US2151072A - Television transmitter - Google Patents

Description

-March 211939. A. v. BEDFORD w 2,151,072

' TELEVISION TRANSMITTER Filed April 25, 1935 Y 2 sheets-sheet 1 R u; N

mfd. 57

qld Vfedford W1' tness:

March 21, 1939. A v vmgm-'0R13 2,151,072

` TELEVISION TRANSMITTER IFiled April 25, 1,955 2 Shee'ts-Sheed- 2 FREQUENCY 10o fr. c.v h 1000 ma..

' y I/vvE/v'ron Alda l B e dford WLfness 'HTTQRNEY .My inventionrelates to television yPaiented Mar. `21, 1,939

2,151,072 TELEVISION TRANSMITTER Alda'V. Bedford, Collingswood, N. J.,rassignor to Radio Corporationv of America, a corporation of Delaware STATES PATENT OFFICE `-App1ication Apriizs, 1935, serial No. 18,136

transmitters and particularlyto transmittersof the type'employingcathode ray'transmitter tubes.

'f Prior to my invention it ,was standard trans-` mitter Ypractice to so design the output circuit of the4 cathode ray vtube that the frequencyv ren sponse curve of the tubewas at. 'I'he picture signal was rthen fed' into an am-plier designed to have 1a kflat frequency-response characteristic whereby` they signal appearing in the amplifier output Wasa true representation of the cndi tions of light and shade of fthepicture.

r`In order to make the frequencyresponse curve ofthe cathode ray tube at,it wasnecessary rto give the output resistor ,of the cathode ray tube anf'impedanc'e, value'approximately equal vto, Vorflessvthan the impedance of the capacity `inshunt to the cathode ray tube outputcircuit `at the highestfrequency to be amplied. Since theinternalgimpedance ofr the preferred' type of cathode ray tube yis yvery high, usually at least ,1A `megohrn, such acircuitrdesign resultedv Y Vinjveryv ineicient operation of the cathode ray Vtube ywhereby a large amount of picturev signal amplification was required. Consequently, the

resulting picturefsignal at the receiverrwas un` satisfactory because of kthe `undue amplification *of stray fields, tube hiss, tube microphonics, rthermal agitation, etc.

Ari object of my invention, theref'ore, is ,to provide an improved ypicture transmittingfcirf r`c uit for supplying substantially undistortedV picture signalsffrorn4 a high impedanceI light-sensiftivedevice. 'f f f provide Va method of and means A further object of my inventiony is'to provide an vimproved. cathode ray tube circuitr for transmittingpictures." y

' Aj further objectA of my `invention is to provide amethod'of. zand vmeansfor reducing noise in signals suppliedfrom a high impedance light-v sensitive device;A .l

f A still further object ofmyy invention is to noisefin picturesigrials obtained rfrom a cathode ray tube of the type in which a light-sensie tive mosaic is scanned by the cathode ray. In' a preferred embodiment of my invention IA employ'a cathode ray transmitter tube of the type which' comprises a light-sensitive mosaic` uponfwhich is' formedvan -optical imager of the picture to be` transmitted.y `Picture signals are produced by scanning the. rmosaicby an electron beam.'` The outputvcircuit ofr the cathode rayl tuber-is coupled to the 'input circuitvof an amplier through an output resistor which is given va resistance value, considerably higherthan the impedance of the capacityin shunt to the outputcircuit at thehighest frequency to be transmitted. f

j Asa result of using this comparatively high response curve is made substantially flat.

for reducing y s claims. (ci. 179-171) output resistance, the lower frequency components of the picture signals are Vamplified much more than they would be by the circuit formerly employed which was designed to have a substantially flat frequency response characteristic. The highest frequency components of the picture signals are amplied approximately the same amount as when employing the former circuit design. y

It will be apparent that the lower frequency components of the picture signal will require a comparatively small amount of amplification and, since' it is in this frequency range that most of the noise appears, the result is that the noises are not amplified to an objectionable deglee.

The building up of the low frequency response in the cathode ray tube output circuit follows a denite law and,`as a result, it is possible to design an amplifier having a complementary' characteristic whereby the over-all frequency- In the preferred embodiment of my invention this complementary characteristic is obtained in one amplifier stage While the other amplifier stages are. adjusted individually to have a at frequency response characteristic.

Other objects, features and advantages of my invention will appear from the following description when taken in connection with the accompanying drawings, in which Figure 1 is a circuit diagram of a preferred embodiment of my invention, Fig`2 is a schematic diagram which is referred to in explaining the theory of operation of the cathode ray tube shown in Fig. 1,

Fig. 3 is a group'of curves which are referred to in explaining the invention, and

Fig. 4 is a circuit diagram of a frequency compensating circuit which may be employed in f place of the compensating circuit shown in Fig. 1.

Referring to Fig. 1, the transmitter circuit comprises a cathodek ray tube l consisting of ran evacuated envelope having an electron gun 2 therein for producing a beamr of electrons, a

second anode 3 and a mosaic 4 of light sensitive elements. An optical image fof the picture to be transmitted, indicated at 6, is projected upon the mosaic 4 by means of a suitable lens system indicated at 1. This forms on the mosaic what may be referredr to as an electrical image of the picture, whereby picture signals representative of the conditions of light and shade of the f picture are produced as the mosaic is scanned by the electron beam. The beam is caused to scan the mosaic 4 by means of deflecting plates or coils (not shown).

The mosaic may be made in different ways as explained in the article by V. K. Zworykin appearing in the Proceedings Aof the Institute of Radio Engineers for January, 1934, pages 16 to 32,

`this article giving ,a more detailed description q of a cathode Vray tube of this type. The mosaic consists of a great number of light sensitive elements which are insulated from each other and which are also insulated from a metallic signal plate 3y upon vwhich they are mounted.

'The circuit forth@ photooieotrio oeil portion of the cathode ray tubetwill be more readily Y capacity of eachlightsensitive element 9 to the v an amplifier signal plate 8 is lrepresented by small condensers I'I.VV 1t willbe apparent that the light sensitive f elements 9 act as the cathode of lzrlvlotoelectrc cells and that the secondanode 3 acts as an anode common to the large number of cathodes.

Referring to both Fig. 1v and Fig. 2, the signal plate 8 is connected through a resistor VR1 and a biasing battery I2 .to theY second anode 3 and toground.V

The resistor R1 and the Ybiasing battery I2Vare connected across the'ca'thode I3 and Vgrid I4 of Y tube I6 which, in theparticular em. bodiment being described, is a screen grid tube of` the 57-type. In this particular'circuit the 'combined capacity'of the signal plate and leads to ground and of the control grid to ground, that is, the total capacity appearing across electrodes 8 and 3, is ZO'micrO-microfarads, this capacity be- 35V ing indicated at C1. 1 Y

If the circuitwere designed to give a fiat frequency-response curve at the rinput circuit ofthe amplifier tube I5, Vas was' the practice. prior to my invention, theresistor R1 would be-given aV valuerof about 8000 ohms. This Vwould be determined by making its value equal tothe capacitive reactance of the cathode ray tube output circuit at the highest frequency tobe amplified; this reactancebeingfequal tor r Y 21rfC1 where the highest frequency is 1000 k. c; K

ohms

seen that when the resistor R1 is given this value Y the frequency response characteristic of the circuit feeding into the rstamplifier tube I6 isv substantially fiat. In accordance with valuek of the resistor yR1 much'higher than is 'permissible ifa flatA frequency response char-f acteristic is to be obtained at the input of 'tube I6. In the specific-circuit beingdescribed, R1 has been given arvalueof 50,000 ohms. By referring to Fig. 3 it will be seen that when `R1 is vgiventhis value the amplification of the lower frequency components lof a picture signal israpproximately six times the amplification which would be obtained when employing the lower resistance value. -It will also be noted that at the higher 'frequencies the amplification f falls off rather rapidly.V If desired, still higher amplification may be obtainedA by giving the resisto-r R1 ay still higher resistance value as will be evident byreferringfto the upper curvel in Fig. 3 which shows the frequency-response `characteristic for the output circuit of the cathode ray tube when R1 has been given a value of 100,000 ohms,

' For the purpose of illustration, only three amplifierstages are shown in the picture amplifier.

The first stage and the third stage are so designed spense characteristic "a plate resistor e load impedance of the tube I1 .is madeveryrlow, Y the plate resistor Rz'having a value of only`200 ohms' whereby thershunting effect ofi-distributed u A capacity `in thel plate circuitis'negligibla The inductancey of the coilV L2 is determined by equa- Y kmyY inventionl make the f fromV the signal then. Y

that their individual frequency-response curves are flat. The second stagefis sodesigned in` accordance Withmy invention that it amplies the Y t high frequencies more thanv the low frequencies Vand compensates for the ,fallingY frequency-refr v Y n ofv the cathode ray tube output circuit caused bythe use of a resistor R1 havf inghig'h impedance. Aswill be 'understood from ardetailed explanation of the circuit given laterV in the specication,the output or `load impedance of the second'stage is, made so" low that this stage provides little if any amplification nof picture signals, the otherv amplifier stagesrbeingvdepended uponVV to give the necessary amplification. A

rThe output circuit of the first amplifier tube I6 includes a peaking coil Lo andV afplate resistorR.

The input circuit of the second amplifier tube I'Iv Y includes a biasing battery? I8"andagrid` resistor Y I9.y T'Ifhe'output circuit ofthe'firsttube is' cou pled to theY input circuit of the seconditube through a coupling condenser 2 I It Ywill'be noted* that the peaking'coil Lo has avalue of 5l0 micro: Vhenrys and that the plate resistor R has a value ofY 6360 ohms, these values'beingy `thefproper onesi response characteristic up to 1000 where the I6, indicated at Co, is 25 mmf.

The output circuit s'ating tube I1 includes lan inductance coil L2 and R2. `It willbe noted that the tions which jwill K be referred "to hereinafter. The third'amplifier stage comprises an amplifier of the secondforcompen-W .tube 12,2"having an input circuitrwhichincludesl I a biasing battery 23 and a" grid resistor-24.l The 4,0k

outputfcircuit of the second amplifier stage` is 1 coupled to the input circuit of the third amplifier stage through [a coupling condenser'Z.

.. The output circuit of the tube 22'1ncludesran inductancefcoil 271Y andYY a plate resistor'28, these,

impedance units having proper values to givethevv v thirdamplifier stagea flat frequency response i By referring tothe curves in Fig3, ite-will be Y,

characteristic j'ust as in the Vcase of the'first ani-fv j plifier stage.

The invention willv be' better understood tgfttfH `ferring to the following equations, in'which vece.l

torial values are used: Y

1 If e1'is Ythe alternating-current voltage' imi-iY pressed Vupon the input circuit oftherfirstLampli-A,

Vfier tube I6, i1 the alternatingcurrentv supplied` 1 Y K plate 8, e the Yimpedance-'Yof. the outputl circuit ofthe cathodefrayitube, Rtthe If e2 is the alternating current voltage appear-V ing acrossthe'output electrodes` ofthe secondy tube Il, i2 the alternating'current appearing inV plateA circuit of this tube, R2 theresistance .of

Vresistor R2, and the inductance vof the coilL2` 'I e2: inferi-.125VH12)A Y Y f i It wm be noted thaty in Equation 1111 the v capacity. across the output circuit of the tube IT Y is ignored but this is permissiblebe'cause the i1n` Y to give the `first amplifier stage .a yflat'frequency r4fromfEquation (I) we get.

f impedance lbeing low impedance. y

pedan'ce of thehload circuit!v isf madeV very low.`

and f=frequency in cycles per second.. v

If weassume that thev first amplifier 'stage has been: given. a. linear frequency-response' characteristic, then e f is=Ke1y k(III) Where K' is a constant, since vthe load impedance yof the compensating tube I1V is negligiblek corn` pared with its internal or plate impedance, and since the coupling circuitlbetween tube |'6 and tube l1 has a linear response.

` Substituting in Equation (III) the value of ei 2-K11/R1+f21ffci (IV) "Substituting the above value of i2 is Equation (II) we get K i then ez=Kiii where K1 is a constant. If e2=Kii1,

an undistorted picture signal isy impressed upon theinput circuit of the last amplifier 22. This follows sinceri is substantially independent of the load across thecathode ray tube, the load compared with the tube vThat e2=Kii1 under the'conditions assumed, may be proved as follows:r 1 n f By the above assumption substituting inA equation (vf.) n

Clearing .the equation of they` term,

1 RTI, 'l- (21T f CO2 Withihe values Lof Ri, ci, `ami R2 indicated in Fig. 1, we may calculate the required value of L2 as follows:

= 20X 10-2X 50,000X 200 =200 106 =200 microhenrys In determining the proper value of L2, the value of Ri is determined by considerations previously explained, the value of C1 is xed by the structure of the cathode ray tube and the first amplifier tube, and the value of R2 is determined by selecting a value that will make substantially negligible the effect of the capacity across the output electrodes of the amplifier tube Il at the highest frequency to be amplified.

As previously stated, the first and third amplier `stages should have a substantially flat frequency response characteristic. If peaking coils are used as illustrated, this characteristic may be obtained by making the plate resistor R, equal to the reactance of thev distributed capacity Co across the output circuit of the amplifier tube I6 at the highest frequency fo to be amplified, 1,

and bymaking the reactance of the peaking coil La equal to 1/2 the reactance of the distributed capacity at the said frequency. That is,

'I'he above method of calculating the plate resistor and peaking coil values is based upon the assumption that the plate impedance is very high llat frequency-response curves for best results,

reasonably good results in some cases may be obtained by. adjusting the amplifier stages as a group toy obtain an overall frequency-response curve which is approximately at.

. It will be apparent that in accordance with my invention the cathode ray tube works into a load circuit which is predominately capacitive over the greater part o-f the frequency range of the transmitter. This is shown by the falling characteristic of the upper curves in Fig. 3. The resistor Ri is required only as a grid leak.

Attention is directed to the fact that the picture signals are amplified sufciently before they are impressed upony the compensating stage to prevent the signal voltage from fall-ing to a value lower than its input value at any point in the Y circuit. In other words, theoverall gain at any point in the amplifier for any frequency in the amplifier frequency range should not be less than one. Otherwise, the ratio of signal to noise will drop to a value which is too low and the quality o-f the picture will be impaired.

In the circuit illustrated, which is designed to pass frequencies up to 1000 k. c., the gain in the first stage isI about 7.5 while` the gain in the second stage or compensating stage is about 0.24 at the lowest frequency. Therefore, at the lowest frequency, the overall gain of the two stages is 7.5 .24=1.8.

, In practice, .itzis generally desirableto employ two or more amplier stages preceding the compensating stage, each having a fiat frequencyresponse curve, in place of the single stage l5 shown in Fig. 1. If 57 type tubes areused and the' amplier is `topass frequencies kup to 1500 k. c., at least two amplifier stages should precede the compensating stage.

By designing a circuit in accordance with my invention, the signal to noise ratio is maintained at a high value asI compared with that obtainable in previous picture transmitters. As a result, I

can obtain a good picture instead of a very poor one.

A compensating network of the type described comprising an inductance coil and a resistor is the preferred one to use in practicing my invention since maximum gain is obtained in the compensating stage by its use. However, a capacityresistance network, such as shownrin Fig. 4, may

be substituted therefor and a marked reduction Yin noise obtained as compared with the noise in pe-dance resistor 34 is connected across the output electrodes of the tube 3| through a coupling condenser 36 and a conductor 3l. If circuit values of the order indicated on the drawings are employed, the compensating stage will have a frequency-response characteristic complementary to that of the cathode ray tube output circuit.

It can be shown that Vthe Ynetwork of Fig. 4,

gives proper compensation if there isY thelfollow- 'ing relation between circuit elements.

Then if RQ is given a value of 10,000 ohms C =R1C1 50,000X20 mmf.

From the foregoing description it will be apparent that various modifications may be made in my invention without departing from the spirit and scope thereof, and I desire, therefore, that mmf.

only such limitations shall beplaced thereon as` a photoelectric cell device having output electrodes and having high internal impedance, an amplifier comprising at least one amplifying stage having an output circuit and a compensating stage having an input circuit, said input circuit being coupled to said output circuit, and means for coupling said photoelectric cell Ydevice to said amplifier stage through a circuit which, because of distributed capacity, kis predominately capacitive over at least eighty percent of the range of frequencies to be amplified, the resistance of said circuit being effectively connected across said electrodesI and at the lowest frequency in said range being at least ten times as great as the capacity reastance of said circuit effectivelyin shunt to said electrodes at the highest frequency to be'amplied whereby the picture signal output from said device at'the low frequency end of said quency end produced in the portion of* said sysband is large compared with noise at said low frei tem immediately following said device thereby giving a good signal Vto noiseratio in the output of said system, said amplifying stage having a.

substantially flat frequency-response characteristic over said range, and saidcompensating stage having a frequency-response characteristic which is complementary to that of said photoelectric cell device. l

2. In combination, a light sensitive devicehaving high internal impedance, and having outputV electrodes, an electric discharge tube having an input circuit and an output circuit, a grid leaky resistorincluded in said input circuit, said output electrodes being connected to said input ,cir-

V,cuit whereby said resistor is in series with saidV internal impedance, a compensating electric discharge ktube of the screen grid type having a-con-V trol grid, Va cathode and a plate, means for `impressingsignals appearing in said output circuit upon said' control grid and said cathode, a plate resistor VYand an inductor in series, means for applying voltage to said plate through said. plate resistor and said inductor, and a utilization cir-` cuit connectedga'cross saidplate resistor and said.

inductor, said resistors. and Asaidinductor having.`l

wherevr e k fr n R1=the resistance of said4 grid leak resistor, Rz=the resistance of said plate resistor,

Lz=the inductance of rsaid inductor, and C1=the capacity effectively in shunt to said output electrodes. Y

3. The invention according toclaim 2 charac-Y terized in that the value R2 is small compared with the plate impedance of said'compensating tube. Y

ture signals are producedcovering a certain frequency range and wherein there is more Ynoise from certain sources at the low frequency end of 4. In a picture transmitter system wherein pic.- Y'

said range than at the high frequency end thereof, the combination of a photoelectric cell device of the high vacuum type having high internal impedance and having outputr electrodes, anampliiier tube havinginput electrodes, a loading network which couples said output electrodes Vto said.

input electrodes, said network having an effective resistance R1 and a shunt capacity YC1,R1 atV the lowest frequency to be'amplied being atleast ten times the capacity reactance of saidgnetwork at the highest freguencyto be amplified, ay compensating circuit having a` frequency response V,characteristic ywhich is complementary to that ofV said photo-electric cell device, and means for supplying the output `of said amplifier tube tof Ysaid compensating circuit, said certainsources 5. In combination, a light-sensitive devicehaving high'internal impedance and having output electrodes, an electric discharge tube havingV an input circuit and an output circuit, a loading network which couples said output electrodes to said input circuit, said network having an eiective resistancey R1 which' is in series with said internal impedance and havinga shunt capacity C1, a compensating electric discharge tube of the screen grid type having a control'grid, a cathode and plate, means for impressing signals appearing in vsaid output circuit upon said control grid and said cathode, a compensating circuit including a resistor and an, inductor in series connected to have *y signals impressed thereacross from said screen grid tube, and a utilization circuit connect-` ed Aacross said resistor and said inductor,'v said resistor and said inductor having the relation 6. ,In a system for amplifying picture signals covering a wide frequency band, in combination, a photoelectric cell device of a high vacuum type having high internal impedance and having output electrodes, an amplifier tube having input electrodes, a loading network which couples said output electrodes to said input electrodes, said network having an effective resistance and a shunt capacity, said effective resistance being so large as compared with the capacity reactance of said network at the highest frequency to be amplified that said network presents a capacitive reactance over the upper 80 percent. of the frequency band to be amplified whereby the picture signal output from said device at the 10W frequency end of said band is large compared with noise at said low frequency end produced in the portion of said system immediately following said device thereby giving a good signal to noise ratio in the output of said system, a compensating circuit having a frequency response characteristic which is complementary to that of said photoelectric cell device, and means for supplying the output of said amplier tube to said compensating circuit.

ALDA V. BEDFORD.

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