US2507721A - Amplitude modulation - Google Patents

Description

Patented May 16, 1950 2,507,721 AMPLITUDE H eT R nssell R Law, Princeton, NQJ assignogr to Radio? ene ate A e i iqrrsiieii ee i l Anp eafiom l c mbet 1 8, erial (016332-63 12 Claims This invention relates tolan plitude modulation,

and more particularly tqa m cd ulator for p req ncy m fie An object of this invention is tot devise a mod ulator circuit which will give a greater depth;

of modulation, or a larger change; of power, in a radio frequency power amplifier, than that 'giv'en 1 by circuits used heretofo; e,' for a given modula on i na Another object is -to devise a circuit includin a radio frequency amplifier and amodulatorso interconnected that there is a substantiallv constant flow of direct current to the cathodes andtherefore also to the anodes of the modulatoranm the amplifier, the direct current being-switched from the amplifier tube to the modulator tube.

and vice versa at the modulatingfrequency, when a modulating signal is applied; to produce modulation of the radio frequency carrier,

The foregoing and other objectslot thev invention will be best understood from the-following,

gramv of a practical. embodiment of: this inven tion;

Now referring to. Fig. 1, a; suitable source of:

modulating signals, such as ;video signals forex;

ample, is connec ed- Q nPi F fi lm li l i a 2 e n ere eflerl sh wn-H eed I) isconnected directlyto the control grid ii o ffla triode 4 which serves asa video modulator. Ti' i-;

ode; 4 may be, and preferably is, a coax lecaivity triode of the type described in detail in the are ticle by Law et al., RCA Review, vommgvnnq Anode. 8 is grounded for radio frequencies through condenser 9, and is also connectedto the positive terminal of a D50. or 3 power sup ply [0, the negative terminal of which is grounded as shown. Cathode 5' of tube 4 is connected directly to a point I! which will bereferred tolater.

Tube 4 i used t hod -modul a nded-grid triode or electron discharge device? [2 which thus operates as a cathode-modulatedra dio'frequency power amplifier; This triode prererahly is of the coaxial-cavity type describedin the Law et al. article, A developmental televif sion transmitter for 500-;9QQ megacycles lished in RCA Review, volume No, 4 ,j P Inhe 19 asesfie fiz, an Q udS a cavity [2a and a cathode cavityl2b, separated through: v i,

by a metallic partition I'Zc Which includes the igfoftriode l2. Grid" I3 "is" grounded at i i; Cathode lief-"electrondis device 5 2 'is connected through 'an- "opeii chai ing nf cavityj some point 11,7 w'hileanode it of said-deviceis connected through an opening cavityliio' tolthe positivejterininalof power sup-f plyd B," said anode "being grounded "for 'alt'ernat ing: currents through condenser-i7. The'lead-to the cathode I 5 and the dead" to the anode I 6 i are; illustrated as passine through the usualchoke" apertures "into the: respective cavities, the cathode 'bfeing therefore 1 bypassed or grounded'to the cavit'yror R F only;

RgFf input energy from a suitable source not shown; is' applie'dfto theRFF power amplifie including tube" IZby r'neans of a coaxialline the inner conductor" of""whichl terminates in coupling loop l 9 electromanetically coupledinto c thode cavity-I251} Modulated i-F output en: ergy abstracted from the 'R-F power amplifier by means of a coaxial "line 20; the inner conduc I tor ofwhidh terminates-111 a; coupling loop 21 electromagnetically "couple'd int'o "anode cavit'y 2a; U U V, l According to-this invention, bias is suppliedtothe two tuoes 'I-Z-and M-iby co'nn'ecting th'e-posi tiifter lnifial Of a DI-C'l' DiBS SbIIIQ ZZ through a constant curren't network 23' to the" commoncathode point orte'rminal u Network-23jis i1- lustrated as an inductance which is splitint'osections 23a, 23b ana zae of pr'ogressivek, in:

creasing size;- for a seaoil which will hereinafter} appearj The negative terminal of current source; 21 grounded" at' '24 to complete the -D."- C'.' an'- ode-cathode-circuifito the negative ter minal of pow r supply {I source" 22 biases cathodes 5 and lifpesitively with respect to ground or with respect to grounded grmgra; "Bias source 1 isso v adjusted thatf'cfathcd 5 is-also "positive -with ie'f p lm dj a V. a l r t K, M

Ordinarily; the grounded-grid triode i2 is opere ted under-Class 0 conditions to-gi've' good eifi cienc'y, said trio'de therefore operating as" a Class C rounde'd grii'i amplifier; Under Class C conditi'on s; the instantaneous "cathode "potential" swings below the grid-potential level or ground,"

' sum-at the grid is e'fictivlv positive with'respectto the cathodeffor 'oiil ya fraction of-th'e cycle" of- RAF" appliedvoltage, and 1 during this fraction a momentary pulse of current is. drawn f rorn the-cathode l5and injected into the grid; anode Space; "if theiiistantaheous anode 'potential is below 'theD; CIValuethereofwhen this pulseof current arrives at the anode; a portion or" the -D.C. power from source -i G is converted into AwC. energy, thus providingamplificationof the R-F inputener gv. Therefore, "thepower level of the R-E output appearing in lead 20 varies iii' the same directi'o'ii as} and substantially proportionally i to, the level of the DZ- CJ power 'appliedirorn .s'ources'fiq and 22- (these 'two sources being effectively in series in the anode-cathode circuit of tube I2) to tube l2. Such D.-C. power is substantially directly proportional to the product of plate voltage and plate current in this tube, so that as the direct plate current of tube [2 is increased, the power level of the R-F output in lead 20 is increased also. Thus, by variation or modulation of the D.-C. anode current to tube I2, amplitude modulation of the R-F carrier is produced, this modulated R-F carrier appearing in lead 20.

This action may be explained in another Way, as follows: When the positive bias, with respect to ground or grid l3, on cathode I is increased, the duration of the momentary pulse of current drawn from cathode l5 and injected into the grid-anode space of tube I2 is decreased (because the time during which the instantaneous cathode potential is below the grid potential level is lessened), thus decreasing the portion of the D.-C. power which is converted into A.-C. energy and also decreasing the level of the R-F output power. Conversely, when the positive bias (with respect to ground) on cathode I5 is decreased, the R-F output power is increased.

In operation, the bias source 22 is so adjusted as to give the desired sum-total direct current to the cathodes 5 and I5 of the modulator 4 and amplifier l2, this total being appropriate to the particular characteristics of the tubes utilized. Inasmuch as this current flows through the series-connected inductances 23a, 23b and 230, video frequency changes in current are substantially prevented by the flywheel effect of the inductance, and the combined direct currents to the modulator 4 and amplifier [2 will remain substantially constant. The inductance 23 is chosen o as to have practically no D.-C. resistance, the current drawn through said inductance being substantially constant through its inherent action; the inductance 23 may therefore be termed a constant-current network.

The current flowing from source 22 to the two cathodes 5 and i5 is held substantially constant by the action of inductance 23, the division of current between the two cathodes depending on the character of the modulations. When a positive modulating signal is impressed on the grid 3 of the modulator 4, an increased flow of current through the modulator tube results, but, inasmuch as the total current from source 22 is held constant by the action of network 23, the current to the cathode l5 of amplifier I2 is reduced, reducing also the R-F output power in lead 20. Conversely, a negative signal applied to grid 3 has the effect of decreasing the current taken by modulator 4 and thereby increasing the current delivered to the amplifier l2, increasing the R-F output power in lead 20. Thus, amplitude modulation of the R-F carrier is produced.

In effect, then, the sum of the direct currents to the two cathodes 5 and I5 from source 22 and/or source II] is held constant; this also holds the direct current to the two anodes substantially constant. Modulation at video frequencies serves to switch the current from the amplifier [2 to the modulator 4 and vice versa.

The advantages of using a constant-current network in the common bias lead to the cathodes 5 and I5 should now become apparent. If an impedance in which an IR voltage drop could occur, and in which the series current could change, were used in the common cathode bias lead, the current change in the R-F power amplifier tube produced as a result of modulation would not be as great as that which is produced by modulation when a constant current network is used in the common cathode bias lead. To make this clearer, assume that a series resistance of ten ohms is in the common cathode bias lead, and that a current of one ampere is flowing to cathode 5 and a current of one ampere to cathode l5. Thus, two amperes of current are flowing through the resistor, giving an IR drop of twenty volts therein, biasing the two cathodes twenty volts positive with respect to ground. If, now, a modulating signal is applied to grid 3 such that the current to cathode 5 goes to zero, the current to cathode [5 remains substantially one ampere and the IR voltage drop in the series bias resistor falls to ten volts. This change in the IR drop or cathode bias, from twenty volts to ten volts, would at first glance seem to be sufficient to produce a maximum change of current in R-F amplifier tube l2, and therefore also a maximum amount of amplitude modulation of the R-F carrier. However, the plate current-bias characteristic curve of a triode such as triode I2 is nonlinear and does not have a slope such that when the bias is halved the current is doubled. Therefore, the ratio of the currents in the R-F amplifier tube with the serie resistor circuit connection is substantially less than one-to-two from the initial situation above to the final situation above, giving a less than maximum amount of amplitude modulation of the R-F carrier.

If, now, instead of a series resistor in the oathode bias lead, a constant-current network is used in this lead, when the one ampere of current originally flowing to cathode 5 is reduced to zero in response to a modulating signal, this one ampere of current is added to the one ampere flowing to cathode I5, making a total of two amperes flowing thereto. This is so because with a constant-current network in series in the cathode bias lead, the total D.-C. flowing is maintained substantially constant at its original value of two amperes and cannot change in response to modulating signals. Thus, with the constant-current circuit connection, the ratio of the current in the R-F amplifier tube is substantially one-totwo from the initial situation to the final situation, giving the maximum possible change of power in the R-F amplifier or the maximum possible amount of amplitude modulation of the R-F carrier.

The reason for splitting the series inductance 23 into sections 23a, 23b and 230 of progressively increasing size or progressively increasing inductance is to provide relatively high impedance over the video band. In a practical embodiment, section 230 was a large iron-core filter chok of the kind commonly used in 60-cycle rectifier power supplies. Sections 23b and 23a were successively smaller coils designed to resonate at successively higher frequencies. Thus, 23b had an inductance such that, in combination with its distributed capacitance, resonance occurred at approximately one-half megacycle, while 23a in turn had an inductance such that in combination with its distributed capacitance resonance occurred at approximately 20 megacycles. Tests of this combination indicated an impedance of many thousands of ohms throughout the entire video band. So long as this impedance is large compared to Rp/1+,Lb, the gain is substantially constant and equal to ,u/ 1+,u, where Rp and refer to the characteristics of triode [2. The high value of impedance of the combination 23a, 23b, 230 is so spotter la'rgeas to give substantially constant gain as T embodiment of this invention, parts the same as tl'ios'e of Fig. l are referred to by the same referonce-"numerals.

modulator triodes 4 and l. Anode 21 of triode the two grids 3 and 3 of modulator triodes 4 and 4; a-suitable bias being applied thereto through resistor 8. The cathode-coupled stage 26 ahead of the modulator triodes l and iserves to lower Video input signals are applied to'thecontrolgrid 25 of a triode 26 whichis cathode-coupled to the two parallelly-connected the-"efiective input capacitance presented to" the video amplifier by th set of tubes 4 and thiscathode follower stage having a low input ca pacitance.

Thetwo modulator tubes 4 and 4 are connect'ed in parallel, having their grids 3 and 3' connected to cathode 38, as previously described,

and-having their cathodes ii and 5' connected together and to a point I I.

A resistor 35 is connected in series with anode Gffrom it to-the positive potential source, while a resistor 32 is connected in series with anode '6". fr'omit to the positive potential source, these resistors; as well as resistor 28 in series with anode 21, being provided for the purpose of suppressing parasitic oscillations. The plate ends of resistors 3t and 32 are carefully by-passe'd to ground by mica condensers 9 and 9' of several micr'ofarads capacitance, the condenser 29 similarly' by-passing the plate end of resistance '28 to round.

As in-Fig. 1, a positive bias is supplied to point I I through a constant-current network or inductance 23. Although in Fig. 2 the inductance is shown as-a single section, it is desired to bemade clear that actually inductance 23 is split into a plurality of sections, as previousl described inconnection with Fig. 1.

Apair of triodes I2 and i2 are connectedin push-pull to serve as an R-F power amplifier, point ll being connected to the nodal point'of a U-shaped conductor which connects cathodes l5 and'15". Grids i3 and 43 are connected together and to ground at it, While the direct cu'rrentlea'd to the anodes l6 and It is'led off'from the nodal point of a U-shaped conductor con meeting these anodes. A condenser l7 by-passes theanodes to ground for alternating currents,

la'ting'signal is impressed on the first tube '26-, the cathode as ofthis tube becomes more positive due to the drop of the biasing voltage developed acros's'resistor 8 due to increased current flow in tube 259 This increases the positive voltage ontiie' grids 3 and 3' of the modulator tubes and results in'an increased flow of'current therein. Biiti sincethe' total current is heldconstant by As an example, a current change of more while R-F input energy is applied by meansof cathode-current" flo'w*causingmodulationof th the action of inductance' 23', the current' to theamplifier tubes '12 and"l2 -is rel:llieed, givihg 1;, reduced R. Fpower-output in lead -20? com versely, a negative signal applled'to the first'trf- 0de26 has the-eliect' of"decreasing the current" taken by the modulatortubes 4 -and- 4 thereby-' increasing the current delivered to theamplifier tubes' l2 and I2 and'increas'ing the"R'=-F"power"" output in lead 20. Thus; amplitude modulation of the R F'carrieris produced.

Although it isto be understood that this inven tionis not to be limited thereto, the following *1 rather typical values are givenfor'anoperative" embodiment of this invention." D.-C. bias ontlie commonly-connected cathodes 5 and "l 5 was '80 volts positive with respect to ground. In view of" the hightrans'c'onduct-ance of the modulator tube; approximately 0.15 ampere pervolt at the o der ating point, the voltage diiference between the control grid 3 and cathode-El ot themodulator tube is only a few volts. Thus, the cathode 30 of the preceding stage is only .a few volts-more negative than the conjoined cathodes of the modulator and amplifier. The value 01 resistor it for this first stage is determined by the imped ance band width requirementsrequired to matchthe output of the preceding video amplifier. -W=ith# a resistor 8 of approximately ohms-inthe case of current of 0.5 ampere,-a biassupply-.01 approximately- 4l0 volts is-requirecl'forthis'tube 26. All stages of the circuit operate from-acorn mon anode supply which was operated-at from- 800 to 1,000 volts. Each ol -the resistors 28-, 3land 32" has a value of-approximatelyww --ohrn's,"": the'condensers 9, 9' and 29 beingmica condensers 1 each of several microiarads capacitance.

From all of the above, it'will be-seen that the objects of the invention have been accomplished? With the circuit of-this invention; a maximuni depth of amplitude modulation of an R=F carrier has been achieved "bylca'thode modulation of an- R-F "power amplifier, this being "accomplished through switching of the direct cathode current from-the modulatortube to -the'amplifier-tuba and vice versa} at modulating frequencies and in response to modulating signals applied -to the input of'the modulating system;

What I claim as my invention is:

1. In a transmitting system": a high frequency stage to be modulated; said stage includin'g' an electron discharge device having atleast'a cathode and an anode; variation of theanode -cathode current'flo'w causing modulation of the highfrequencycarrier applied to such stage; a modulator stage; said last name'd stage including an 'elec-" tron discharge device having a cathodaan anode," and a control electrode;'means'for'applying modulating signals to said control elec'trode-to va'r'y the anode-cathode current flow in said "lastnamed device; means connecting the cathodes of said 'devicestog'etherand the anodes of said devices together in order to' cathode-modulate said high frequency stage; means connectingthe anodes of said "devicesto a source of positive" potential; a common circuit'for connectingboth of the cathodes to a single source of current" which supplies current to both cathodes; and means in said common circuitfor maintaining substantially constant-the total current"flow-in-' the two'devices.

2. In a transmitting systemra high frequency stage-to be modulated, said stage including-an electron discharge device having atleast a cathode and "an anode, variation of' the anode high frequency carrier applied to such stage; a modulator stage, said last-named stage including an electron discharge device having a cathode, an anode, and a control electrode; means for applying modulating signals to said control electrode to vary the anode-cathode current fiow in said last-named device; means connecting the cathodes of said devices together and the anodes of said devices together in order to cathodemodulate said high frequency stage; means connecting the anodes of said devices to a source of positive potential; a common circuit for connecting both of the cathodes to a single source of current which supplies current to both cathodes; and a constant-current network connected in series in said common circuit to maintain substantially constant the total current flow in the two devices.

3. In a transmitting system: a high frequency stage to be modulated, said stage including an electron discharge device having at least a cathode and an anode, variation of the anode-cathode current flow causing modulation of the high frequency carrier applied to such stage; a modulator stage, said last-named stage including an electron discharge device having a cathode, an anode, and a control electrode; means for applying modulating signals to said control electrode to vary the anode-cathode current flow in said lastnamed device; means connecting the cathodes of said devices together and the anodes of said devices together in order to cathode-modulate said high frequency stage; means connecting the anodes of said devices to a source of positive potential; a common circuit for connecting both of the cathodes to a single biasing source of current which supplies current and biasing voltage to both cathodes; and means in said commoi circuit for maintaining substantially constant the total current flow in the two devices.

4. In a transmitting system: a high frequency stage to be modulated, said stage including an electron discharge device the flow of current through which may be varied to modulate the high frequency carrier applied to such stage; a modulator stage, said last-named stage including an electron discharge device the flow of current through which may be varied by modulating signals; means for applying modulating signals, the frequencies of which may vary over a band, to said last-named device to vary the flow of current therethrough; a common circuit for connecting both of said devices to a single source of current which supplies current to both devices; and an inductance connected in series in said common circuit to maintain substantially constant the total current flow in the two devices, said inductance being split into a plurality of sections of different inductance values to provide a high impedance throughout the band of frequencies of said signals.

5. A modulating system, comprising a first electron discharge device having input electrodes and output electrodes, means for applying modulating signals to said input electrodes, a second electron discharge device having an anode, a cathode, and a control electrode, means for applying high frequency carrier energy to said second device and for abstracting modulated high frequency energy therefrom, means connecting the output electrodes of said first device to the oathode and anode of said second device in order to effect cathode-modulation of said carrier energy by means of said second device, means connecting said control electrode to a point of reference potential, means connecting the anodes of said first and second devices to a source of positive potential, a common circuit for connecting the cathodes of said first and second devices to a single source of current and bias voltag which biases said cathodes positively with respect to said reference potential, and means in said common circuit for maintaining substantially constant the total current flow to the cathodes of said two devices.

6. A modulating system, comprising a first electron discharge device having input electrodes and output electrodes, means for applying modulating signals to said input electrodes. a second electron discharge device having an anode, a cathode, and a control electrode, means for applying high frequency carrier energy to said second device and for abstracting modulated high frequency energy therefrom, means connecting the output electrodes of said first device to the oathode and anode of said second device in order to effect cathode-modulation of said carrier energy by means of said second device, means connecting said control electrode to a point of reference potential, means connecting the anodes of said first and second devices to a source of positive potential, a common circuit for connecting the cathodes of said first and second devices to a single source of current and bias voltage which biases said cathodes positively with respect to said reference potential, and a constant-current network connected in series in said common circuit to maintain substantially constant the total current flow to the cathodes of said two devices.

7. A modulating system, comprising a first electron discharge device having input electrodes and output electrodes, means for applying modulating signals, the frequencies of which may vary over a band, to said input electrodes, a second electron discharge device having an anode, a cathode, and a control electrode, means for applying high frequency carrier energy to said second device and for abstracting modulated high frequency energy therefrom, means connecting the output electrodes of said first device to the cathode and anode of said second device in order to effect cathode-modulation of said carrier energy by means of said second device, means connecting said control electrode to a point of reference potential, means connecting the anodes of said first and second devices to a source of positive potential, a common circuit for connecting the cathodes of said first and second devices to a single source of current and bias voltage which biases said cathodes positively with respect to said reference potential, and an inductance connected in series in said common circuit to maintain substantially constant the total current flow to the cathodes of said two devices, said inductance having a high impedance throughout the band of frequencies of said signals and having a low D.-C. resistance.

8. A modulating system, comprising a first electron discharge device having input electrodes and output electrodes, means for applying modulating signals, the frequencies of which may vary over a band, to said input electrodes, a second electron discharge device having an anode, a cathode, and a control electrode, means for applying high frequency carrier energy to said second device and for abstracting modulated high frequency energy therefrom, means connecting the output electrodes of said first device to the cathode and anode of said second device in order to effect cathode-modulation of said carrier en- 75 ergy by means of said second device, means connecting said control electrode to a point of reference potential, means connecting the anodes of said first and second devices to a source of positive potential, a common circuit for connecting the cathodes of said first and second devices to a single source of current and bias voltage which biases said cathodes positively with respect to said reference potential, and an inductance connected in series in said common circuit to maintain substantially constant the total current flow to the cathodes of said two devices, said inductance being split into a plurality of sections of different inductance values to provide a high impedance throughout the band of frequencies of said signals.

9. A modulating system, comprising a first electron discharge device having an anode, a cathode, and a control electrode, means for applying modulating signals to said control electrode, a second electron discharge device having an anode, a cathode, and a control element, means coupling said control element to the cathode of said first device in order to control the flow of current in said second device by said signals, a third electron discharge device having an anode, a cathode, and a control grid, means for applying high frequency carrier energy to said third device and for abstracting modulated high frequency energy therefrom, means connecting the cathodes of said second and third devices together, means connecting the anodes of each device to a source of positive potential in order to effect cathode-modulation of said carrier energy b means of said third device, means connecting said control grid to a point of reference potential, a common circuit for connecting the cathodes of said second and third devices to a single source of current and bias voltage which biases said two last-named cathodes positively with respect to said reference potential, and means in said common circuit for maintaining substantially constant the total current flow to said two last-named cathodes.

10. A modulating system, comprising a first electron discharge device having an anode, a cathode, and a control electrode, means for applying modulating signals to said control electrode, a second electron discharge device having an anode, a cathode, and a control element, means coupling said control element to the cathode of said first device in order to control the flow of current in said second device by said signals, a third electron discharge device having an anode, a cathode, and a control grid, means for applying high frequency carrier energy to said third device and for abstracting modulated high frequency energy therefrom, means connecting the cathodes of said second and third devices together, means connecting the anodes of each device to a source of positive potential in order to effect cathode-modulation of said carrier energy by means of said third device, means connecting said control grid to a point of reference potential, a common circuit for connecting the cathodes of said second and third devices to a single source of current and bias voltage which biases said two last-named cathodes positively with respect to said reference potential, and a constant-current network connected in series in said common circuit to maintain substantially constant the total current flow to said two last-named cathodes.

11. A modulating system, comprising a first electron discharge device having an anode, a cathode, and a control electrode, means for applying modulating signals, the frequencies of which may vary over a band, to said control electrode, a second electron discharge device having an anode, a cathode, and a control element, means coupling said control element to the oathode of said first device in order to control the flow of current in said second device by said signals, a third electron discharge device having an anode, a cathode, and a control grid, means for applying high frequency carrier energy to said third device and for abstracting modulated high frequency energy therefrom, means connecting the cathodes of said second and third devices together,means connecting the anodes of each device to a source of positive potential in order to effect cathode-modulation of said carrier energy by means of said third device, means connecting said control grid to a point of reference potential, a common circuit for connecting the cathodes of said second and third devices to a single source of current and bias voltage which biases said two last-named cathodes positively with respect to said reference potential, and an inductance connected in series in said common circuit to maintain substantially constant the total current flow to said two lastnamed cathodes, said inductance having a high impedance throughout the band of frequencies of said signals and having a low D.-C. resistance.

12. A modulating system, comprising a first electron discharge device having an anode, a cathode, and a control electrode, means for applying modulating signals, the frequencies of which may vary over a band, to said control electrode, a second electron discharge device having an anode, a cathode, and a control element, means coupling said control element to the cathode of said first device in order to control the how of current in said second device by said signals, a third electron discharge device having an anode, a cathode, and a control grid, means for applying high frequency carrier energy to said device and for abstracting modulated high frequency energy therefrom, means connecting the cathodes of said second and third devices together, means connecting the anodes of each device to a source of positive potential in order to efiect cathode-modulation of said carrier energy by means of said third device, means connecting each of said anodes to ground for frequencies on the order of the frequency of said carrier energy, means connecting said control grid to a point of reference potential, a common circuit for connecting the cathodes of said second and third devices to a single source of current and bias voltage which biases said two last-named cathodes positively with respect to said reference potential, and an inductance connected in series in said common circuit to maintain substantially constant the total current flow to said two lastnamed cathodes, said inductance being split into a plurality of sections of different inductance values to provide a high impedance throughout the band of frequencies of said signals.

RUSSELL R. LAW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,152,753 Terman Apr. 4, 1939 2,432,720 Brown Dec. 16, 1947 FOREIGN PATENTS Number Country Date 424,470 Great Britain Feb. 21, 1935

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