US2387685A - Voltage generator - Google Patents

Description

OC. 23, 1945. R. w` SANDERS 2,387,685'

VOLTAGE GENERATOR Filed Feb. 15,'1945 BSVJJOA HBHILOIH U mE-FI.

BSVIIOA 01331530 ESVDOA INV E N TO R ONBFIOBEH HOLVTIIOSO Patented Oct. 1945l VOLTAGE GENERATOR Robert W. Sanders, Fort Wayne, Ind., asslgnor to Farnsworth Television and Radio a corporation of Delaware Corporation,

Application February 15, 1943, Serial No. 475,861

(ill. Z50-36) 'l Claims.

This invention relates to the generation of periodic voltages and particularly to voltages of complex wave form such as saw-tooth and related forms.

According to conventional practice in the operation of cathode ray tubes, there is customarily provided a voltage of sawtooth wave form for deilection purposes. When a saw-tooth voltage is applied to one set of cathode ray tube deflection elements, an electron beam is deilected across the screen of the tube alternately in two opposite directions. Usually the electron beam is started at one side oi the uorescent screen of the tube and is moved in a linear manner at a uniform relatively lo'w velocity across the screen to the opposite side. The beam is then returned to its starting point at a uniform rel atively high velocity. These two deflections of the electron beam are effected under the control or" a voltage applied to one set ci ole-fleeting elements. In order to secure the two uniform deflection velocities, this voltage has a saw-tooth wave form. voltages applied to the other set of deflecting elements cause deilections oi the electron beam in a direction normal to that of the deflection effected by the rst set of deilecting elements. For many uses oi cathode ray tubes. it is necessary to secure a highly accurate and constant time relation between the two deflection voltages.

Certain arrangements heretofore have been devised to produce the desired time relation ha1 tween the deflection voltages. For example, one expedient for this purpose is the use of two self oscillating voltage generators which are periodically controlled by a means common to both. However, between the control periods, each gen crater operates independently or the other and, consequently, departures from the desired time relation are possible. For uses o cathode ray tubes, such as oscilloscopes, where an effect to be observed occurs repeatedly at the same point in a time cycle, it is necessary to secure a high degree oi precision in the time relation between the deflection voltages. Otherwise, the trace or other indication on the cathode ray screen will shi-ft in position on the screen.

The object oi the present invention, therefore, is to provide an improved deflection voltage generator for cathode ray tubes by means of which it is possible to secure a precise, positive synchronization between the horizontal and vertical deflection systems.

Cil

Another object oi' the invention is to provide an improved method of generating a saw-tooth wave which comprises the forming into unidirectional energy oi saw-tooth wave form of a series of en orgy pulses derived from a periodically modulated alternating current.

ln accordance with the present invention, there is provided a generator of periodic waves of a predetermined form comprising a means for periodically modulating an alternating current to produce a voltage of predetermined frequency and predetermined amplitude. The modulated alternating current Iis impressed upon an oscilu latory circuit which is designed to respond only to a voltage of the predetermined frequency and the predetermined amplitude. Thus, a series of sharp pulses of energy is developed by the oscillatory circuit, the pulses recurring at a periodicity equal to that of the modulating frequency. The energy pulses are applied to a wave-forming circuit to produce a wave oi? the predetermined iorm which has a periodicity equal to'that oi the modulating frequency.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

ln the accompanying drawing:

Fig. l is a partly schematic circuit diagram of a portion of a superheterodyne receiver embodying the invention;

Fig. 2 is a detail of a frequency-changing element:

Fig. 3 is a graph of the resonance characteristics of the oscillatory circuit;

Fig. fl illustrates graphically, the frequency time variation of the frequency-modulatedalternating current, the voltage time relation of the generated saw-tooth wave, and the impulse energy time relation of the oscillatory circuit; and

Fig. 5 is a view of the cathode ray tube screen.

Referring now more particularly to Fig. i of the drawing, there is shown schematically the ilrst and second intermediate frequency amplifier and the signal frequency amplifier portions of the superheterodyne receiver embodying the present invention in a preferred form. A receiver of this character is frequently used for the panoramic reception of all radio signals in a predetermined frequency band. After amplifying the received radio frequency signals and converting them to a rst intermediate frequency in the present receiver, the converted signals are impressed upon a ilrst intermediate frequency amlator il wherein' vthe intermediate frequency signais are mixed with a frequency generated by a local oscillator for the purposeof converting them to a second intermediate frequency. There is providedin the local oscillator a vacuum tube l2 which serves the dual vfunction of aiding in the generation of the local oscillations and of mixing these oscillations with the first intermediate frequencies. The frequency of the locally produced oscillations is governed by a frequency-determining circuit i3 which is coupled between the input and output circuits of the oscillator portion of the tube l2 by condensers it and l5 and which includesan inductor i6 and" a variable condenser lll. A leak from the oscillator grid is provided to. ground through a resistor is. The capacity of the condenser I is continuously varied by means to be described, whereby the frequency of the locally generated oscillations is varied between two limit frequencies as determined by the constantsof the frequency-determining circuit I3.

The intermediate frequency signals thus proate frequency amplifier i9. This amplifier is tuned to pass a narrow band of frequencies, prefa detector il and the resulting'signalsare subse quently amplified by a vertical deiiector ampli-V The output of the first intermediate frequency amplifier IG is connected to an oscillator-moduduced then are amplified by a second intermediuein",y sayv g/amgof 'a second. 5 This result-may lie'.v secured by properlyshaping the l condenser lplates in the manner described. v'Having' attained said otherextremeY .value of condenser capacity, re,-

suiting in the generation of one of the limit fre-v quencies, further rotation 4of the rotor plates changes the capacity to said rst extreme value, in, say 1/soo of a second. The cycle isl repeated Y. continuously, whereby the frequency of the local oscillator is varied in a substantially linear man'- ner with respect to time at the rate of 30 cycles l per second.

Thus, vit is seen that it is possible to achieve a linear change in frequency withrespect to time from one`limit frequencyto a second limit frequency at a relatively low rate of change. Also, by reason of the apparatus described, it is possible to obtain a linear change of frequencies with respect to time from the second limit frequency to the first at a relatively high-rate of change. The graphic plot of such a frequency change may be representedby the curve 38 of Fig. f4. Those portions of the curve having vthe lesser slopes rep#` resent the frequency change of the local oscillator atthe relatively low rate, while the portion of the curve having the greater slopes represent the frequency change ofthe oscillator at the relatively high rate. g

vThe parameters of the oscillatory circuit 23 are adiustedjto produce a sharp resonant response by this circuit to an impressed voltage ofa predetermined frequency and a predetermined .amplitude.` The resonance characteristic of this circuit is represented in Fig. 3 by a curve 39. The frequencygat which theresponse by the oscillatory /circuitoccurs may be any predetermined fre er 2l. This amplifier is designed to function at The varying frequency' potential generated by the local oscillator is couple'dinductively to'an os-l cillatory circuitl 23 comprisinganlyinductor'Zd and The amplified signals then l.

a condenser 25. The couplingis secured by propverly disposing the inductors Vid andfl with re spect to each other.. There is .connected tothe cscillatory circuit 23 a forming circuit 2B 'comprising a parallel arrangement of a .r esistance v2l and a -v condenser'28. f

The lenergy pulses developedby the oscillatory circuit 23 are impressed upona rectifier circuit including a diode rectifier tubeZSan'd .a'forming circuit 30 comprising a parallel arrangement of 1 a resistance 3l anda condenser`32. The'op'posit'e polarity varying magnitude unidirectional potentialswhich are derived 'respectively from vthe wave forming circuits 28'and 430, havesaw-loth fOrmS vand are amplified by a horizontal defiector amplifier 33. VThe amplifiedsaw-toothyoltage wave is connected to the horizontal deflection elements of the cathode` ray tube 22 to control the horerally tlfleshapesv illustrated .in Fig.v 2. A- statorplate 34 has a wedge-like shapegv A rotor platel 35 has'a periphery which is substantially spiralquency in the band of frequencies determined by the two limit frequencies.' AIt ispreferable, how

- ever, to produce the resonant response by the os- Vcillatorycircuit at one ofthe limit frequencies.v

:f For 'the purpose ofthis` description assume that the response is at thev higher o f :the two limit frequenCeS.

i When an energy pulse of the character repre-- sented by'thecurve 3 9 of Fig; 3 is applied tothe rectifier circuit of Fig. 1, theresult is therapid y charging ofthe condensers 28 and- 32 in there` spective'forming circuits 26 vand 30; The condensercharges then are dissipatedV at a lower-rate through the associated resistances 2l. a`n'd'3l,` Y Consequently, the voltages derived from theformf lng circuits change substantially linearly with-respect to time at a relatively high rate from one value to a second value byfreas'on vof the charging of the condensers. These voltages then change from the second value to theflrst value substa 1 1ti'H denser charges through: .the resls'tances.. These shaped with a radially extending portion joining the two ends of the spiral. The rotor plates are tor 31. With the motor V3 'l operating at a substantially vconstant rate of 1800 R. PgM., for example, the capacity of the condenser is varied from one extreme value to the other extreme valconnected to a shaft 3'6 which is rotated by a ino- 'voltages areofopposite polarity and, when simultaneously amplified by the amplifler 33, produce a saw-tooth form yoltagehavi'ng a value whichv is.

substantially double that of either.

The wave form of the ampliedvvoltageis relJe resented by the curve l0-.of Fig.: f1. Those portions of the curve having the .greiterslopes represent fthe charging of the'condensersj-'Zlifand .32 landthe portions ofthe curve having the-lesser'slopes represent the dissipation of the-condenser charges.v

through the resistances lan'drl. Also in'Fig. 4 is a graphic representation ofthe time-spaced energy'pulses developed by theoscillatory-circuit 23. These pulses Vare represented by the curves 4l and the time relationbetween successive energy lpulses and the peaks of the saw-tooth voltage derived from the Wave-forming circuits may be seen. The energy impulses represented by the curves 4| occur coincidentally with the peaks of the saw-tooth voltage wave 40. Thus, it is seen that the periodicity of the saw-tooth deflection voltage wave is equal to the periodicity of the energy pulses developed by the oscillatory circuit.

Referring now to the operation of the system, assume that it is desired to receive panoramically all radio signals in a band of frequencies between the limit frequencies of 3.25 megacycles and 3.5 megacycles. The radio frequency amplifier stage of the receiver is tuned lto pass all frequencies within this band. If it is also assumed that the first oscillator by means of which the first intermediate frequency signals are produced, generates a wave having a frequency of 5.42 megacycles, there Will be impressed upon the first intermediate frequency amplifier I0, intermediate frequencies between the limit frequencies of 1.92 megacycles and 2.17 megacycles. It is seen that the band width of the radio frequency signals and also of the rst intermediate frequency signals is 250 kilocycles. All signals within the frequency band 'between 1.92 megacycles and 2.17 megacycles are amplified by the ilrst intermediate frequency amplifier I0. They then are applied to the oscillator modulator il by which they are converted successively to the second intermediate frequency. If it is assumed that the second intermediate frequency is to be 295 kilocycles, the local oscillator including the tube i2 must vary between the limit frequencies of 1.625 megafcycles and 1.875 mega- 1 cycles. This variation is also seen to be within a frequency band of 250 kilocycles and is periodically effected at the assumed rate of 30 cycles per second.

In order to clearly understand the following portion of the description, reference will be made to Fig. 5. There is shown in this ligure of the drawing the fluorescent screen 42 of the cathode ray tube 22. The trace made by the electron beam upon this screen is represented by the line 43. For convenience in interpreting the trace made upon the screen there may be provided a scale 44 which is graduated in units representing the radio frequencies of the received signals. In the assumed case the limit frequencies are 3.25 megacycles and 3.5 megacycles which are indicated respectively on the screen by numerals corresponding to these frequencies.

As the variable condenser Il of Fig. 1 varies by reason of the linear manner with respect to time that the oscillator frequency change is effected, the spacing on the cathode ray tube screen 42 of any vertical trace from the starting point of the horizontal deflection of the electron beam is a measure of the frequency deviation of the signal producing the vertical trace from the frequency represented by the starting point of the horizontal deflection. Since such a point represents the lower limit frequency of the band of frequencies under observation, the horizontal position of the vertical trace is an indication of the radio frequency of the signal producing the trace. The wave form of the vertical trace corresponds to the tuning of the vertical deector amplifier 2i. Consequently, it is seen that, for every signal in the band of frequencies under observation, there is produced on the screen 42 of the tube 22 a vertical trace displaced horizontally from the side of the screen by an amount corresponding to the difference of the radio frequency upon which the signal is modulated from one of the limit radio frequencies. For example, in the case assumed, a signal having a radio frequency of 3.375 megacycles will appear as the tracev 45 at the center of lthe screen for the reason that such a frequency lies mid-way between the two assumed limit radio frequencies of 3.25 megacycles and 3.5 megacycles.

It has been found that, when working within the range of frequencies specified in the assumed example used for illustrative purposes, an oscillatory circuit which has a resonance characteristic capable of effecting the development of short energy impulses at a predetermined frequency is one in which the inductor 24 has a value of 287 microhenrys and the condenser 25 has a value of 25 micromicrofarads.

It will be obvious to those skilled in the art that it is within the scope of the instant invention to permit a substantial deviation from linearity in the variation of the oscillator frequency by means of variable condenser il. Insofar as the derivation of a saw-tooth voltage wave from a frequency-modulated alternating current in accordance with the instant invention is concerned, it is immaterial what the shape of the from the capacity producing an oscillator fre/ quency of 1.625 megacyeles to the capacity producing an oscillator frequency of 1.875 megacycles, all first intermediate frequency signals are converted successively in point of time to the assumed second intermediate frequency of 250 kilocycles. Each time that the second intermediate frequency is producedl there is a voltage resulting therefrom which is detected, amplified and then applied 'to the vertical deflector elements of the cathode ray tube 22. Each of these voltages effects a vertical deflection of the electron beam and thereby produces a vertical trace such as 45 upon the screen 42 of the tube. Since the oscillator frequency is varying continuously with respect to time through the assumed frequency spectrum and is effective, not only to produce the second curve representing the frequency time relation of the oscillator variation is, so long as there is generated periodically the predetermined frequency at which the oscillatory circuit 23 develops an energy pulse. Of course, in cases where the oscillator frequency is varied for a purpose such as that described, it is desirable that the variation be substantially linear with respect to time. However, since the apparatus utilizing the linear frequency variation for the production of the second intermediate frequency forms no part of the present invention, it is not contemplated that the saw-tooth wave generator be limited to the control by a linearly varying oscillator frequency. Also, it is obviously within the scope of the invention to develop the time-spaced energy pulses by the periodic amplitude-modulation of an alternating current wave of the predetermined frequency.

It is seen that, by reason of the arrangement provided bythe instant invention, it is possible to secure positive synchronization between the vertical and horizontal deflection voltages for a cathode ray tube which is of a high order of precision throughout the entire ranges of such voltages. Such synchronization is necessary when using a cathode ray tube for a purpose such as that described, namely, as the indicating element in a panoramic radio receiver. In order for the trace produced upon the screen of the tube by a signal of a given frequency to be repeated on successive horizontal deections of the electron beam, and upon each repetition to appear at the same point on the screen, positive synchronization between the two deflecting systems must be of a high order of precision. It will be obvious that these requirements are satisfactorily met by the instant invention.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a saw-tooth wave generator, an oscil lator for generating a wave having normal amplitude and frequency characteristics and having coupled thereto a resonant network to determine the free oscillation frequency, means coupled to said oscillator for repeatedly varying one of said characteristics of said wave through a given range of values, whereby once during each cycle of variation to produce a voltage of predetermined frequency and predetermined amplitude, an oscillatory circuit coupled to said resonant network and adjusted for response substantially only at said predetermined frequency and said predetermined amplitude, whereby to develop a series of energy pulses of a periodicity equal to the production periodicity of said voltage of predetermined frequency and amplitude, and means including a Wave-forming integrating circuit coupled to said oscillatory circuit for converting said energy pulses to a unidirectional wave of sawtooth form.

2. In a saw-tooth wave generator, an oscillator, means including said oscillator for periodically frequency-modulating an alternating current wave in a predetermined band of frequencies, an oscillatory circuit sharply tuned for resonance at one of said frequencies in said band, means for impressing said frequency-modulated wave upon said oscillatory circuit to develop a series of energy pulses vof a periodicity equal to the periodicity of said frequency modulation, and means including an integrating circuit for converting said energy pulses to a periodic unidirectional wave of saw-tooth form.

3. In a saw-tooth Wave generator, an oscillator, means for controlling said oscillator to generate periodically analternating current wave modulated in frequency in a band of frequencies determined by two limit frequencies, an oscillatory circuit sharply tuned for resonance at one of said limit frequencies, means including said oscillatory circuit for deriving from said frequency-modulated wave a series of energy pulses of a periodicity equal to the periodicity of said frequency modulation, and a rectifier including an integrating circuit for converting said energy pulses to a periodic unidirectional wave of saw-tooth form.

assaesa 4. In a saw-tooth wave generator, an electronic oscillator having a frequency-determining circuit, means including said oscillator and controlled by said frequency-determining circuit for generating periodically an alternating current wave modulated in frequency in a band of frequencies determined by two limit frequencies, an oscillatory circuit sharply tuned for resonance at one of said limit frequencies, means including said oscillatory circuit for deriving from said frequency-modulated wave a series of energy pulses of a periodicity equal to the periodicity of said frequency modulation, and a rectifier including a saw-tooth wave-forming circuit for converting said energy pulses to a unidirectional wave of saw-tooth form.

5. In a saw-tooth wave generator, a circuit sharply tuned for resonance at a predetermined frequency, an oscillation generator including a frequency determining circuit, motor driven means for continuously varying the constants of said frequency-determining circuit to effect the periodic generation by said oscillator of a predetermined band of frequencies including said predetermined frequency, means for coupling said generator to said tuned circuit to effect the development of a series of periodic energy pulses by said tuned circuit by the resonance thereof at said predetermined frequency, means for demodulating said energy pulses, and means including a saw-tooth wave-forming circuit connected to said demodulating means to effect the generation from said energy pulses of a wave having a saw-tooth form and a frequency equal to the frequency of said pulses.

6.111 a saw-tooth wave generator, a circuit sharply tuned for resonance at a predetermined frequency, an oscillator tube having input and output circuits, a frequency-determining circuit connected between said'input and output circuits and having a variable condenser, a driving motor for continuously varying -the capacity of said condenser to effect the periodic generation by said oscillator of a predetermined band of frequencies including said predetermined frequency, means for impressing said band of frequencies upon said tuned circuit to effect the development of a series ofperiodic energy pulses by said tuned circuit by the resonance thereof at said predetermined frequency, a rectifier connected to said tuned circuit, means including said rectifier for demodulating said energy pulses, and means including a parallel arrangement of resistance and capacitance connected to said rectifier to effect the generation from said demodulated pulses of a unidirectional wave having a sawtooth form and a frequency equal to the frequency of said pulses.

7. The method of generating a saw-tooth wave which includes the steps of, effecting a periodic frequency modulation of an alternating current wave, deriving periodically from said frequencymodulated alternating wave a series of energy pulses, and converting said energy pulses to unidirectional energy of a saw-tooth Wave form.

ROBERT W. SANDERS.

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