d functions in conjunction with resistor R and condensers I5 and C to filter out alternating currents in the control circuit. The purpose of the circuit just described is, as previously indicated, to derive a uni-directional control voltage whose amplitude is proportional to that of the incoming carrier wave at the detector and apply such control voltage to the radio frequency amplifier or amplifiers ahead of the detector.

It will be noted that no normal biasing voltage is provided for the control grid of detector D, which device operates in the manner of the usual grid-leak detector by virtue of the elements utilized herein for control purposes. The manner of operation of a grid-leakdetectoris well known and it is deemed unnecessary to describe such action herein. AIn the embodiment of Fig. l resistor I2 is connected as illustrated, the screen grid conductor being connected to the resistor intermediate its ends. This method of connection interposes a part of the resistance in a screen grid-cathode circuit While the remainder of the resistance is connected in the screen grid potential supply circuit. n'ected as shown at I6, in the usualmanner.

Resistor I2 serves to increase the unidirectional current drawn from the screen voltage supply, thereby reducing the steady screen voltage below that of source II in such a way as to allow for a greater voltage swing in the plate load before plate over-loading is reached. -This method of controlling the detector characteristic by increasing the overload limits of the system results in so-called linear detection. I have also found that the specific circuit arrangement with resistor I2 connected/as illustrated enhances the variable mu characteristic of the detector tube, thereby enhancing the characteristic obtained by virtue of the pentode construction of the tube. While this construction of the tube has the advantage already mentioned,which it is desired to obtain, viz., variable mu. characteristic, it has other advantages. The auxiliary grid or fifth element of the tube has the same potential as the point of the filament or cathode to which it is connected. The effect of this grid is to influence the plate or anode in such a manner as to minimize secondary electron emission from the plate which is possible in the ordinary screen grid tube when used as contemplated herein.

Since no biasing potential is applied to the control grid of the detector, this element will remain at substantially zero potential during inoperative periods when no carrier wave is being received.l Practically no current flow takes place in the grid-cathode circuit of the detector at this time and only a very small voltage is established across resistor R and, therefore, substantially no q control voltage is applied to the control grid of tube V.

Assuming now that a carrier wave is present and excites the system, tube V, being substantially unbiased, will initially operate over the maximum amplification portion of its characteristic curve to give the desired amplification of the incoming wave. During one-half cycle of the carrier wave when the potential across the secondary of transformerT1 has the polarity indicated on the drawing and the tube D is positively charged, the grid will attract electrons emitted from the filament or cathode and grid rectification current will flow in the input circuitv of the detector tube through resistor R. The voltage set up Cil A by-pass condenser is conl across theresistor by this current f ow will have the polarity indicated on the drawing so that a carrier wave, condenser C will be gradually charged until the peak voltage of the applied wave is reached and this condenser will discharge very slightly through resistor R during the alternatehalf cycles when the voltage applied to the control grid goes negative and no grid rectification current flows through resistor R..

Should the amplitude of the incoming carrier increase to a value greater than the predetermined value, a correspondingly increased grid rectification current will flow in the input circuit of tube D, causing a' proportionate increase in the control voltage set up across resistor R. This increased biasing voltage being applied to tube V will decrease its efliciency and cause a decrease in the amplitude of the carrier wave applied to tube D.

Asstated above, as the carrier amplitude increases, the control grid is driven more negative. 'This causes the uni-directional screen grid and plate currents to decrease, allowing the energization potentials of both these elements to increase. y

In Fig. 2, there is illustrated a modified system in which the detector tube D takes the form of a tetrode or ordinary screen grid tube. In this instance, the screen grid is connected to the anode I supply source through resistors l1 and I8 to provide the desired potential onthe screen grid.

A by-pass condenser I9 is connected as i11us` trated.r Otherwise the system is similar to Fig. l, the modied portion only being shown in Fig. 2. I have found that the desired variable mu characteristic may be imparted to the detector by virtue of the arrangement of resistors I] and i8, as illustrated. By means of this arrangement also, the output and sensitivity of the detectorr are automatically' adjusted. With a 135J von supply the screen grid should have a potential of l volts for the desired maximum sensitivity. With the network illustrated, `a potential of 30 volts on the screen grid is generated before overloading occurs. .Without the network a screen grid voltage of 60 volts would be had and too much plate current would be drawn because of the action of the screen grid. The resistor l1 is used in this circuit to prevent overloading due to the above explained actions, with the following effect. 1

As the 4carrier increases, the control grid goes negative, the plate and screen currents decrease, and the plate and screen potentials rise. The increased plate current due to this rise causes an increased drop in potential of resistor I1, which prevents the screen potential from becoming excessive, and interfering with the plate circuit.

In Fig,4 3, there is illustrated a still further modified system in which detector D takes the form of a triode or three-element tube.- In this instance, resistor i1 alone serves to impart to the detector the desired variable mu characteristic. Thus, the resistor serves the purpose of preventing overloading in the-manner above described. As in Figx2, only the portion of the system which has been modied is illustrated in Fig. 3, the remainder of the system being the same as illustrated in Fig.` 1 and the automatic volume control action being the same as has already been described in connection with Fig. 1.

vWave variations.

voltage to the output circuit.

-It is important to note that in the grid-leak detector systems shown and described herein, condenser C. functions as the usual grid condenser of the grid-leakv detector and resistorR serves as the usual grid-leak to relieve the con- 5 trol grid of accumulated electrons after carrier The. detector circuit utilized has the inherent high gain of grid-leak detectors ndin addition provides a substantial amount of control energy which may be utilized to control the carrier wave output volume from the radio frequency amplifier tubes in the manner described whilebeing capable of supplying a. large The specifically arranged screen grid tube circuits of Figs. 1 and 2 have all of the inherent advantages of such tubes when used as detectors of modulated carrier Waves, at the same time eliminating the defects and overload disadvantages of such tubes that have heretofore rendered them impractical as linear grid-leak detectors at moderate plate potentials. When two or more carrier frequency amplifiers are used, the application of the derived unidirectional control voltage to more than one of such amplifiers will obviously enhance the automatic volume control action.

Reference may now be had to Figs. 4 and 5 for an explanation of the action of a vvariable mu detector as distinguished from the ordinary type of detector having a fixed amplification factor. In Fig. 4, there is illustrated the plate current and grid current characteristic curve of an ordinary grid-leak detector having a fixed ampli cation factor. The corresponding curves of Fig. 5 35 are those of a variable mu detector, such as those of Figs. 1 to 3. It will be understood, of course, that these curves are only generalizations of the action which takes place in therespective detectors, but they will serve the purpose of the pres- 40 ent disclosure. As is Well known, in a grid-leak detector, the detecting action occurs because of the sharp bend of the grid current curve near the origin 0 of the curve, which, .of course, represents zero grid voltage. In order to provide an I output from the detector having minimum distortion, the plate current curve should have no v very pronounced or sharp bends or points of curvature; As is evident from the platecurrent curve 0f Fig. 4, theI ordinary tube having ya xed ampli- 50 fication factor has a plate current characteristic curve with a sharp bend or point of curvature at A. As a result, when the modulation passes beyond pointA, the signals are distorted.

Comparing'the plater current curve of Fig. 5 55 with that of Fig. 4, it will be noted that the former has no sharp bends or points of curvature andv consequently no marked distortion is introduced at any point. vConsidering the point A on this curve, which corresponds to the point above conpassing of the. modulaton beypnd the point will not cause a material increase of distortion of the signals. Let us assume that in each of the detecsidered on the curve of Fig. 4 it will be noted that tors under consideration,.the modulation envewill differ largely from the negative swings in 7o their control of the plate current, thus, generati ing a large proportion of second harmonic wave. On the other hand, in the variable mu detector,

' whose action is depicted in Fig. 5, only a slight difference occurs in the positive swings ofthe 75 incoming oscillations as compared with the negative swings.

An additional advantage of the variable mu detectois that as the carrier increases, the increased bias causes the demodulation efficiency to decrease, thus providing automatic volume control in the detector. This action occurs due to the fact that an'increase in bias causes the modulation envelope to operate over a portion 'of a curve which has a small slope, as Will be apparent from a consideration of Fig. 5. For example, if the carrier level is low, the plate circuit is controlled by a signal operating on a portion of the curve having a steep slope. If, however, the bias is high, the plate current is controlled by a signal operating on a portion of the curve having a small slope where the transfer efliciency of the detector is low.

This feature is particularly advantageous when this invention is embodied in the rst detector of a superheterodyne as the bias from the volume control thus controls the transfer eiciency of this detector.

-comprising a pentode vacuum tube having input and output circuits, means for applying signals to said input circuit of such intensity that rectification occurs in both said circuits, said tube having the characteristic of continuously varying amplification over the normal range of amplitude of said signals, and means external to said tube for increasing the degree of said characteristic, whereby the rectification in said output circuit is maintained relatively small as compared with the rectification in said input circuit.

2.- In a radio apparatus, a'grid-leak detector comprising a vacuum tube having a cathode, a irst grid, a second grid, a third grid, and an anode, said grids being spaced between said cathode and said anode in the order named, means for applying a variable unidirectional voltage between said first grid and said cathode, said voltage varying in accordance with the incoming signal-modulated carrier wave, means for maintaining said third grid substantially at the same potential as said cathode, a source of unidirectional voltage connected to said cathode and said anode, means comprising a resistor connected` to the positive terminal of said source and to said second grid whereby the voltage supplied to said second grid increases as said first means causes said rst grid 'to become more negative, thereby diminishing the reduction in current flowing in said anode connection caused by the said increase in said negative voltage.

3. In a signaling system, a grid-leak detector comprising a multi-element vacuum tube having a control element, a cathode, an anode, an anodic grid and an auxiliary element, an input circuit connected between said control element and said cathode, an output circuit connected between said cathode and said anode, an energizing circuit for said anodic grid, means for applying signals to said input circuit, means in said input circuit for forming a uni-directional voltage determined by the amplitude of said signals, means comprising a relatively great resistance in said anodic grid circuit for making said anodic grid responsive to said voltage to eiTect substantial variation of the amplification factor of the detector, and means including said auxiliary element for increasing the operating range of said detector.

4. In a signaling system, a grid-leak detectorv comprising a multi-element vacuum tube having a control element,` a cathode, an anode, an anodic grid and an auxiliary element, an input circuit connected between said control element and said cathode, an output circuit connected between said cathode and said anode, an energizing circuit for said anodic grid, an amplifier tube the output of which is coupled to said input circuit for transferring signals thereto, means in said input circuit for forming a unidirectional voltage determined by the ampitude of said signals, means comprising a resistance in said anodic grid circuit for making said anodic grid'respon'sive to said voltage to effect substantial variation of the amplification factor of the detector, means including said auxiliary element for increasing the operating range of said detector, and means for controlling the amplification of said amplifier by the said voltage.

AN'I'HON A. LEONARD.

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