US2141338A - Detector circuit - Google Patents

Description

Dec. 27, 1938. l.. E. BARTON DETECTOR CIRCUIT Filed May 8, 1937 TH T 3m A u .wu

Patented Dec. 27, 19.38v

Egli-LESS UNITED s TArEs PATENT oEEicE DETECTOR CIRCUIT'v Loy E.,Barton, Collingswood, N. J.,assignorV to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation fofDelaware' Application May 8, 1937, `Serial No. 7,141,561

v 12 claims. (111.250,-20)

`'Ihis invention relatesto arnovel detectorrcir- -.cuit for a radio receiver or the likeembodying .gart. The present invention is directed specifical- 1y to the .detectorjsystem comprising lthe trans- -formerfT, thedetectorf tube D, and the associated. 5 circuits. The detectorsystem-may be .coupled to" thelast stage ofthe intermediate frequency amy-plier .or;to a driver tube, `as represented by the :automatic volume control and is useful, forexample, in a superheterodyne radio receiver. More 5wspecical1y, the invention relates to radio-receiv- 'ing'.systems. in which it is desired .to obtain both an*V audio signal Vand an automaticvolumecontrol signal from the detector circuit, and .in which it is desired to obtain the-automatic volume conf -lO ltrol signal from a primary circuitY and to obtain -the audio signal from a secondary circuitasso- *ciated with the primary circuit. i

One obj ect of the invention is to provide a` circuit of this class Whose selectivity orresponse v4l5 characteristic is decidedly better'xthan -those of .prior devices, particularly in that theY selectivity Icharacteristic curve has ak flat Yblunt nose land steep Sides, thus giving higher fidelity and better adjacent channel attenuation than ,prior devices ofthe same general class.

Another object of the invention is to provide 1;

n a-device Vof this character wherein the response Y characteristic of the automatic Volume control system is broader than that of the ydetectorcircuit so that the automatic volume control system rbecomes operative before anoutput signal is ob-v tained as the receiver is tuned to a-desiredstation.

' A further object of the invention is .to Iprovide I :lo a device of the stated character `wherein the selec- Vtivity characteristic of the detector circuit is symmetrical with respect to the carrier frequency.

' A still-further object of :the invention is to provide afdetector circuit of the type specied which may employ a'conventional doublediodetriode vacuum tube' and in which the deleterious yeffectsoi the inherent interelectrode capacities in a tube of this type may be alleviated.

YThe inventionmay be more clearly understood .1?) -by referenceto the accompanying drawing in which there are illustrated a preferred .form of the-invention and certain'characteristics thereof.

`In the-drawing: Fig. 1 is a diagrammatic illustration of a supertion; f

' Fig. 2 is a graph illustrating certain character- 'y istics'of-the invention; and

Fig. 3'illustrates a specific form of the transthe terminals ra and d thereof. 50

.3o former employed'inthe detector circuit.

Referring to Fig. 1, there. is shown a typical `voltage which is built up across resistor R1 by radio receiver of 'the superheterodyne type which,

heterodyne radio receiver embodying-the invencomprising resistors R and R1 and the radio fresince the construction and operation of Isuch a: receiver is well understood by those skilled-.in the tube" V. 5- y 'Iv'heprimary winding P yand lthe secondary;10 winding S ofthe transformer TareV tappedat in- ;termediatepoints/b. and' e, respectively,... each of Which-points should preferably be at the electrical centerV or :midpoint ofthe particular winding.

4the output electro-de or .anode of the tube V. 'Ihlmidpointofftheprimary Winding is also con- .wnected tofground through a path having low im- 20 pedance at radio frequency, which-path may consistof a separate connection to ground through a blocking condenser, as il1ustrated,.or may comvv`prise a suitable fllterin vthe'energy supply circuit Vfor the tube V. The entire primary Winding is 25 tuned torthecarrierrfrequency by means of a condenser connected thereacross, as `well understood.

The center-tapped secondary .winding S, which is electromagnetically. coupled to the primary Winding-is also tuned to the carrier frequency by` means of afcondenser connected across the Winding, as illustrated. The-extremity or terminal d -of:.the.. secondary -windingis connected to the anode Vof `the diode .d1 of the detector tube D. 35

The anode-of the other diode d2 of the detector' tube is connected to the terminal a of the primary 'winding through .the small blocking condenser C1.

It will bev understood, of course, vthat this connection to the primary winding, as well as the` connection of the anode of tube V to the primary winding,imay be made to a tappedpointnear the Y endoi the primary windinginstead of at an end thereof. The midpoint e of the secondary Vwinding -is connected to the diode circuit or audio load quency by-pass condensers C. The primary and secondarywindings are preferably polarized so asv to minimize the potential difference between d1, is supplied to the rgrid-cathode or input circuit. ofthe triode portion of tube D through cone.

denser C2, potentiometer P and condenser C3. A suitable bias may be supplied toI the grid of tube D through the grid-leak resistance R2. The amplified audio signal from tube D is supplied to the usual audio amplifier, as illustrated.

The anodel'of the diode d2, which may be-termed the A. V. C. (automatic volume control) diode, is connected to the automaticjvolumecontrol circuit comprising the resistors R3 and R4 ,and bypass Vcondenser C4, which circuit is connected to a portion of the radio receiver ahead of the tube V. 1-

in a manner to control the amplification or gain of the system. Since automatic volume control systems are now well understood, this circuit requires no detailed explanation. The connection of the circuit to the portion of the radio receiver to be controlled may be made in accordance with any of the conventional methods and practices. If desired, a delay voltage may be appliedto the automatic volume control system through the resistance R4. Y' A Y From the above description, it vwill be seen that the incoming modulated carrier signal is supplied to the portion of the primary winding P between the points a and b and is thus transferred to the circuit of diode d1 comprising the portion of the secondary S between the points Vd and e. The audio signal derivedby the rectification action of the diode dr is supplied to the triode amplier portion of tube D, as aboveV described.Y At the same time, a portion ofthe signal energyfrom tube V is supplied to the diode d2, the load circuit of which includes the resistor R4. The voltage built up across this resistor is utilized for the purpose of automatic volume control as above mentioned.

It has been found that in a circuit of this character, the capacity between the anodes of the diodes d1 and d2 and the stray capacity between the leads Vconnected thereto, introduce a fortuitous capacity coupling between the primaryV winding P and the secondary winding S, as vindicated at C5.

' several micro-nuorofarads and tends'to disturb the selectivity Vcharacteristic ofthe transformer T, causing it to become non-symmetrical. A typical illustration of the non-symmetrical selectivity characteristic curve which is thus produced is shown at A in Fig. 2, 'wherein the abscissa represents the frequency of the testsignal and the ordinate represents on a logarithmic `scale the amplitude of the input signal required to produce a standard output signal.

v be obtained, for example, using the circuit of Fig.

l, by connecting la `signal 'generator to the grid of tube V and determining the amplitude and frequency of a signal from the signal generator required to produce a certain d c. voltage across the load resistance in the circuitof the audio diode d1. For example, a micro-ammeter might be connected in the circuitV of diode d1 and the output of the signal generator Vadjusted so asto produce a certain reading of the micro-ammeter. It has been found further that increasing Vthe capacity between the anodes of the diodes d1 and d2 tends to make the characteristic curve more non-symmetrical; Y

In accordance with the present invention, this eiect is reduced by introducing a capacitance Ce between Vthe opposite extremities or terminals,

c and f of the primary and secondary windings.

It is found that as this capacitance is increased,

the response or selectivity characteristic becomes more symmetrical `and there-is finally produced the symmetricalV curVe'B of Fig. 2. If the capac- Such capacitance may amount to Such curve might Y ity is increased further beyond the value giving the symmetrical curve, the curve again becomes non-symmetrical but on the Vopposite side of the axis as illustrated by the curve E in Fig. v2. It has been found ythat where both the primary and secondary windings are center-tapped, the capacitance Ca should be substantially equal'to the fortuitous. capacitance C5.Y Ihe position of the tapped points b and e may be varied; however, under these circumstances the Ycapacitance C6 will have a value different from that of the for-- tuitous capacitance C5.

VIn addition to the advantage above noted, the circuit of the invention materially improves the shape of the selectivity characteristic as may be seen from Fig. 2. As compared with a characteristic curve obtained by using two tuned circuits coupledl tothe same degree and otherwise comparable, it is found that the symmetrical curve B produced by the circuit of the invention is flatter over the `useful operating range and, therefore, the circuit gives better fidelity; and, furthermore, the sides. of the curve B aresomewhat steeper, thus improving the adjacent'channel attenuation. Both of these features, viz. Ythe flat blunt nose andthe steepsides of the curve B,

' rived from theV primary circuit.A It will be noted also that the selectivity of the automatic volume control circuit is broader than that of the detector circuit, as a result Yof which the gain of the radio receiver is reduced before theaudio signal builds up to its vnormal value .as the receiverris tuned to a strong station. Consequently, momentary blasting of the receiver, which mightotherwise occur in Ythe interim during which the A` V. C.

voltage is building up, is effectively avoided.

A further feature of the invention resides in thefact that by connecting the input and output circuits of the transformer T to taps on the primary and secondary windings, theY impedance presented by the transformer to the tube V and to the diodes d1 and dz'may be reduced to a certain extent, thus permitting more eicient matching between the several circuit elements without requiring the use of non-standard parts.

- In Fig. 3, there is illustrated Va speciflcform of the transformer T by the use of which satisfactory results have been obtained. VIn this instance,` the primary and secondary windings each comprises twopie-wound sections or coils each comprising 110 turns of 7-41 litzendraht wire universallyl wound in the 'same direction on a tube G one-half inch in diameter. In each case, the total inductance of the two sections or coils was 1290 micro-henries, and the condenser employed with each of them was of suflicient value to tune the circuitto resonance at a vfrequency of 465 kilocycles. VThe capacity of each padding ycondenser was approximately 100 "micro-microfarads. The mutual inductance betweenV the primary and secondary windings was 53 microhenries which, in combination with the capaci- Ycuits. TheV two sections of the primary windingyas well as those of the secondarywinding,

were* spaced apartwabout ione-sixteenth .of an inch, and a comminuted iron core was used with each winding lor coil as shown at H and I, re-

'and tap points of thewindingsare clearly identified. The connectionsbetween the two pies was usedV as'theicenter tap'; 'Y The entire transformer wasfmountedi in a conventional cylindrical housing one and three-fourth inches in diameter.

In thespeciiic device mentioned, a type 'l5 detector tube was employed, and the several components ofthe A. V. C. and audio load circuits were given conventional values- .A type .'78 tube was used for-V and bothtubes .were operated at their ratedvoltages The coupling capacity Ci 'and the kcapacity C6 were Ybuilt into the padding vcondenser itself, .the capacity C1 being obtained `by adding 'an additional plateto the padding condenser, while the capacity Cs was obtained by the proper spacingand proximity of the sol- Cil points on said windings,

dering lugs on the condenser. .e In. addition tothe. above, the following values were assigned to 'certainr of the elements:

' C1=110 micro-microfarads R,=,`100,000 ohms C 110 micro-'microfarads R1=1/3 `meshm P 1 megohm R2`=1.5jmegohms C4 `:0.15 microfarad R3=1 megohm R4=% megohm l C5=C6=2 micro-microfarads It will be understood, of course, that the in- -vention is not limited-tothe specific embodiment illustrated and described herein nor to the values cf the various elements given as aspecific eX- ample. The invention is capable of Vvarious embodiments or modifications which may be resorted to without departing from its scope.

I claim:

1. In a signaling system, a transformer having a primary winding and a secondary winding, means for tuning said windings `Ato a desired frequency, means for supplying signal energy to part cf said primary winding, a diode rectifier connected across part of said secondary Vwinding so as to receive signal energy therefrom,` means for deriving rectified signal from the circuit of said rectifier, a second diode rectifierconnected to said primary winding, there being a fortuitous capacity coupling introduced between said windings bysaid diode rectifiers, means providing a low impedance rpath between intermediate capacitive coupling means between said windings for reducing` the leffect of said fortuitous coupling,'and means for deriving rectified signal fromsiaid second diode rectifier,

2. In a signaling system, a transformer having a primary winding and a secondary winding, means for tuning said windings to a desired frequency, means for supplying signal energy to said primary winding, a diode rectifier connected in circuit with a Dart of said secondary winding so as to receive signal energy therefrom, means foi` deriving rectified signal from the circuit of said rectifier, a second diode rectifier connected ,tofaipart -of-said primary winding, there being a -fortuitousfcapacity coupling introduced between said windings by Vsaid diode rectiers, means providing a low impedance path between intermediate pointsA of said windings, capacitive coupling -means between ysaid windings for reducing the effect..of.said fortuitous. coupling, and means for y.derivingrectified signal from said second diode rectifier.

3,. In a signaling system, a transformer having a primary l.winding and. a secondary winding, ymeansforttuning saidv windings to a desired frequency, means Vfor supplying signal energy to part of said. primary-.winding a diode rectifier connected in circuit with a part of said secondary winding. soas toi receive signal energy therefrom,.avload impedance in circuit with said diode .rectier for deriving a rectified output signal therefrom,..a secondv diode rectifier connected to a part of said primary winding, there being a forltuitous capacity coupling introduced between said.

windings by said diode rectifiers, means providing a lowimpedance path between intermediate points of'said-windings, capacitive.V coupling means between 'saidwindings for reducing the eect of vsaid'fortuitous4 coupling, and ra load impedance in circuit with saidsecond diode rectifier for deriving' a rectified signal therefrom.

:4. In a signaling system, a transformer having a .primary .winding and a secondary winding, means forv tuningsaid windings lto a desired frequency, vmeans for..supplying signal energy to part of said-.primary winding, a diode rectifier connected. in circuiti with a part of said secondary winding so as to receive signal energy therefrom, a. loadv impedance in circuit with said diode rectifier kfor deriving-La Vfrectied output signal therefrom, a' second diode rectifier connected to ari'part of .said primary winding, said windings being pola1ized'to.minimize the potential between .theV parts of said windings to which said .diode rectifiers are connected, there being a fortitouscapacity coupling introduced between said windings by saiddiode rectifiers, means providy ing alow impedance-path 'between intermediate points of said windings, capacitive coupling means between said windings for reducing the effectv of saidfortuitous coupling, and a load impedance in circuit with said second diode rectifier for deriving a -rectied signal therefrom.

5. In a signaling system, a transformer having a primary winding and a secondary winding, means for tuning saidy windings to a desired fre- Yquenc'y', lmeans` for supplying signal energy to said `primary winding, a vacuum tube including two 'diodes in `tliesame-erivelope, means connecting one of the diode rectiers of said tube in' circuit Awith a part of said secondary winding so as to receive signal'energy therefrom, a load impedancev iri circuit with said diode rectier forderiving a rectified' output signal therefrom,

means'. connectin'g'the other diode rectifier of from said other diode rectier, and means for utilizing said last-named signal to control the amplitude level of the signals transferred by said system.

6. In a signaling system, a transformer having a primary winding and a secondarywinding, means for tuning said windings to a desired frequency, a source of signals connected between an extremity and an intermediate point of said primary winding, a vacuum tube including two diode rectiers in the same envelope, means connecting one of the diode anodes of said tube to an extremity of said secondary winding', means including a load impedance connecting an intermediate point of said secondary winding to the cathode of said tube, means for deriving aY rectified signal from said load impedance, means connecting the other diode anode of said tube to the said extremity of said primary winding, there being a fortuitous capacity coupling introduced between said windings by said diode ancdes,'means providing a low impedance path between the said intermediate points of said windings, Ycapacitive coupling means between the other extremities of said windings for reducing the effect of said fortuitous coupling, and means for deriving a rectified signal from` said other diode rectifier.

7. In a signaling system, a transformer having a primary winding and a secondary winding, means for tuning said windings to a desired frequency, a source of signals connected between an extremity and an intermediate point of said primary winding, a vacuum tube including two diode rectiers in the same envelope, means connecting one of the diode anodes of said tube to an extremity of said secondary winding, means including a load impedance connecting an intermediate point of said secondary winding to the cathode of said tube, means for deriving a rectied signal from said load impedance, means connecting the other diode anode of said tube to the said extremity of said primary winding, there being a fortuitous capacity coupling introduced between said windings by said diode anodes, means providing a low impedance path between the said intermediate points of said windings, capacitive couplingmeans between the other extremities of said windings for reducing the effect of said fortuitous coupling, and means for deriving a rectified signal from said other diode rectifier, and means for utilizing said lastnamed signal to control the amplitude level of the signals transferred by said system. v

8. In a signaling system, a transformer having a primary winding and a secondary winding,Y

means for supplying signal energy to a part of said primary winding, a space discharge device having a plurality of electrodes, one of said electrodes being connected to a part ofsaid secondary winding so as to receive signal energy therefrom, another of said electrodes being connected to a part of said primary winding, there being a fortuitous capacity coupling introduced between said windings by said electrodes, means providing a low impedance path between intermediate points of said windings, and capacitive coupling means between said windings for reducing the eifect of said fortuitous coupling.

9. In a signaling system, a transformer having a primary winding and a secondary winding, means for supplying signal energy to a part of said primary winding, a space discharge device having a diode rectifier and at least one additional element, means connecting said element in circuit with a part of said secondary winding so as to receive signal energy therefrom, said diode rectifier being connected to a part of said primary winding, there being a fortuitous capacity coupling introduced between said windings by the interelectrode capacitance of said diode rectifier and said element, means for providing a low impedanceV path between intermediate points of said windings, capacitive coupling between said windings for reducing the eifect of said fortuitous coupling, means for deriving a rectified signal from said diode rectifier, and means for utilizing said rectified signal to control the amplitude level of the signals transferred by the said system.

10. Ina signaling system, a transformer having a primary winding and a secondary winding, means for tuning said primary winding to a desired frequency, means for tuning said secondary winding to said frequency, a source of signal energy connected between an extremity of said primary winding and an intermediate point thereof, means for deriving a signal from a portion of said secondary winding between an extremity of the secondary winding and an intermediate point thereof, means providing a low impedance path between the said intermediate points of said windings, a capacitive connection between the said extremities of said windings, and capacitive coupling means between the other extremities of said windings.

ll; In a signaling system, a transformer having a primary winding and a secondary winding, means for tuning said primary winding to a desired frequency, means for tuning said secondary winding to said frequency, a source of signal energy connected between kan extremity of said primary winding and an intermediate point thereof, .a load circuit connected between an extremity of the secondary winding and an intermediate point thereof, means for deriving a signal from said load circuit, means providing a low impedance path between the said vintermediate points of said windings, a capacitive connection between the said extremities of said windings, and capacitive coupling means between the other extremities of said windings.

12. In a signaling system, a ing a primary winding and a secondary winding, means for tuning said primary winding to a desired frequency, means for tuning said secondary winding to said frequency, a source of signal energy connected between an extremity of said primary winding and an intermediate point thereof, a load circuit including a diode rectifier connected between an extremity of the secondary winding and an intermediate point thereof, means for deriving a rectified signal from said load circuit, means providing a low impedance path between the said intermediate points of said windings, a capacitive connection between the said extremities of said windings, and capacitive coupling means between the other extremities of said windings.

yLOY E. BARTON.

transformer havi

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