US2190513A - High frequency circuits - Google Patents

Description

Feb. 13, 1940. G. w. FYLER 2,190,513

HIGH FREQUENCY CIRCUITS Filed March 24, 1936 CARRIER WAVE GENERATER AT MAXIMUM FREQUENCY, REACTANCE VALUE OF I9= APPROX-Y2, RESISTANCE 0F 30, AND RESISTANCE 0F Z0=|NPUT REACT/MICE 0F TUBE [2.-

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. w |o0poo I gaaqooo' Geor ge F ler, FREQUENCYCYCLESPER.SECOND J W aw H i s Attorney.

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Patented Feb. 13, 194O Y I U ITED STATES PATENT OFFICE 2,190,513 "HIGH FREQUENCY omo'urrs George W. Fyler, Strati'ford,v Conn., assignor to General Electric, ,Company, a corporation of New York I I a Application March. 24, 1936, Serial No. 70,567 teams. (01. 179-171 My invention relates to high'frequency cin-v cuits and more particularly to high frequency apparatus of .the short wave high powentype; Specifically my invention pertains to untuned amplifier systems adaptablejior use in amplifying with a minimum of distortion signal currents having component frequencies extending over an exceedingly wide range,'such for example assignal currents produced by a television...signal J" source wherein the signal current frequency rangeextends from a low audio frequency to-a high radio frequency.

It is an object of myinvention to provide an improved amplifier systemcapable of amplifying signal currents having frequencies extending over an exceedingly Wide range with substanftially the same transmission efficiency for all fre quencies within the range. 1

It is a further object of my invention to pro.- vide an amplifier capable of operating in the above manner in which uniform time delay for voltages of all frequencies within the operating range is produced in each amplifier stage.

above objects by providing an improved coupling impedance network between successive stages. of a cascadeconnectedjamplifier which gives a flat amplifier characteristic and a uniform time delay for all frequencies Withinthe desired operat 35 amplification of frequencies in the low portion of the frequency range. This value has been de-' y termined by calculation and experiments to be sufiicient to produce at the highest frequency which it is desired to amplify an inductiveyre- 40 value of the coupling resistance,=where"the coupling resistance has a yalue equal to the input reactance into which *the coupling impedance network operates.

Thenovel'features'which I believe to be characteristic ofny invention are set forth with particularity in the appended claims. Myinvention itself, however, both astoits organization andmethod of operation together withffurther objects and advantages thereof will best be under'stood by reference 'to .the following specification takenin connection with-the accompany ing drawing in which Fig. 1 illustrates a transmitting system having'my inventionembodied therein; Fig. 2 illustrates the. equivalent circuit.

In accordance with my invention I attain the for a portion of the circuit shown in Fig. 1; Fig. 3 illustrates a graphical method of determining "the correct impedance values for certain of the elements illustrated in Fig. 1; Figs. 4 and 5 111118? trate certain characteristics of the operation of I my invention. I

Referring to Fig. 1 of the drawing, I have illustrated my inventionas applied to a television transmitter of conventional design. As shown, the transmitter comprises a radio frequency carrier Wave generator I connected to supply its output through a driver stage '2, a push pull class C connected power amplifier system 3 and the wind-v ings l and ii of a coupling transformer t to the conductors l and 8 of an antenna transmission line system.

The carrier wave output from the powerampli-fler is modulated in accordance with the signal oscillations produced by a signal current source 9. These signal oscillations are amplifiedfby the cascade'class A connected amplifiers to, H and I2 and the constant current modulator amplifierstage I3. and are impressed on the output circuit of the power amplifier 3' through a: modulator coupling impedance network Hi. This network comprises an air core inductor l5 'havijng a very low distributed capacity shunted by aresistance l6 and connected in series with an lIOIl. core reactor ll having an inherently high distributed capacity. "One terminal of-the net- 9 i work 14' is :jtapped to the midpoint l8 offthe winding ,4 and the other terminal is connected to the positive side of a high voltage source (not ,shown) which is provided to supply energy to the zespective anode circuits of the transmitterSysem.

vIn order to neutralize the efiect of stray capacity in the carrier power amplifier on'lthe I ,-modulator stage an inductance coil i8 is proactance approximately equal to one-half the.

vided which'is connected in the modulator stage 40 plate circuit inthe manner illustrated. It will .be 1 I as compared to the distributed capacity of the;

reactor I'l and accordingly the value of inductancecoil 18" may be small. The source of signal oscillations 9 may comprise a photo-electric cellsystem, a source'of light, and a scanning disk employed in atelevisionitransmitter, all suitably arranged to scan I the object to be televised with a spot of light; "The: signalcurrent frequencies which are pro- 1 sued February 14, 1939.

of modulation. If certain of the frequencies arediscriminated against, asfor'example, the frequencies in the high portion of therange, frequency distortion results which produces a blurred image or lack of definition when the signals are reproduced in a receiving system. One problem involved in the construction of a transmitter suitable for the transmission of a carrier wave modulated in accordance with television signal currents is that of providing a coupling impedance network I4 between the constant current modulator stage I3 and the output circuit of the amplifier 3 which operates to impress the signal voltages upon the output circuit without attenuation of the high frequency components of the impressed voltages. This problem, together with the solution thereof, is completely described in my Patent No. 2,147,486, is-

Briefly, equalized amplification by'the modulationstage I3 of all of the components in the side-band frequency range is obtained in accordance with the invention described in the aforementioned patent by providing the parallel connected inductance I5 and resistance I6 connected in series with the iron core reactor II across the signal channeland in the anode circuit of the modulator stage I3. The constants of this coupling network are so selected that the overall impedance between the terminals thereof is maintained above a predetermined value for currents of'all frequencies within the wide frequency range.

:Another problem involved in the construction "of an amplifying system capable of amplifying without substantial distortion of frequency components of the signal voltages is that of obtaining a coupling impedance network between the successive intermediate amplifier stages which is capable of impressing the output voltage of one amplifier stage on the input circuit of the succeeding amplifier stage with equalized transmission efliciency forall frequency components con tained in the signal voltages.

In accordance with my invention such equalized transmission efiiciency is obtained by providing in series with .each'of the coupling resistances 20, 20', etc., inductances I9, I9, .etc.,

and so proportioning the impedanceconstants of these elements with respect to the input impedance of the amplifier into which the coupling network operates that attenuation offrequencies in the upper portion of the frequency range is substantially eliminated. The effect of the inductance I9 in the coupling circuit will more readily be understood by reference to Fig. 2 wherein the equivalent cirin the anode circuit of the amplifier II.

cuit included between the amplifiers II and I2 is illustrated. This equivalent circuit. includes a source of electromotive force E1 which of course corresponds to the electromotive force generated The source of electromotive forces E1 may be considered as connected in a circuit including a recircuitconnections to these electrodes.

stantaneous currentflowing in the branch circuit.

inductance L and resistance R0 connected inseries across the terminals of the condenser'C. The

resistance R0 includes the coupling resistance between the grid and cathode of the discharge device I2 and comprises the resistance 20 and the non-inductive resistance: of the inductance coil 10 I 9. The inductance L of course comprises the in ductance of the coil I9. Thecapacitance C is the capacitance between the grid and cathode of the discharge device I2 and that included in the including the resistance R0 may be represented The inby the reference character I1 and the instantaneous current flowing in the .branch circuit inthereference character I2. Obviouslythe including the capacitance C may be indicated by I stantaneous current traversing the resistance R is equal tothe vector sum of I1 and I2.

' The value of Ru and of the inductance L necessary to provide a constant ratio of E1/E2 for equalized amplification at all frequencies within the desired operating frequency, range, maybe determined by the graphical method illustrated inFig. 3 from rived.

In Fig. 3, E2 is drawn to scale along a hori zontal axis in the manner illustrated. This volt;-v

which the curves of Fig. 4 are deage is assumed to be constant in determining the relation between E1 and E2 at any desired frequency. With voltage E2 fixed in magnitude and direction, it is necessary to obtain thevector sumof IlRp and I2Rp at any frequency in order'to determine the value of E1. 'Since E2 is constant and is the voltage across the series circuit L and R0 at all frequencies, the locus of the vector I1R as the frequency isvaried from zero to infinity is'the semi-circle 2|. Further, the tangent of the angle 6 between the vector IiR andthehorizontal reference axis is proportional to fre- I quency. By a trigonometric calculatlonit may .be determined that the distance from 0to the intersection of the projection of the vector I1R with the vertical reference axis is directly pro the linear scale marked along the vertical axis from 0 upward may directly be calibratedto read v:

fportio'nalto tangent 6. Hence it will beseen that directly proportional to frequency. Hence the linear scale may extenddownwardly from 0 and being proportional to frequency will also be proportional to the vector I2Rp, it being understood:-

that this vector is in. leading quadrature has relation :to the voltage vector-E2.j L v Withthe. three vectors E2, IlRp and I2R thus determined in phase and. magnitudefor any given frequency'the voltage that that frequency is of course the vector sum ofthethree vectmrs'. The angle ebetween the vectors E1 and E2 corre spondsto the phase shift between the voltages E2 and E1 for thefrequency, being considered. For

any given'combination of circuit constants the locus of the vector E1 may be determined by drawing downward from the semi-circle 2I voltage vectors of proper magnitude to be proportional to theselected frequencies .corresponding to the points selected on thesemi-circle 2|. Such a locus may be as shown bythe curve 22. The locus of, E vectors corresponding to a constant 'va1ue-0fE1/E2 is of course an a'rcas indicated at 23 having a center at the right endof the vector E2. and a. radius equal tothe sum ofthe vector Ez-.

.andthe diameter'ofthesemi circle 2|.

' With-theabove pictorial representation of the voltage relation between the sourcevoltage E1 and the input voltage'Ez fixed-for a givenset of f are shown in Fig. Hey the curves'M to 20 inclusive. Phase shift curves corresponding to the circuit constants which give the transmission cfficiencycurves M- to 29 inclusiva'are illustrated at 24' to 29' inclusive. In this figure the curves M to inclusive show the transmission emciencies for the circuit constants given in the following table and the curves 24 to 29* inclusive I show the corresponding phase shift betweenthese voltage vectors as a function of frequency:

' Ro L Micro Curve ohms hemies L 24 andll 2000 640 2010 at 500,000 cycles. 25am! 2 2000 240 I 750st 500,000 cycles. 26 and 26. 1000 160 1.000 at 1,000,000 cycles. 1 27 and 27 1000 67 420 at 1,000,000 cycles; 28 and 28 500 500 3,112,000,000 cycles. 29 and 29 i 500 16. 7 210 at 2,000,000 cycles.

1 C Micro- I i 9"? microfarads w,

24 and 24' 1.60 .2000 at 500,000 cycles. 25 and 25 160 2000 at 500;000 cycles. 26 and 26 160 1000 at 1,000,000 cycles. 27 and 27 160 1000 at 1,000,000 cycles, 28 and 23 1.60 500 at 2,000,000 cycles. 29 and 29. 160 500 at 2,000,000 cycles.

Each of thecur'ves shown in Fig.4 was deter-- mined by the graphical method illustrated in Fig. 3 and these curves were found to check approximately with corroborating tests conducted on circuits containing/these constants. Unpredictable stray capacities of the circuits, and the change of impedance of resistances with frequency, tend to prevent accurate checking of calculations and test results.

By reference to the curves of Fig. 4 it willbe observed that to obtain flat equalization over the desired frequency range it is. necessary that the resistance Rd should be approximately equal to the capacitive reactance between the input electrodes of the discharge deviceiinto which the coupling impedancenetwork operates at the high- .est. frequency to bev amplified, if equalized amplification over the entire frequency range is to be obtained. It will further '.be seen that the inductance L must be of such value that the re-" actance thereof at the highest frequency to be amplified is equal approximately to one-half the 'value of the resistance R0 whichis connected in ,series therewith.

Thus, consider the curves 24 and 25 by way of example,where the desired frequency range ex-- tends to a value of approximately.500,000 cycles per second. At this frequency the capacitive reactance of the condenser C is equal approximately to 2000 ohms. With 640 microhenries an inductance value for the inductance coil E9 the reactance thereof isequal approximately to 2010 ohms. It will be observed that with this value of inductive treactance a decided hump is produced in the voltage ratio curve inthe upper portion of frequencies within this operating range.

uniform time 'delay','the phase shift should'be.

the frequency range asshown in curve 24. However, if the inductance value of the element 19 be selected to be 240 microhenries corresponding to an inductive reactance of 750 ohms at afrequency of. 500,000 cycles per second equalized amplification is obtained, as shown in curve 25,

over the entire frequency range extending to the desired value of 500,000 cycles per second. Contrasting curve 25 with curve 21 it will be seen that the latter curve shows that equalized amplification is obtained over a frequencyrange extending to. 1,000,000 cycles per second. At this frequency the capacitive reactance of C is'a'pproximately 1000 ohms. From the above table of circuit constants the values of resistance R0 and L necessary to produce this curve are given I as'l000 ohms and 67 microhenries respectively.

vAs is noted above the curves 24 to 29' illustrate the phase shift between voltages E1 and E2 as a function of frequency. It will of course b'e understood that phase shift angles may be'converted into time delay intervals representing the time lag between the voltage E1' and E2 at selected For proportional to frequency.

i 'It' will further be understood that if phase dis- Uniform time delay over a wide.

tortion is to be prevented this time delay should plifier of a television transmitter does not cause distortion of thetransmitted picture signals for purposes of reproduction, butsimply delaysthe entire picture. I have found experimentally and mathematically calculated that flat equalization of the response characteristic, illustrated for.

example in curves 25, 21 and 29 of Fig. 4, utilizing the circuits described herein and the relationship given for the circuit elements and the input capacity of the succeeding amplifier, is accompanied by uniform time delay, illustrated by the phase shift curves25, 21 and 29' of Fig. fl. In a six-stage amplifier having a flat response over a range extending from 20 to 1,000,000'cycles, the phase shift has been found to correspond to a time delay of .53 microsecond.

To further emphasize the importanoef'of selecting the circuit constants of the coupling net- I work to conformto the principles described above,

plification characteristics of the signal amplifier system included in the transmitter shown inFig. l'is illustrated. It will be seenthat substantially uniform amplification is obtained for all frequencies extending from 20 cycles tov 1,000,000 cycles. I

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications in the circuit may bemade, and I contemplate by the appended claims to reference may be had to Fig. .5 wherein theamcover all such modifications as fall within the 1 true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patentof the United States, is:

1. In combination; an electron discharge amplifier having input electrodes comprising a cathode and control grid, a source of oscillations having component frequencies extending over a wide range, and an impedance network for impressing said oscillations on said input electrodes including a series connected resistance and inductance connected across said source and in shunt to the input capacity of said amplifier, said resistance having a value substantially equal to the input reactance of said amplifier at the highest frequency in said range and said inductance having ;a reactance value at the highest frequency in said range equal approximately to one-half the value of said resistance, whereby substantially equal transmission efficiency is obtained for all of said component frequencies.

2. In combination, an electron discharge amplifier having input electrodes comprising a cathode and a control grid, a source of oscillations having component frequencies extending over a Wide range, and a coupling impedance network for impressing said oscillations on said input electrodes, said network including a resistance having a value equalto the reactance between said input electrodes at the highest frequency in said range and an inductance connected in series with said resistance, said inductance and resistance being in shunt to the input capacity of said amplifier, said inductance having a reactance value at the highest frequency in said range equal approximately to one-half the value of said resistance. i H 3. In combination, an electron discharge am plifier having input electrodes comprising a cathode and a control grid, a source of oscillations having component frequencies extending over a range from a low audio frequency to a high radio frequency, and an impedance network for impressing said oscillations on said input electrodes including a series connected resistance and inductance connected across said source and in shunt to the input capacity of said amplifier, said resistance having a value'equal to the reactance between saidinput electrodes at the highest frequency/in, said 'range-;'and said inductance,

having a reactance value at said highest fre-,-, quency equal approximately toone-half the value of said resistance.

4. In a systemfor amplifying currents having frequencies extending over a Wide range and comprising a plurality of cascade-connected am-' y 1 plifiers, a coupling impedance network for im pressing said currents from one of said amplifiers on the next succeeding amplifier including a resistance having a value approximately equal to the input reactance of the succeeding stageat the highest frequency to be amplified, and means for equalizing the, amplification of currents of I all frequencies within said range said last-named means including an inductance connected in series with said resistance, said inductance'and;

resistance being connected in shunt to the input capacity of said next succeeding amplifier, said inductance having a reactance value at the highest frequency to be amplified equal approximately to one-half the resistance value of said resist-f ance.

plifier having input electrodes comprising a oath wide range, and a coupling impedance network '25 5. In combination, an electron discharge am;

for impressing said oscillations on said input electrodes comprising inductance and resistance connected in series with each other across said source and in shunt to the input capacity of said amplifier, said resistance having a value substantially equal to the input reactance of said am;- plifier at the highest frequency in said range'and said inductance having a reactance value at the highest frequency in said range equal approximately to one-half the value of said resistance, whereby uniform time delay for transmission through said network of currents having, any

frequency in said range is produced.

GEORGE W. FYLER;

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