US2492090A

US2492090A - Automatic frequency control circuit for television deflecting systems

Info

Publication number: US2492090A
Application number: US58034A
Authority: US
Inventors: Harland A Bass
Original assignee: Avco Manufacturing Corp
Current assignee: Avco Manufacturing Corp
Priority date: 1948-11-03
Filing date: 1948-11-03
Publication date: 1949-12-20
Anticipated expiration: 1966-12-20

Description

Dec. 2%, 1949 H. A. BASS, 2,492,099

AUTOMATIC FREQUENCY CONTROL CIRCUIT FOR TELEVISI ON DEFLECTING SYSTEM Fild Nov. 3, 1948 2 Sheets-Sheet 1 INVENTOR.

HARLAND A BASS.

ATTY.

Patented Dec. 20, 1949 imill" D STATE-S1:

fr em osmcs TEMS- Bananas. Bassiitunt Healthy; Ohio, assignor Manufacturing ('lor-poration; Cincin- 9, r nati; Ohio; at cnrporation of Delaware Application November 3, 1948, Serial No. 58,034

3 cams; (01. 315-27) The present invention relates to ir-nprovefnents in automatic frequency control systems of the type employed in the deflection systems of television receiver circuits. While not limited to utility i-na horizontal deflection system the invention is of'particular advantage therein.

Reference is made to the autoinatic irequency control system shown in Fig. 11 1 following page 278 and described at pages 294 et' seq} or the book Television Simplified, by Kiv'er, second edition; DI Van Nostrand-Inl'c New York; 19% wherein horizontal synchronizin (sync) pulses and horizontal oscillator output'sign'a'ls are'ap plied to a comparator orhorizontal sync discriminator and utilized to derive a corrective control potential. This control potential has a magnitud and polarity proportional to the deviation b'etwe'enthe' oscillator output 's'ig-iial phase and the sync pulse phase. The unidirectional control potential is eniployedto Control a' reactance tube, which causes etearen't'chafiges in" the oscillator tank circuit in such a way as to" eliminate the deviation; TheoSCfllatoniS a sine wave Oscillator Whih Cofitfbls sawtbothwav generator. p

In prior" art circuits in which a sa'wtbotho'scil} later is employed for initiating" the sweep ai rents. the automatic frequency control otential is derived by measuringthe'phas 'raationsnips between synchronizing signals and sawtooth 'sig nals'. This relationship is /er y critical, Because of the short duration of the trailing portion of the sawtooth. It is well known that a more effective' method for obtaining the autcmanc fr'equency control potential is to Ineasureth phase relationships between sine-wave signalsand syschronizing pulse signals. However, in systems which do not provide a separate sine-wav'e'oscillator, no means has heretofore been developed for producing the sine-wave.

A primary object of the invention is to provide an automatic frequency control circuit in" which the above-mentioned separate sine-wave oscillator tube is eliminated.

In the above-mentioned prior artsy'stem; the

horizontal oscillator (sine-wave) circuit is coupled to a horizontal discharge tube, which turn is coupled to a horizontal output tube or} power"'ainplifierl The ainplifieroutputis applied to the horizontal deflectingcoil or yoke of the present invention is to provide a circuit for" utilizing this inductance as'a high=level powersource of data signals, representative 'of the re sponseof the deflecting coil ,'w-hi'ch data-signals are compared with'the sync pulses to develop afrequency control potential; This control p0- tential-is functionally related to'the phase devia; tion between sync signals and deflecting coil current waves and is ernployed so to control an oscillator as to eliminate the deviation.

Fora better understanding of the present invention, together with other and further advantages, capabilities and objects thereof, reference is made to the accompanying drawings and to the appendedclai'ms. In the d wings, Fig, l is a circuit sene'matie efan illustrative referred" form or utomatic frequency control system accordance with the invention; anti iii-g. 2'is a set or curves em loyed-m describing the opera: tion of the invention; V H p The automatic frequency control systein here in shown comprises a generator for gen-earnings deflection signal havin a rreque'ncy least part determined by the unidirectional control"- potential applied thereto. This generator of cathbtie bolipled ifiiiltiviblaltor is per se Well khowntb the art and is of the ge eisitype d scri ed in rior" art literature, inc1'uding "P city, and an article lo'yJ; L. Potter, fiages 'mg'et" Seq, PT'O'C'eedil'ii'iS 0f the Institute; (if R'aiiiG Ell gineers, June 1938, vol. 26 No; 6, published bythe Institute of' Radio Engineers at New York;-

New York. It consists of'a pair of tiiode tub anode resistors l6, H, connected to ahi'gh potential junction point 2?; is returned to ground thro'ug'h a series or re--- sistorsill; l9 and 20% vibrator outputsignals to correspo'hd tothat 6f the synchronizing signals applied to the system" from a sync separator circuit? l 'or other-suitable source of sync pulses; Itis well known to the" art that th frequency ofthe output signals ofa Potter inultivibrator maybe controlled by'adi-- reot current voltageapplie dto the grid of one of the tubes, such as tube I I.

The rul'tivihiator 'is coupled, by a network The grid of triode I l Across capacitor 48*tliiere appears a'c'ontrol potential which'i'sapplied-to triode l l to cause the frequency of the"inulti'- end of trace.

comprising anode load resistor l6, capacitor 22, grid resistor 23, and series damping resistor 24, to the input circuit of a beam-power horizontal output or power amplifier tube 25. Capacitor 26, between plate of triode l2 and ground, is the discharge capacitor across which the sawtooth sweep voltage is developed. The stage comprising high-impedance amplifier tube 25 is per se a well known prior art circuit shown in substance as the horizontal output or power tube circuit of the above-mentioned Fig. 11.1 in the Kiver book. The tube has a cathode bias resistor 28, a screen by-pass capacitor 30, a cathode by-pass capacitor 29, and a screen dropping resistor 3| connected to a high-potential point 32. A filter network comprising series resistor 33 and shunt capacitor 54 is interposed between high

potential points

32 and 27.

The plate circuit of tube 25 is coupled to the horizontal deflecting coil 34 of a picture tube (not shown) through the primary of an output or deflection transformer 35. Plate voltage for the system is supplied from a centering-control potentiometer 31, comprising a resistance having leads connected between +3 and ground, the sliding contact of this potentiometer being connected to one lead of coil 34 and a fixed tap on the resistance being connected to one lead 50 of the secondary of transformer 35, i. e. to the junction of

elements

38, 39, 50.

A balancing capacitor 5! is connected across one section of deflecting coil 34 in conventional manner to balance the distributed capacitances of its two sections.

There is provided in shunt with the deflecting coil 34 a conventional damping circuit consisting of a tube 36, having a cathode and two anodes, and a series capacitor 38. This circuit damps out undesired oscillations in the circuit comprising deflecting coil 34 and its distributed and stray capacitance 52. This circuit also feeds back power to the anode circuit of tube and permits the deflecting coil current to reverse with one half cycle of free oscillation during the retrace or beam-return intervals. To prevent falling ofi of the increase of current appearing in coil 34 during the latter part of the trace or scansion and to achieve greater linearity of the ai1: 1it -de-time characteristic of that current, there are provided a capacitor 39, in parallel with capacitor 38, and a variable inductance 40, between

capacitors

38, 39. The low potential terminal of the primary of transformer 35 islconnected to the junction of inductor and capacitor 39. There is developed in network 39', 49 a linearity control voltage which, as applied to capacitor 38, so varies the conduction of tube 36 as to prevent non-linearity of current flow through the deflecting coil 34 at or near the The time when this voltage occurs is controlled by adjustment of variable delay inductance 40, which is preferably adjustably permeability tuned. This inductance is per se con ventional and well known to the art, being shown in the literature, for example Tourshou Patent 2,440,418.

The plate of discharge device 25 is connected to the positive terminal (+B) of the high voltage space-current source through a D. C. path including the primary of transformer 35, inductor 4D, the cathode-anode circuit of tube 36, deflecting coil 34 and potentiometer 37, so that the average voltage developed across capacitor 38 is such as to boost the power supply voltage and permit the utilization of power recovered from the deflecting coil circuit.

The linearity control voltage across inductance 40 (curve D, Fig. 2) has a major component of line frequency, i. e., the frequency of the multivibrator. In accordance with the invention, there is provided means 'for deriving from this voltage and from the synchronizing-pulses a direct current control potential having an average amplitude and polarity determined by the magnitude and direction of the phase difierence between the linearit control voltage waves and the synchronizing signals. This means includes a secondary inductor 4|, so coupled to inductor 40 as therewith to constitute a transformer. Across the secondary 4| is a pair of series capacitors 42, 43, which tune the circuit of secondary 4| to resonance at line frequency, whereby thereis developed in resonant circuit 4|, 42, 43 a sine-wave voltage of line frequency.

Coupled to this resonant circuit is a horizontal synchronizing discriminator circuit, included in the means for deriving the frequency-control potential having a polarity and average amplitude determined by the direction and magnitude of the phase difierence between the last-mentioned sine-wave voltage and the sync pulses. The comparator or discriminator circuit comprises a pair of diodes 44, 45, each cathode being connected to a terminal of secondary 4| whereby the sine-wave voltage is applied in push-pull to the diodes. Diode 44 has a load resistor H! between anode and a center tap on secondary 4|. Diode 45 has a load resistor 20 between its grounded anode and that center tap. Load resistors l9, 2|! are connected series opposing. The sync separator sync signal output, of negative polarity, is applied between the junction of capacitors 42 and 43 and ground, therefore, in parallel or with the same polarity to the cathodes of both diodes. from tuned circuit 4|, 42, 43 is applied with opposite polarity to said cathodes so that there is developed b this phase discriminator circuit a D. C. voltage which has an amplitude and polarity functionally related to the amount and direction of the deviation between the phases of the sync signals and the deflecting currents. This D. C. frequency control voltage is applied to the grid circuit of multivibrator triode through a network comprising resistor I8 and

capacitors

41 and 48.

Capacitors

48 and 47 form a voltage divider to attenuate rapid changes in the D. C. voltage from the phase discriminator such as are produced by noise or by the vertical synchronizing prises, incompletely separated out in unit 2|. This voltage divider arrangement also removes undesired disturbances caused by phase modulation in the transmitted pulses.

The discriminator per se is well known to the art and is described in the above-mentioned Kiver book, at pages 294-296.

Coming, now to the operation of the system, it will be understood that successive sawtooth voltage waves at horizontal line frequency are developed'across capacitor 23 and applied to the grid of tube 25. This wave has the shape illustrated in curve A, Fig. 2, linearly rising during trace as capacitor 26 charges through the charging circuit including resistor l6, point. 21, resistor 33, point 32, inductance 49, tube 36, coil 34, potentiometer 3'3 and the space current source (not shown), and steeply falling during retrace as capacitor 26 discharges through triode l2.

The wave form of the voltage at the anode The sine-wave voltage derived tubes '25 and 36 are noneonductive. As shown in curve 'C, Fig. *2, the output transformer 35 inverts-this voltage. -Curve-C-shews-the approximate shape-of the voltage --wa-ve required (resistance parameters being neglected as overcome by negative resistance -efiects) --to cause asawtooth wave of current to flow through the deiiecting coil 34. It comprises a series of relatively :long steady portions of positive polarity, representing the .go sweep or trace :period, during which theoutput tube -25 supplies current to :the deflection yoke, and alternate short square Wave :pulses .of negative polarity, representing :the intervals. of retrace. or return ;.time, figuring which theoutput tube 25 plate current .iscut-ofl.

LDuring trace energy'is storedin the magnetic .circuit of coil314, both tubes 25 and 36'being con- 4i ctiv,e, so that there is a linear rise of current 3'4. .Thismeans that the electron current flow through coil 34 during trace is the resultantoi two .electroncurrent flows. is the driving current output oftube 25, electrons fiowingiromthesecondary terminal 50, to ter- 5I, then through deflecting coil 34 .back .tgth ther secondary terminal. Second, there the electron fiowthrough tube 35 and coil 34 ,passing through coil 34 in the reverse direction .from the first-mentioned current flow, representing the release of energy by coil 34, as shown .inan article by Otto H. Schade, RCA Review, pages-.506 et seq., vol. VIII, September 1947, No. .3, New York. The resultant current flow is a sawtooth wave approximately symmetrical with respect. to a zero axis, as shown in Fig. 1 of the Schadearticle, page 507. When the multivibrator .output goes negative, the plate current of tube .25 is suddenly cut off. The magnetic field, WhiGhhas been steadily building up in the output transformer 35 and deflecting coil 34, begins to collapse. The rate of collapse is determined by the resonant frequency of the inductance and capacitance parameters in the deflecting coil system. This system is shocked into oscillation by the .sudden-cut-ofi of tube 25. Initially the .voltage generated by the collapsing field is negative'as appliedto the plates of damping tube .3 p1 =eventing this tube from conducting. There is substantially no load across the'system during onerhalf cycle of free oscillation. At the end of this half cycle, the deflecting coil current reachesragmaximum value in the direction opposite to thatin which it was flowing at the end of the traceperiod. lhe induced voltage then reverses polarity, and the damping tube 36 begins to conduct, .fiy this time the retrace has been complated, and the next trace begins.

purine the retrace period, both

tubes

25 and 33 .are nonconductive, and the current in de-. vfleetingcoil 34 is the resultant of onehalf cycle at free-oscillation of-the tuned circuit comprisin g coil 34 and its distributed capacitance 52. vDuringthefirst quarter cycle the distributed capacitance 52 is charged and the current through GQ l .34 falls to zero. During the next quarter cycle the capacitance 152 discharges and causes currentto flow through coil .34 in a negative sense, Bythe time the capacitance 52 has becgmedischarged, the full half cycle of oscillation has-become executed at the resonant frequency of the-tunedcircuit .34.. 52, and the current through First, there (not shown) coil .34v is at .a maximum in its negative sense.

.Tube 36 now becomes conductive and P events capacitance 52 ,from charging .in .the reyerscdirection. Tube 36 becomes conductive at the start of the. succeedin trace. The energy -.-which .was .developed in the deflecting coil system by the output tube 25.during the-lat er ,part-of'the last trace has vnot been completely dissipated- Very little ,en ergy .waslostduring the .-first half cycle of retrace because the damping -.tube was .nonconductiveand the circuitis designed iorlcw loss. When, after the half :cyclebfmscillation,

thedamping tube doesbegin to conduct,.it,places such alheavyiload across theideflectionqcoil sys tern thatfurther oscillation is prevented. .During the initiation of the above-mentioned 5116" ceeding trace, .the energy field .decays at a :rate

determined by the load of the -.damp.in g tube across the coil. This damping tube causes the oscillatory voltage .toldecay. in. essentially alinear manner. The output tube :25 .beginsito conduct again at the beginning of this successive trace,

.and the. additional power supplied by .the-ioutput tuned .circuit 34, .52 are damped, and energy in storage in I the ma netic fieldiof deflecting 12011.34 at the end -of -.retrace is discharged in .a con trolled manner.

.Thecurrent through damper tube-.35 is made todecrease ata ,morerapidrataduriue scansion oritrace, .for purposesof linearity control,.by ,,developing across-capacitor v.39 .a voltage which is applied to capacitor .38 through the delay in ductor All. The positive peak of this voltage normally occurssubstantially at the .mid point of thescanning portion of ,thedeflection cycle. This voltage across capacitor .39 isv derived from theplate current oftube25 and is illustratedas to wave form in curve D (Fig. 2). This voltage isappliedto capacitor 38, being delayed ,as to phase by inductor .40. .The amount ofwtheidelay can be adjustedlby variation of the inductance parameter of unit-40. ,-Although.it, is not-necessaw that the theoryof op ration cfan-invem tion be known, or that thetheoryof operation of prior art elements included therein be cirplained, applicant believesthatthe.foregoingcX- planation of .the development of the linearity control voltage is correct. Some authoritiesbelieve that the voltage across capacitor 38 varies, due to the charging by the deflection coil kickback and the discharging through the output tube 25. "The rise and fall of this voltage is equivalent to an alternating currentripple in the'anode supply-for the output-tube '25. Those authorities' believe that inductor 40 and capacitor'39 form aphase-shift network and delay=net- WOIk, WhiCh' so shifts-the phase ofthis ripple-as to alter thecharacteristic of the output--tubei-25'. Inductor 40 -is variable to provide the desired improvement in linearity. However the operation of the delay network be regarded, it is well known to the art that a linearity control voltage having a large component of line frequency appears across inductor 40.

It will be seen that the linearity control voltage appearing across inductor 40 has a major component of the same frequency as the current pulses in deflecting coil 34. In accordance with the invention this voltage is used to produce data signals which are compared with the synchronizing pulses to derive the automatic frequency control voltage. The voltage across inductor 40 is converted into a sine wave by coupling inductor 40 to a tuned secondary circuit 4|, 42, 43. A separate sine-wave generating tube is thus eliminated.

The sine-wave signals are applied with opposite polarity to the two diodes 44, 45, tending to make them alternately conductive. The synchronizing pulses are applied with the same polarity to both diodes, tending to render them simultaneously conductive. For purposes of discussion it will be assumed that each sync pulse normally occurs as the sine wave is going through zero.

If the synchronizing pulses arrive (curve F, Fig. 2) when the sine-wave voltage across tuned circuit 42-43 is zero, then each diode 44, 45 (curve G, H, Fig. 2) will receive the same pulse voltage and the same amounts of rectified voltage will appear across load resistors l9 and 20. The total net output from both tubes will be zero, because the load resistors are coupled in opposition.

If the sync pulses arrive when diode 45 is cut off and diode 44 is conducting, the average voltage developed across resistor l9 will be greater than that across resistor 20. This average voltage, over one cycle, will be negative as applied to the grid of tube II. On the other hand, if the sync pulses arrive when diode 45 is conducting and diode 44 is cut ofi, the average voltage across resistor 20 will prevail and will be positive as applied to the multivibrator input. It is understood, therefore, that the double-diode horizontal synchronizing discriminator is a sensitive phase discriminator which develops an output voltage that may be negative, zero or positive (to slow down, maintain at steady speed, or speed up the multivibrator), depending on the phase of the synchronizing pulses with respect to the sinewave voltage generated in the tuned circuit 4|, 42, 43 coupled to the delay inductor 40. In this manner, any undesired deviation between sync pulses and deflection rate is rapidly eliminated.

The multivibrator normally operates at 15,750 cycles per second in a horizontal deflecting system under present standards, and the sync pulses have that frequency. The circuit 4|, 42, 43 is tuned to the large 15,750 cycle component of the voltage appearing across the linearity control inductor 40.

The following parameters were found suitable in one successful commercial embodiment of the invention:

Triodes H, l2 Type 7N7 tube Tube 25 Type 6BG66 Tube 36 Type 5V46 Resistor I3 1000 ohmsv Resistor l5 500,000 ohms Capacitor I4 82 mmf. Resistor ll 47,000 ohms Resistor l6 1 megohm Capacitor 22 0.01 mf. Capacitor 26 150 mmf. Resistor 23 1 megohm Resistor 24 47 ohms Resistor 28 100 ohms Resistor 33 10,000 ohms Capacitor 54 0.05 mf.

Q Capacitor 29 0.1 mf.

Capacitor 30 0.05 mf.. Resistor 3| 50,000 ohms Capacitor 39 0.06 mf. Capacitor 38 0.05 mf.

Capacitor 51 56 mf. Resistor 20 100,000ohms Resistor l9 100,000 ohms, Resistor l8 470,000 ohms, Capacitor '41 0.004 mf..

Capacitor 48 0.05 mf..

Capacitor 42 0.005 mf., Capacitor 43 0.005 mf.

While there has been shown and described what is at present considered the preferred embodiment of the present invention, it will be'obviousto those skilled in the art that various changes and modifications may be made therein without departing from the invention as define by the appended claims.

Having fully disclosed and described my invention, I claim:

1. In an automatic frequency control system for a television picture tube deflecting system, said system being of the type which comprises a deflecting coil, a multivibrator circuit for originating the voltage wave for said deflecting system, an amplifier circuit employing a high impedance electron tube for generating the deflecting current for said coil and a damper tube circuit shunted across said coil and including a delay inductor, said system being controlled as to frequency by a pulse form synchronizing wave, the improvement which comprises a tuned cir- 4; cuit inductively coupled to said delay inductor for deriving a sine wave of voltage synchronized with said deflecting current and a comparator circuit for deriving from said sine-wave voltage and said pulse form synchronizing voltage a voltage for controlling the phase and frequency of said multivibrator.

2. In an automatic frequency control system for a television picture tube deflecting system, said system being of the type which comprises 5 a deflecting coil and signal generator for generating a deflection signal having a frequency at least in part determined by a control potential applied thereto, which control potential is indirectly derived from synchronizing signals, 5 electron beam-deflecting means for coupling said generator to said coil whereby to produce periodic beam-deflecting waves therein, a discharge device for damping out undesired oscillations across said coil, and means for developing a linearity control voltage which varies as a function of the current in said coil, the improvement which resides in the combination of means for deriving from said linearity control voltage and from said synchronizing signals a frequency control potential having a direction and magnitude determined by the direction and magnitude of the phase diff ference between said voltage and said synchronizing signals and means for applying said frequency control potential to said signal generator to eliminate said phase difference.

3. In an automatic frequency control system for a television picture tube deflecting system, said system being of the type which comprises a deflecting coil and signal generator for gen-'- erating a deflection signal having a frequency at least in part determined by a control potential applied thereto, which control potential is indirectly derived from synchronizing signals, electron beam-deflecting means for coupling said generator to said coil whereby to produce periodic beam-deflecting waves therein, a discharge device for damping out undesired oscillations across said coil, and means for developing a linearity control voltage which varies as a function of the current in said coil, the improvement which resides in the combination of means including a phase discriminator for deriving from said linearity control voltage and from said synchronizing signals a frequency control potential having a direction and magnitude determined by the direction and magnitude of the phase difference between said voltage and said synchronizing signals, and means for applying said frequency control potential to said signal generator to eliminate said phase difference.

4. In an automatic frequency control system for a television picture tube deflecting system, said system being of the type which comprises a deflecting coil and signal generator for generating a deflection signal having a frequency at least in part determined by a control potential applied thereto, which control potential is indirectly derived from synchronizing signals, electron beamdeflecting means for coupling said generator to said coil whereby to produce periodic beam-deflecting waves therein, a discharge device for damping out undesired oscillations across said coil, and means including a delay inductor for developing a linearity control voltage which varies as a function of the current in said coil, the improvement which resides in the combination of means for deriving from said linearity control voltage and from said synchronizing signals a frequency control potential having a direction and magnitude determined by the direction and magnitude of the phase difference between said voltage and said synchronizing signals, and means for applying said control po- CFI 1.0 tential to said signal generator to eliminate said phase difierence, said means comprising a tuned circuit coupled to said delay inductor, a phase discriminator having two diodes coupled in pushpull to said tuned circuit whereby the sine-wave voltage developed across said tuned circuit is applied to the diodes in push-pull, and a coupling network for applying said synchronizing signals in parallel to said diodes.

5. In an automatic frequency control system for a television picture tube deflecting system, said system being of the type which comprises a deflecting element and a signal generator for generating a deflection signal having a frequency at least in part determined by a control potential applied thereto, which control potential is indirectly derived from synchronizing signals, electron beam-deflecting means for coupling said generator to said element whereby to produce periodic beam-deflecting waves therein, and linearity control means, the improvement which resides in the combination of means coupled to the linearity control means and responsive to the voltage developed therecross and to said synchronizing signals to derive a control potential having a direction and magnitude determined by the direction and magnitude of the phase difference between said voltage and said synchronizing signals, and means for applying said control potential to said signal generator to eliminate said phase difference.

HARLAND A. BASS.

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

UNITED STATES PATENTS Number Name Date 1,933,219 Nakajima et a1 Oct. 31, 1933 2,363,659 Doba, Jr. Nov. 28, 1944 2,383,822 Schlesinger Aug. 28, 1945 2,412,210 Edson et al. Dec. 10, 1946