Patent GB-587364-A (GB)
Title: Improvements in or relating to thermionic valve circuits
IPC: H03K4/22
Filed: 1944-08-22
Abstract: 587,364. Pulse generating and frequency dividing circuits. WILLIAMS, F. C., and MOODY, N. F. Aug. 22, 1944, No. 16043. [Class 40 (v)] In a circuit employing a plurality of multi-electrode valves, the potentials of the electrodes pass through a predetermined cycle in response to an impulse, the impulse causing a linear-fall in anode potential-of one valve at the termination of which a second valve effects the return of the circuit to its initial condition. The circuit may be used as a divider or for producing a high-speed time base. The circuit may be arranged to rest in a semi-stable condition at the end of the linear fall until re-triggered back to the initial condition by a second set of pulses. In the quiescent state, Fig. 1, valve V3 conducts and due to the potential drop across resistances R8, R9 the pentode V2 is cut-off at the suppressor grid, all the cathode current passing to the screen grid, the anode voltage being held at the setting of potentiometer P due to diode V1 conducting. A negative triggering signal applied to the suppressor grid of pentode V3 cuts off the valve, the anode potential rising rapidly and due to the coupling condenser C5 the suppressor grid of pentode V2 also rises allowing the valve to pass anode current. The resulting fall in anode potential is applied over coupling condenser C4 to cut-off valve V3, diode V4 maintaining the grid just below cut-off. Due to the condenser C3, the grid of valve V2 is driven almost to cut-off, the anode voltage then falling linearly maintaining the control grid at the appropriate negative value due to the feed-back over condenser C3. As the anode potential approaches earth; the rate of fall decreases and the control grids of valves V2, V3 commence to rise, pentode V3 starts to conduct, the fall in anode potential being fed back over condenser C5 to drive the suppressor grid of pentode V2 negative, thereby cutting off the anode current. The rise in anode potential is fed back to the control grid of valve V3 to make the change over extremely rapid. The condensers C3, C4 charge over resistance R2 to bring the circuit back to its initial condition. The duration of the linear fall may be varied by altering the potential to which the grid resistance R6 is connected so that a non-linearfall may be produced. When operating as a dividing circuit, the pulses are applied to the cathode of diode VI since the circuit cannot then be retriggered until the valve V1 is again conducting. The circuit may also be triggered by negative pulses applied to the grid of valve V3, preferably over an isolating, diode. In a modification, Fig. 3 (not shown), the screen grid of valve V2 is connected to the junction of resistances R8, R9, thus avoiding heavy screen grid current during the quiescent period and allowing the use of tetrodes instead of pentodes. In a further modification, Fig. 4 (not shown), the feed-back from valve V3 to valve V2 is accomplished by the use of a common cathode resistance, valve V3 normally being biassed by a diode to conduct, thereby biassing off valve V2. In the circuit shown in Fig. 5; the tetrode V13 is maintained conducting in the quiescent state, the low anode potential maintaining the cathode of diode VI I below earth so that the valve conducts and biasses off tetrode V12. Negative pulses applied to the cathode of diode V14 cause the anode potential of valve V12 to fall and hence the control grid of valve V13, the anode potential rising and carrying the cathode of diode V11 sufficiently positive for it to cease conducting with the result that tetrode V12 commences to pass anode current. The anode potential of valve V12 falls linearly due to the feed-back across condenser C6 until earth potential is approached. The potential of the control grid of valve V13 rises and the fall in anode potential again cuts off valve V12 over the diode V11 to complete the cycle. In order to reduce the time of triggering, the diode V10 maintains the control grid of valve V12 just above cut-off and since this would result in an unduly low resistance value for resistance R15, an inductance L is employed to offer a high impedance during the linear fall. The circuit shown. in Fig. 6 operates as a divider to count triggering pulses, the number counted being determined by the setting of potentiometer P. Normally, valve V18 passes grid current and valve V17 is biassed off at the suppressor grid. A negative pulse applied to the cathode of diode V15 causes tetrode V18 to cut-off and remove the paralysing bias from the suppressor grid of valve V17 so that it passes anode current, the anode potential then falling linearly. When valve V17 conducts, the anode potential falls and hence the control grid due to condenser C8, the resulting decrease in space current causing the cathode potential to fall. When the anode potential of valve V17 approaches the cathode, the feed-back decreases and the control grids of valve V17, V18 commences to rise, valve V18 conducting and causing V17 to cut-off again at the suppressor grid. The anode potential of valve V17 rises back to the setting of potentiometer P until the next triggering pulse is effective to initiate the next cycle. In a further modification, Fig. 8, the circuit is triggered by pulses applied to the cathode of diode V21 and after the anode potential of valve V22 has fallen linearly to cathode potential, the circuit is held in a semi-stable state until retriggered by a pulse applied to the anode of diode V19. The frequency of the retriggering pulses may be an integral multiple of the triggering frequency. During the quiescent period, valve V23 passes grid current, the low anode potential maintaining valve V22 cut-off at the suppressor grid. The negative triggering pulse lowers the anode potential of valve V22 and the control grid potential of valve V23, the resulting rise in anode potential. causing valve V22 to conduct and the anode potential to fall in a linear manner. Due to the condenser C9, the sudden fall in anode potential drives the control grid negative almost to cut-off, the cathode potential also falling below earth. The bias developed across diode V19 remains sufficient to prevent positive retriggering pulses, applied to the anode of valve V19, from being effective. The anode potential of valve V22 continues to fall until the cathode potential is approached when the control grid, anode and cathode potentials rise to a semi-stable state, the control grid and cathode assuming the value of the quiescent condition, the cathode of diode V19 rises nearly to the anode potential and due to the long time constant circuit C10, R35 the grid V23 is maintained at this potential, the valve remaining biassed off. The bias across diode VJ 9 is thus reduced and the next retriggering pulse is effective to cause valve V23 to conduct, and the circuit to restore to the normal condition. Specification 582,758 is referred to.