US2201978A

US2201978A - Frequency control circuits

Info

Publication number: US2201978A
Application number: US237052A
Authority: US
Inventors: Alda V Bedford
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1938-10-26
Filing date: 1938-10-26
Publication date: 1940-05-28
Anticipated expiration: 1957-05-28
Also published as: GB534749A; FR989233A

Description

May 28, 1940., v v. BEDFORD 2,201,978

FREQUENCY CONTROL CIRCUITS Filed Oct. 6, 1938 OUTPUT ll. 15 8620 z940- POWER L/IVL' HG-Z a.

I Bnventor Hlda V: Bedford attorney Patented May 28, 1940 PATENT OFFICE 2,201,978 FREQUENCY CONTROL CIRCUITS Aida V. Bedford, Collingswood, N. J., aulgnor to-Badio Corporation of America. a corporation of Delaware Application October 26, 1938, Serial No. 237,052

10 Claims.

My invention relates to the frequency control of oscillators and particularly to television systems which include synchronizing impulse generators which must be locked in with or operated in a fixed time relation to a power line supply such as the usual 60 cycle supply.

A system of the above-mentioned type is described and claimed in my Patent No. 2,137,010, issued November 15, 1938, and assigned to the Radio Corporation of America. While this earlier system was found to be fairly satisfactory, it had certain objectionable features, one of these features being that it did not act as quickly as desired in correctingfor frequency or phase changes because of the filtering in the circuit. At the same time this filtering was necessary in order to smooth out the control impulses and make the final control voltage as close to a D.-C. voltage as possible. Too little-filtering would permit the control voltage to drop in value between control impulses, and the frequency of the oscillator being controlled would change accordingly.

It is accordingly an object of my invention to provide an improved control or lock-in circuit for a synchronizing impulse generator or the like.

It is a further object of my invention to provide a control or lock-in circuit for a synchronizing impulse generator or the like which will not have the above-mentioned objectionable features.

It is a further object of my invntion to provide an improved means for and method of looking in an oscillator with a controlling frequency source.

It is a further object of my invention to provide means by which impulses of varying height produce a voltage wave having uniform voltage during the interval between each two consecutive pulses which is proportional to the value of the immediately preceding pulse.

In practicing one'embodiment of my invention, asine wave oscillator provided with an automatic frequency control circuit is employed, the frequency of this oscillator being divided by suitable frequency dividers to produce electrical impulses occurring at the frequency of the controlling source or at the rate of 60 per second in this case where the 60 cycle line is the controlling source. The 60 cycle impulses and the 60 cycle waves from the power line are added and utilized to charge a condenser through a time. However, a special discharge circuit forthe condenser is provided, this circuit being made effective to discharge the condenser only during the occurrence of the control impulses. Thus, if the oscillator frequency starts to change, .there is a shift in phase between the control impulses and the 60 cycle linesupply with a resulting change in the height of the combined impulses whereby the condenser receives an additional charge or is discharged a certain amount depending upon whether. the height of the combined impulses is increased or decreased.

By applying the condenser voltage to the automatic frequency control or A. F. C. circuit the oscillator is brought back to the predetermined desired frequency.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a circuit diagram of one embodiment of my invention, and

Figures 2a to 2d are curves which are referred to in explaining the invention.

Referring to Fig. 1, I have shown my invention applied to a television system of the type disclosed in my above-mentioned copending application wherein there is a synchronizing generator comprising a high frequency oscillator I which feeds into a chain of frequency dividers Ii, I2, I3 and It to produce impulses occurring at the picture frame frequency, that is, at the rate of 60 per second. The oscillator It also feeds into a frequency divider ii to produce impulses occurring at the line scanning frequency.-

The frequency dividers may be multivibrators, blocking oscillators, or the like.

For improved stability of operation. the master oscillator l0 preferably is a sine wave oscillator provided with an A. F. C. circuit i'l, although it may be a different type of oscillator such as a blocking oscillator which has its frequency controlled by varying the grid bias as described in my above-identified copending application.

In the specific example illustrated, the master oscillator I0 operates at double the line frequency and the frame frequency is derived by dividing the master oscillator frequency in odd number steps as described and claimed in my copending application Serial No. 726,258, filed May 18, 1934,

entitled Television systems, and assigned to the' Radio Corporation of America. The frequencies at which the several units operate in the system illustrated are indicated on the drawing by way of example.

The oscillator It may be of conventional design comprising a tuned circuit l8 having an intermediate point connected to the plate voltage supply and having points on either side thereof connected to the control grid l9 and to the screen grid II, respectively, of the oscillator tube, the screen grid 2| acting as the oscillator anode. The oscillator it is coupled to the frequency dividers through the tube plate 22, elec- Referring now to my obtaining the control voltage, electrical impulses from the frequency divider I4 are supplied over a conductor 23 andthrougha transformer secondary 24 to a rectifier 28 such as a diode. These 60 cycle impulses are shown at a in Fig. 2a.

At the same time, a 60 cycle voltage is supplied from the power line to the rectifier 26 through a;

transformer 21 comprising the primary 29 and the secondary 24. This sine wave voltage is shown at bin Fig. 212.

Since the sine wave voltage and the 60 cycle impulses are fed into the control circuit in series, they add as shown at c in Fig. 2c and charge a condenser 29 to a value depending upon the height of the combined impulses. The voltage across condenser 29is fed to the A. F. C. circuit I'l whereby a change in voltage across condenser 29 causes a change in frequency or phase of the oscillator I0.

An important feature of my invention is that the condenser 29 is also provided with a discharging circuit which is rendered operative to efiect a discharge only for the duration of an impulse from the frequency divider I4. This circuit comprises a vacuum tube 3| having a cathode 32, a control grid 33 and a plate 34. The plate-cathode discharge path of the tube 3i is connected across the condenser 29 through a comparatively high impedance resistor 36.

The tube 3i is normally biased beyond cut-off by means of a suitable biasing means such as a battery 31 connected to the grid 33 through a resistor 38. The tube 3| may be made selfbiased. if desired, by means of the usual cathode resistor and shunting condenser. In order to make the discharging circuit effective for the duration of a control impulse, the 60 cycle impulses from the unit I4 are impressed upon the grid 33 of tube 3| through a conductor 4i and a coupling condenser 42. The impulses from conductor 4|, whlch'are the same as impulses a, except possibly in amplitude, render the tube 3I ccnducting periodically whereby the condenser 29 may discharge therethrough a certain amount if the potential relations permit such a discharge. This will be better understood by considering the operation of the circuit, assuming that the voltages a and b shown in Figs. 2a and 2b are impressed upon the rectifier tube 26.

Referring to Figs. 2a to 20, it will be seen that the first impulse of curve a'and the first cycle of curve b add to give an impulse I of medium height. It is assumed there has been a series of impulses less than this height whereby the condenser 29 has a voltage thereacross slightly less than the peak value of impulse I. The voltage across condenser 29 is shown by the curve in Fig. 2d.

Upon the occurrence of impulse I,a voltage is impressed through diode 26 and across condenser 29 which is greater than the condenser voltage whereby the condenser 29 receives an additional charge. Meanwhile the tube 3i has been conducting for the duration of impulse I but, because of the high impedance of the discharging circuit as compared with the charging circuit, it has substantially no effect. The result is that the voltage across condenser 29 rises to the level a: (Fig. 2d) and the frequency or phase of oscillator I is immediately corrected. Obviously, this correction may be either for a change in the oscillator frequency or in the power line frequency but, in practice, it is usually for a change in OI- cillator frequency. In either case the oscillator I0 is held tightly locked in with the power line supply.

Now assume a relative phase shift of impulses a and D such as to produce an impulse 2 of decreased'amplitude. The voltage of condenser It is now greater than the charging voltage and the condenser 29 discharges through tube 3| to bring the'control voltage to the level 11 (Fig. 2d). The condenser discharge stops at the level 1 because this is the voltage at which charging of the condenser 29 through the diode 26 begins and cancels any further discharge through tube 3 I.

Similarly, a phase shift in the other direction produces a control impulse 9 of still larger amplitude whereby the voltage of condenser 29 is brought to the level z.

It will be evident that the change in control voltage is practically instantaneous and that the control voltage remains constant between successive control impulses. As a result, the horizontal scanning lines are evenly spaced from the top to the bottom of a picture whereas they would be spaced differently from top to bottom if the control voltage fell off in value between successive control impulses, this being accompanied by a change in oscillator frequency.

It will be understood that any one of several well known A. F. C circuits may be employed for controlling the oscillator I0. The particular A. F. C. circuit I! which is illustrated operates to vary the reactance of the tuned circuit I8 whereby the frequency of the oscillator is changed.

The A. F. C. circuit I'I comprises a vacuum tube 46 of the variable mu type having a cathode 41, a control grid 48 and a plate 49. The tube is provided with a self-biasing resistor The grid 48 is maintained at a biasing potential determined by the biasing battery 52, the voltage across condenser 29 and the drop across selfbias resistor 5i. I

Remembering that the intermediate point of the inductance in the tuned circuit I8 to which voltage is supplied is at A.-C. ground potential, it will be seen that the tube 46 is connected in shunt to the upper portion of this coil. Current from the tuned circuit I8 is fed through a resistor 53 and a condenser 54 and the resulting voltage across condenser 54 which is slightly less than 90 degrees out of phase with the current, is applied to the grid of tube 48 whereby it functions as a reactance across tuned circuit I8. Thus a change in the bias voltage on grid 48 changes the frequency of the oscillator I0.

It may be noted that the impulses a preferably are of substantially greater amplitude than the sine wave voltage b although this has not been indicated on the drawing. The impulses a need not occur at the same frequency as the wave 12 but they should have a harmonic relation to wave b. For example, assuming wave b to occur at 60 cycles they may occur at 60 cycles or 30 or 20 cycles. The wave b, of course, need not be a pure sine wave but preferably it is a wave with sloping sides.

I claim as my invention:

1. In a control circuit for maintaining the output of an oscillator in a fixed time relation to a source supplying an output at a controlling frequency, a condenser across which there is to appear a controlling voltage, a rectifier, means for adding said outputs to produce a composite wave, means for charging said condenser through said rectifier by said composite wave, a discharging circuit connected across said condenser which circuit is substantially an open circuit until acted upon by a control means, and a control means for making said discharge circuit effective to discharge said condenser periodically and only during the period said composite wave is of maximum amplitude.

2. The invention according to claim 1 characterized in that said discharging circuit comprises a vacuum tube and a resistor in series, said tube being biased substantially to cut-ofi until acted upon by said control means.

3. The invention according to claim 1 characterized in that said discharging circuit includes a vacuum tube having a control grid, and further characterized in that means is provided to impress said oscillator output upon said grid.

4. In combination, an oscillator which oscillates at a comparatively high frequency, a source of voltage which is at least approximately sinusoidal in wave form and which occurs at a controlling frequency, means for deriving from the output of said oscillator voltage impulses which occur at a frequency which has a harmonic relation to said controlling frequency, means for adding said impulses and said controlling voltage to produce a composite wave, a condenser, a rectifier, means for charging said condenser by said composite wave through said rectifier, a discharging circuit connected across said condenser which circuit is substantially open circuited until acted upon by a control means, a control means for making said discharging circuit effective to discharge said condenser in response to the occurrence of said impulses and for the duration thereof only, said discharging circuit when effective to discharge said condenser having a higher impedance than said charging. circuit, and means for controlling the frequency of said oscillator in accordance with the voltage across said condenser.

5. The invention according to claim 4 characterized in that said discharging circuit includes a vacuum tube having a control grid upon which said impulses are impressed to lower the platecathode impedance of the tube.

6. In apparatus comprising an oscillator and a control circuit which includes a condenser, the method of controlling the frequency of said oscillator in accordance with the frequency of a: controlling source which producing electrical impulses from said oscillator, producing electrical waves having sloping sides from said source, adding said impulses to the sloping sides of said waves to produce a composite control wave, charging said condenser periodically by said control wave when its amplitude exceeds the voltage across said condenser and holding the charge on said condenser between the occurrence of successive electrical impulses, discharging said condenser during the occurrence of said electrical impulses when the amplitude of said control wave is less than the voltage across said condenser, and controlling the frequency of said oscillator in accordance with the resulting voltage across said condenser.

7. In a controlling or indicating device, a first source of signals, a second source of signals having periodic peaks, a condenser, means for applying to said condenser only during said peaks comprises a voltage which is determined by the phase relation of the signals from said sources whereby the charge of said condenser is altered only during each peak, means for preventing discharge of said condenser between the occurrence of successive peaks, and means for causing discharge of said condenser during the occurrence of said peaks if the voltage across said condenser exceeds said first voltage.

8. In a device for controlling frequency, a first source of signals, a second source of signals having periodic peaks, successive periodic peaks occurring during substantially the same part of the cycle of the first source, a condenser, electronic means for altering the charge of said condenser in a first direction in response to the amplitude of the signal from said first source during said peaks being greater than the amplitude during previous peaks, a second electronic means for altering the charge of said condenser in a second direction in response to the amplitude of the signal from said first source during said peaks being less than the amplitude during previous peaks, both of said electronic means being active to change said charge only during said peaks, and means for controlling the frequency of one of said sources of signals in accordance with the voltage across said condenser.

9. A circuit'for producing a voltage representative of the phase relaion between the output of a first signal source and the output of a second signal source, said outputs having a harmonic relation to each other, said circuit comprising a condenser across which said voltage is to appear, a charging circuit including a rectifier, said second source being connected to charge said condenser through said charging circuit, a discharging circuit including a rectifier, means for deriving from said first source voltage impulses of greater amplitude than the output of said second source, means for making said charging circuit efiective to pass a charging current only in response to and for the duration of each of said voltage impulses, and means for simultaneously making said discharging circuit efiective to discharge said condenser only during said voltage impulses and in response to the charging voltage being less than the voltage across said condenser.

10. A circuit for producing a voltage representative of the phase relation between the output of a first signal source and the output of a second signal source, said outputs having a harmonic relation to each other, said circuit comprising a condenser across which said voltage is to appear, a charging circuit including a rectifier, said second source being connected to charge said condenser through said charging circuit, a,

discharging circuit including a rectifier, means for deriving voltage impulses from said first source, means for making said charging circuit efiective to pass a charging current only in response to and for the duration of each of said voltage impulses, and means for simultaneously making said discharging circuit effective to discharge said condenser only during said voltage impulses and in response to the charging voltage being less than the voltage across said condenser.

ALDA V. BEDFORD.