US2114938A - Condenser relaxation circuit - Google Patents

Description

April 19, 1938.

o. SQPUCKLE 2,114,938

CONDENSER RELAXATION CIRCUIT Filed April 17, 1934 INVENI'OR OWEN s. PUICKLE w mw ATTORNEYS Patented Apr. 19, 1938 UNITED STATES PATENT ()FICE CONDENSER RELAXATION CIRCUIT Britain Application April 17, 1934, Serial No. 721,053 In Great Britain May 5, 1933 6 Claims.

This invention relates to electrical time base circuits and systems for producing waves of potential of the type known as condenser relaxation circuits.

In circuits or systems of this type as usually arranged, a controlling condenser is connected through an impedance to a source of continuous voltage so as to be charged up at a rate regulated by the aforesaid impedance, a discharging de- 10 vice connected directly across the condenser being arranged to bring about a sudden discharging of the condenser at intervals. In an alternative arrangement, the condenser, instead of being connected across the discharging device, is connected across the aforesaid impedance so that the condenser is discharged at a rate regulated by the said impedance and suddenly charged at intervals. In both cases the condenser is alternately charged and discharged, the change of charge in one direction being regulated so as to produce a continuously varying potential having a desired characteristic whilst the change of charge in the other direction is in the nature of a sudden restoration of the state of charge on the condenser to a preset value. For convenience, the impedance which controls the rate of change of charge in one direction so as to produce a varying potential having a desired characteristic will be referred to in this specification as the charging device, and the device by which the charge on the condenser is suddenly restored to a preset value at intervals will be referred to as the discharging device, and it will be understood that in case the alternative arrangement described above is adopted, the charging device may have the effect of reducing the actual charge on the condenser and the discharging device may have the effect of increasing the charge on the condenser.

40 Circuits or systems of'the kind to which this invention relates are used in connection with electrical oscillographs for providing a time base deflection. They are also used for controlling electrical scanning devices in television systems.

45 In these applications, considerable difiiculty has the operationof a gaseous discharge valve tends to v mionic valves.

' In the case where a very high frequency of operbe upset when it is called upon to discharge at varying time intervals as is required in certain applications to television systems.

The object of the present invention is to provide an improved discharging system employing hard thermionic valves which will be more satisfactory in operation and will be capable of discharging the controlling condenser more suddenly than the known arrangement described above. 10

According to a subsidiary feature of the invention, in order to enable the anode circuit of the second valve to be supplied from the same source of continuous voltage as is used for charging the regulating condenser, the cathode of the first 15 valve is connected to that pole of the regulating condenser to which the charging device is connected, the anode of the first valve being connected through the resistance in the anode circuit of that valve to the positive pole of the source of 2 continuous voltage. 0

The invention will be more particularly described with referenceto the accompanying drawing in which:

Figure 1 is a circuit diagram showing the preferred arrangement for carrying the invention into effect when used for controlling a cathode ray oscillograph employed for examining wave forms, Fig. l is a fragmentary figure showing a modification of the arrangement shown in Fig. 1, and

v Figure 2 illustrates another modification. 40

Referring now to this drawing, a circuit of the well known type for rectifyingalternating currents is shown on the left hand side while the deflecting plates of a cathode ray oscillograph CRO are shown on the right hand side. The po-; tential on one of these oscillograph deflecting plates, viz. the plate Pr, is derived from a selected one of the controlling capacities C1, C2, C3 and C4.

ation is required however no condenser such as C1, C2, C3 or C4 need used, the stray capacities being relied on instead. The selected controlling capacity is alternately charged and discharged as hereinafter described so as to cause the beam of cathode rays to execute a traversing movement over the screen.

The capacity C1, C2, C3 or C4 is charged by means of a current regulating device such as the pentode P1 which may be a screened tetrode, which is connected in series with the capacity to be charged and has a screen whose potential can be adjusted by means of the potentiometer R4, the control grid being tied to the cathode, which is connected to the negative pole of the supply. While this arrangement is preferred it is to be understood that the control grid may be used in the normal manner instead of the screen for adjusting the anode current of the valve.

The discharging of the condenser is brought about by a triode T1 assisted by the pentode P2 the former being connected through a resistance R1 across the condenser to be discharged and the latter having its anode connected to the grid of the triode T1. The control grid of the pentode P2 is connected to its cathode through a grid biasing resistance R and the anode resistance R2 is so adjusted that the control grid of the valve T1 is normally biased to a high negative value.

In the quiescent condition, which is of course not a lasting one, that is to say with the controlling capacity discharged, the grid of the triode T1 is very negative due to the large current flowing in the anode circuit of the pentode P2. The controlling capacity, e. g. C1, then charges, at a rate determined by the anode current of the valve P1, until the cathode of the triode T1 reaches a low enough potential to pass anode current. The resulting change of voltage across the resistance R1 is communicated to the control grid of the pentode P2 through a condenser C with the result that the control grid of P2 becomes negative thus reducing the anode ourrent of P2 and making the grid of the triode T1 more positive. The resulting further increase in the anode current of T1 increases the fall of potential on the grid of the valve P2 so that the efiect is a cumulative one and the grid of 'T1 rises rapidly to a positive value with respect to its cathode. The anode current of the valve T1 therefore rapidly reaches a high value so that a rapid discharge of the controlling capacity. is brought about. The time constant of the circuit CR is so chosen that the control grid of the pentode P2 remains negative up to almost the end of the discharge of the controlling capacity. When the controlling capacity is discharged however, the anode potential of the triode T1 rises with the result that the grid of the pentode P2 is forced towards zero making the grid of the triode T1 become very negative and thus resetting the circuit for the next cycle of operations.

While the arrangement described above is preferred it is to be understood that, if desired, the arrangement may be reversed by altering the position of the condenser to be charged and discharged, e. g. C1, to that shown at C in Fig. 1a, when the condenser will he suddenly charged'by the two valve T1 and P2 and relatively slowly discharged by the valve P1, and such an arrangement is intended to be included within the scope of the claims.

Included in the anode circuit of the triode T1 of this resistance the rate of discharge of the trolling condenser. By the normal bias on the grid of the'valve controlling capacity can be altered. The lower the value of the resistance R1 the greater will be the rate at which the controlling capacity is discharged. On the other hand, the higher the value of the resistance R1, the more rapidly will the 5 discharging current rise to its maximum value and the more suddenly will it be cut off at the end of the discharging period. The resistance R2 should be adjusted to set the amplitude of the time base sweep.

The maximum value to which the anode current of the valve T1 rises during the discharge of the controlling capacity, is limited by reason of the fact that the flow of grid current in the valve T1 results in a fall of potential across the resistance R2. Where it is desired to obtain 'a more rapid discharge of the controlling capacity than is practicable with the arrangement illustrated in Figure 1, the arrangement illustrated in Figure 2 may be employed. arrangement, the place of the resistance R2 is taken by a thermionic valve V whose anode is connected to the positive voltage supply and whose cathode is connected directly to the con- In this trol grid of T1 and through a biasing resistance Rv to the anode of the valve P2. The valve V has a control electrode connected directly to the anode of the pentode P2.

With this arrangement, during the charging of the controlling condenser, the anode current of of the valve P2 is reduced and acumulative rise of potential on the grid of the valve T1 occurs as has already been described with reference ing to the arrangement of the valve V however, the reduction of the anode current of the valve P2 results in a reduction of the fall of potential acrossthe resistance Rv and consequently in a When a small rise of anode to Figure 1 of the accompanying drawing. Ow-.-f%0

reduction of the impedance of the valve V. The; 4 .5

, rise of'potential on the grid of the valve'T1fis consequently accelerated. 'In addition the limiting effect of grid current inthe valve T1 on the rate of discharge of the controlling capacity is minimizedo-wing to the reduced impedance of the valve V. The discharging current conse-- "quently rises to a higher maximum value than would be the case if the impedance of the connection between the positive potential point and the control grid of the valve T1 remained at itsz normal value during the discharging of the con- By adjusting the resistance V and therefore the normal impedance of this valve can be varied. This results in a variationroo tapping on the potentiometer R4 and the latterthus controls the anode current of the: pentode P1 and hence the speed of the time base sweep.

The wave to be examined is impressed across the terminals W1 and W2 of which W1 is connected to the gun GN of the oscillograph whilst. .70

W2 may be connected to the deflector plate S of the oscillograph.

In the circuit arrangement shown, the wave to be examined is also employed as a synchronizing signal for the time base sweep. For this purpose-.75

a triode T2 is provided for amplifying the synchronizing signal received from W2. The grid of the triode T2 is connected to the point W2 via a tapping on a potentiometer R3. The anode of the triode T2 is connected to a resistance R5 and to'the screen of' the pentode P so that the resistance R5 and triode T2 form a two part potentiometer the impedance of one side of which (viz. that formed by the triode T2) varies with the instantaneous amplitude of the synchronizing wave derived from W2. There is thus applied to the screen of the pentode P2 9. synchronizing wave which may be an amplified or an attenuated copy of the wave to be examined according to the adjustment of the potentiometer R3.

- This synchronizing wave takes controlof the discharge of the controlling capacity and causes this discharge to occur at the same point on the curve of the examined wave form on each occasion so that the individual traces are superposed. It will be understood that by suitable adjustment of the resistance R2 the amplitude of the time base sweep may be set so that the frequency of the discharge becomes a sub-multiple of the frequency of the synchronizing wave. Thus although when a synchronizing signal is employed the value of the resistance R2 does not determine the exact amplitude of the time base sweep, it selects one from a number of possible amplitudes at which the discharge might occur for any given frequency and amplitude of the synchronizing signal.

The possibility of amplification of the synchronizing signal allows perfect synchronization to be maintained even when the amplitude of the examined wave form is very small. Since this amplification does not afiect the wave applied to the plate S of the oscillograph the possibility of distortion due to the amplification necessary for synchronization is avoided.

In the arrangement just described, the potentiometer R3 is fed with a small amount of energy from the circuit being examined and a suitable percentage of this is applied to the grid of the triode T2 thereby altering its conductivity and altering the potential applied to the screen of the pentode P2. While this arrangement is preferred it is to be understood that the synchronizing signal'may if desired be applied directly to the control grid or screen of P2 instead of to the control grid of the triode T2. Alternatively it may be fed directly to the valve T1.

With the arrangement illustrated in Fig. 1 the positive crests of the wave to be examined produce a decrease in the impedance of the triode T2 and consequently a decrease in the screen voltage of the pentode P2 giving rise to an increase in the potential on the grid of the discharge triode T1.

In order to permit of centering the image on the screen of the oscillograph two potentiometers Q1 and Q2 are connected across the H. T. supply, the tappings being connected to the appropriate deflecting plates (Pys, Pms). To enable the potentials on the plates to be made positive or negative with respect to the gun GN the latter is itself connected to the junction of the two resistances Rs, R1.

Connected between the anode or gun GN of the oscillograph, and the source of H. T. potential is a condenser C5 which has the effect of providing a low impedance path for the capacity current through the oscillograph during the fly-back or rapid return of the oscillograph spot, the low impedance path having the effect of preventing the the accompanying drawing can be employed with very little modification for producing the scanning potentials to be applied to the oscillograph of a cathode ray television system. For this purpose the system composed of the valves,T1, P2

may be arranged to discharge the controlling capacity when the potential across the latter reaches a preset limit, the valve T2 being replaced by a fixed resistance. Alternatively a synchronizing signal derived from any suitable source -may beapplied to the grid of T2 or directly to .the control grid or screen grid of P2 for the purpose of synchronizing the discharge. In the case of a velocity modulated television system, the television signal may be applied to the control grid or other appropriate electrode of the.

valve P1 for the purpose of modulating the anode current of this valve.

I claim:

1. A relaxation oscillator comprising a condenser, a direct current source, and an impedance, one terminal of said condenser and one terminal of said impedance each being connected to said direct current source and the other terminals of said condenser and impedance being joined, a hard thermionic valve having an anode, cathode and grid, a second impedance, an anode circuit for said valve connected between the junction of said condenser and the first said impedance and a point in said direct current source having a different potential than the point to which the first said impedance is connected, said anode circuit comprising the space path of said valve together with said second impedance connected in series with and on the anode side of said space path, a third impedance, a second hard thermionic valve having an anode, cathode and grid and having its cathode connected to a point in said direct current source and its anode connected through said third impedance to a point in said direct current source having a higher potential than the point to which the cathode of said second valve is connected, means for coupling the anode of said first valve to the grid of said second valve, and means for coupling the grid of said first valve to the anode circuit of said second valve.

2. A relaxation oscillator according to claim 1, in which the cathode of the first mentioned valve is connected to the junction of the said condenser and the first mentioned impedance, and the anode of the first mentioned valve is connected through the impedance in the anode circuit of that valve to the positive pole of said source.

3. A device in accordance with claim 1 in which said third impedance comprises a fourth impedance and a third valve having an anode cathode and grid, the fourth impedance being connected between the anode of the second valve and the cathode of the third valve, the anode of the third valve being connected to said higher potential point in the direct current source and there being a connection from the anode of the second valve to the grid of the third valve and a connection from the cathode of the third valve to the grid of the first mentioned valve.

4. A relaxation oscillator according to claim 1 wherein the second mentioned thermionic valve has a screen grid and wherein a synchronizing signal is applied to said screen grid through a two-part potentiometer one. side of which consists of a thermionic valve whose impedance is made to vary with the instantaneous amplitude of the synchronizing wave.

5. In an electrical time base circuit or wave producing system of the type referred to, a controlling condenser, a direct current source for charging said condenser, a charging impedance, said impedance and said condenser being connected in series across said source a hard thermionic valve, a resistance, the anode of said valve being connected through said resistance to the positive pole of the said source and its cathode connected to the junction of said charging impedance and said condenser, a second therrnionic valve having an anode, a cathode and grid, a second resistance, said second valve having its cathode connected to the negative pole of said source and its anode connected through said second resistance to the positive pole of said source, a leak resistance,said grid for the second valve being connected through said leak resistance toa point in said source, a connection between the anode of the second valve and the grid of the first valve. a direct current stopping condenser, and a connection through said direct current stopping condenser between the anode of the first valve and the controlling grid of the second valve, the time constant of the said stopping condenser and leak resistance so functioning that the control grid of the second valve remains negative until close to the end of the discharge of the controlling capacity.

,6. Anielectrical time base circuit as claimed in claim 5, wherein the resistance connected in the anode circuit of the second thermionic valve is adjustable for the purpose of adjusting the normal bias on the grid of the first valve to set the amplitude of the time base sweep.

OWEN STANDIDGE PUCKLE.

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