US2577972A - Circuit arrangement for stabilizing the amplitude of electrical oscillations - Google Patents

Description

ec. 11, 1951 E cp LEYTQN 2,577,972

CIRCUIT ARRANGEMENT FOR STABILIZING THE AMPLITUDE OF ELECTRICAL OSCILLATIONS Filed Oct. 27, 1949 A fwd: Curran t lauds afanhbZ- Patented Dec. 11, 1951 CIRCUIT ARRANGEMENT FOR STABILIZING THE AMPLITUDE OF ELECTRICAL OSCIL- LATIONS;

Eric McPhail Leyton, Wimbledon, London, England, assignor' to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain:

Application October .27, 1949, Serial No. 123,832 In Great Britain November 13, 1948 4 Claims.

This invention relates to drive circuit arrangements for applying oscillations to a variable load, for example, for applying oscillations to the modulated valve of a radio-frequency modulating circuit.

In one form of radio-frequency amplitudemodulating circuit, the radio-frequency oscillations are applied from a driving valve to the control electrode-to-cathode circuit of the modulated valve so that the control electrode of the latter valve is driven positive with respect to the cathode during part of each cycle, the modulating signals being also applied to said control electrode-to-cathode circuit. In order to obtain linear modulation it is important that the amplitude of the radio-frequency oscillations should remain approximately constant, but with this form of circuit there is, however, the disadvantage that as the modulation varies the control electrode-to-cathode impedance of the modulated valve also varies due to variations in the current drawn by its control electrode, thereby varying the load which is presented to the driving valve and hence the amplitude of said oscillations. There have been prior proposals for stabilising the amplitude of the radio-frequency oscillations involving the use of ballast resistances, but such proposals require considerable power dissipation in the ballast resistances.

The object of the present invention is to provide an improved drive circuit arrangement for applying oscillations to a variable load, with a view to mitigating the disadvantage referred to.

In accordance with the present invention there is provided a drive circuit arrangement for applying oscillations to a variable load comprising a valve having oscillations applied to its input circuit and having its output circuit coupled to said load, the arrangement being such that in operation said valve is driven to cut-off by the negative excursions of the oscillations applied to said input circuit, and said load being so coupled to said output circuit as to present such a high impedance to said valve that the positive excursions of said applied oscillations bring said valve into a condition of low output impedance, whereby variations in the minimum anode potential due to variations in said load are reduced and the amplitude of the potential variations applied to said load is rendered more constant.

In accordance with one application of the present invention there is provided a modulating circuit comprising a modulated valve and a driving valve arranged to .feed carrier oscillations to said modulated valve to, be modulated therein, whereby said modulated valve presents an impedance to said driving valve which varies in dependence upon the modulation, and wherein said modulated valve is so coupled to the output circuit of said driving valve as to present a high impedance thereto, and said oscillations are applied to the input circuit of said driving valve in such manner that said driving valve is cut-off by negative excursions of said oscillations and is brought to a condition of low output impedance by positive excursions of said oscillations, whereby the amplitude of the oscillations fed to said modulated valve is rendered substantially constant despite variations in the impedance presented by said modulated-valve to said driving valve. 1

In order that the said invention may be clearly understood and readily carried into effect, the

same will now be more fully described With refer-' ence to the accompanying drawing, in which:

Figure 1 illustrates diagrammatically one example of a radio-frequency modulating circuit embodying one example of the present invention, and

Figure 2 comprises characteristic curves of one of the valves of the circuit illustrated in Figure 1.

Referring to the drawing, the circuit illustrated is an amplitude-modulating circuit of a television transmitter and comprises a source I of radio-frequency oscillations, the frequency of said oscillations being for example in the region of 50 to inc/second. The source I is coupled to the control electrode of a driving valve 2 by a blocking condenser 3 and resistance 3. The valve 2 has a grounded cathode and is arranged to op erate as a class B or a class C amplifier. It has an anode load impedance comprising a circuit 5 tuned to the frequency of the oscillations the source I, the circuit 5 being constructed in known manner as a line circuit. The circuit. 5 is connected to the positive terminal '5 of an T. source. supplying for example a D. C. potential of 5 kilovolts. An intermediate tap on the ductance of the circuit 5 is coupledby a blocking condenser 8 and radio-frequency choke 9 to the cathode of a modulated valve H2. The. valve it is arranged to operate as a class B or a class C amplifier, and its control electrode is grounded via a source of the modulating signals, indicated in block form at H, the modulating signals in this example being television video signals interspersed with synchronising signals. The anode load of the valve I0 is in the form of a tuned line circuit. lisimilar to the circuit 15, the amplitudes .senting a very low impedance.

modulated radio-frequency oscillations being derived from the circuit I2 in any suitable manner.

The valve 2, which is shown as a triode, is arranged to be or the kind which has a very low output impedance at its anode when operated at low anode potentials and highly positive control electrode potentials. Figure 2 illustrates a set of idealised characteristic curves for a valve possessing this property and it will be assumed that the curves are applicable to the valve 2. The abscissae of the curves represent the anode potential of the valve expressed in kilovolts while the ordinates represent the anode current expressed in amperes. The numbers at the righthand end of the curves indicate the appropriate control electrode potentials. It will be seen that the slopes of the curves, for low anode potentials and highly positive control electrode potentials are very steep, and convergent on a single almost vertical line l3, shown dotted, this line being sometimes referred to as a diode line and repre- Moreover, the modulated valve I is so coupled to the anode of the driving valve 2 by the circuit 5 that the load impedance presented at said anode is very high, this impedance being for example represented, for a given amplitude of modulating signal from the source II, by the reciprocal of the slope of the full line H in Figure 2. Due, however, to amplitude variations of the modulating signals the impedance presented at the anode of the driving valve 2 is liable to vary, say, between the limits represented by the chain-dotted lines and I6. In order to reduce the undesired regulation of the output of the driving valve which such impedance variations would be liable to produce, the radio-frequency oscillations from the source i are arranged to be of large amplitude, so that the valve 2 is driven hard and the maxirna of the applied oscillations lower the anode potential of said valve to such a low value that the valve 2 operates efiectively along the line l3 and hence with a very low output impedance at its anode. Variations of the load impedance then produce substantially no variation in the potential minima set up at the anode of the valve 2. This is illustrated in Figure 2 which shows that if the maxima of the radio-frequency oscillations from the source I drive the control electrode of the valve 2 to a potential of the order of 500 volts positive, and the load impedance varies between the limits represented by the lines l5 and IS, the potential minima at the anode only vary between the values represented by the points I! and It. The potential maxima at the anode of the valve 2 have the value of the D. C. potential at the terminal 1, represented by the point IS in the present example, since the valve 2 is driven to cut-off during negative half cycles of the oscillations from the source I. It can thus be seen that the amplitude variations in the output potential set up at the anode of the valve 2 are only a small fraction of the mean amplitude. By coupling the anode of the valve 2 to the cathode of the modulated valve [0 by means of a tuned line circuit, very tight coupling with little leakage reactance is achieved between the anode of the valve 2 and the cathode of the valve ID. The amplitude of the radio-frequency oscillations fed to the cathode of the valve In is thus maintained sensibly constant.

The valve 2 may, for example, be an ACT 26 valve which is a forced air cooled triode valve manufactured by the M. 0. Valve Company Limited, Brook Green, Hammersmith, London, W. 6, England. It will, however, be appreciated that other valves such as tetrode valves, may alternatively be employed provided they have the aforesaid property illustrated with reference to the valve 2. The valve In may be. for example, a CAT 21 valve, which is a water cooled triode manufactured by the aforesaid M. 0. Valve Company Limited. Moreover, while in the modulator circuit illustrated only one driving valve 2 and one modulated valve I0 is shown, in practical applications a pair of driving valves and a pair of modulated valves will usually be employed, operated in push-pull. Furthermore, the invention is not limited in its application to television transmitting apparatus.

What I claim is:

i. A drive circuit arrangement for applying oscillations to a varying load, comprising an electron discharge valve having a cathode, a control electrode and an output electrode, said valve having a low impedance for low values of potential difference between said output electrode and said cathode, a load, means coupling said load in circuit with the output electrode of said valve with said load presenting a high impedance to said valve as compared with the firstmentioned impedance, and means for applying large amplitude oscillations between the control electrode and cathode of said valve to cause halt cycles of one polarity to drive said valve to cathode current cut-off and half cycles of opposite polarity to lower the potential difference between said output electrode and said cathode to a low value with said valve in a condition of low impedance, whereby variations in the minimum potential at, the output electrode of said valve due to variations in said load are reduced and the amplitude of potential variations applied to said load is rendered substantially constant.

2. A modulating circuit comprising a driving electron discharge valve having a cathode, a control electrode, and an output electrode, said valve having a low impedance for low values of potential diirerence between said output electrode and said cathode, a modulated electron discharge valve, means coupling said modulated valve to said driving valve with the modulated valve presenting a high impedance to said driving valve as compared with the first-mentioned impedance, a source of carrier oscillations, means for applying said carrier oscillations between the control electrode and cathode of the driving valve to drive said modulated valve with said carrier oscillations, and means for modulating the impedance of the modulated valve to modulate said carrier oscillations, the said source being proportioned to produce carrier oscillations of large amplitude to cause half cycles of one polarity to bring the driving valve to cathode current cut-oil and hall cycles of opposite polarity to lower the potential diiference between said output electrode and said cathode to a low value with said driving valve in a condition of low impedance, whereby the amplitude of the carrier oscillations fed from said driving valve to said modulated valve is rendered substantially constant.

3. A modulating circuit comprising a driving electron discharge valve having a cathode, a control electrode, and an output electrode, said valve having a low impedance for low values of potential diiierence between said output electrode and said cathode, a modulated valve having a cathode, a control electrode and an output electrode, means coupling the output electrode or said drivins valve to the cathode of said modulated valve with said modulated valve presenting a high impedance to said driving valve as compared with the first-mentioned impedance, a source of carrier oscillations, means for applying the carrier oscillations between the control electrode and cathode of said driving valve to drive said modulated valve at its cathode with said carrier oscillations, means including a source of modulating signals connecting the control electrode of said modulated valve to ground, whereby the impedance of said modulated valve is varied in dependance upon the modulating signals, said source being proportioned to produce large amplitude oscillations to cause half cycles of one polarity to bring the driving valve to cathode current cut-off and half cycles of opposite polarity to lower the potential difference between the output electrode and cathode of the driving valve to a low value with said driving valve in a 7 4. A modulating circuit according to claim 3, said means coupling the output electrode of said driving valve to the modulated valve comprising a line circuit in the lead to the output electrode of said driving valve tuned to the frequency of said carrier oscillations, and a connection to the cathode of said modulated valve from a point in said line circuit positioned to cause a voltage step-down between said output electrode and said cathode.

ERIC MCPHAIL LE'YTON.

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

UNITED STATES PATENTS Number Name Date 2,393,785 Leeds Jan. 29, 1946 2,436,802 Glessner Mar. 2, 1948

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