Patent GB-653916-A (GB)
Title: Improvements in pulse generator circuits
IPC: H04N5/12
Filed: 1948-05-21
Abstract: 653,916. Pulse generating circuits. HAZELTINE CORPORATION. May 21, 1948, No. 13835. Convention date, June 7, 1947. [Class 40 (vi)] [Also in Group XL (b)] In a generator for producing pulses of constant duration and limited amplitude the pulse repetition frequency is maintained substantially constant by means of a circuit which is responsive to changes in the D.C. energizing current of the generator and maintains a controlled relationship between the average value of this current and the average value of the energizing potential. The Figure shows the application of such generators as the line and frame synchronizing pulse generators in a television receiver. The line and frame synchronizing signals derived from the television receiver 10 are applied to the phase control units 14 and 13 respectively which compare the phase of these signals with the outputs of the scanning generators 17 and 16 respectively to produce D.C. potentials which are applied to the pulse generators 15a and 15 to maintain a constant phase relationship, as described in U.S.A. Reissue 22,055. The frame pulse generator 15 comprises a blocking oscillator circuit in which the duration of the generated pulses is maintained constant and is determined by the opencircuited delay line 26 included in the grid cathode circuit. The amplitude of the pulses is limited by the resistor 25 which has a value which is much larger than the anode-cathode impedance of valve 18 in the conducting condition. The resonant circuit 21, C is tuned to a frequency which is very much higher than the pulse repetition frequency of the generator and as the anode current starts to flow is caused to "ring" when the regenerative coupling between windings 21, 24 produces a negative resistance in the grid circuit. The grid potential then increases with the amplitude of the H.F. oscillation until the control grid of valve 18 becomes positive with respect to its cathode during positive peaks when the tuned circuit is damped and a pulse is initiated which travels down the delay line 26 and after reflection at its distant end appears at the grid circuit to bias the valve 18 to cut-off, thus completing the cycle. The cathode circuit comprises a resistor capacitor network 27, 28, the time constant of which is short in relation to undesired variations in the D.C. current flow in valve 18 and is long in relation to the pulse repetition frequency. Variations in the average value of this current produce a varying bias voltage across network 27, 28 which in combination with the voltage produced by the potential divider 29, 30 which is thus dependent on the value of the D.C. supply potential determines the grid bias voltage of valve 18. In this way changes in the effective slope of valve 18 or changes in the supply voltage which would effect the pulse repetition frequency produce a change in the average value of the D.C. current in the valve, thus changing the effective grid-bias and maintaining the P.R.F. constant. The D.C. control potential derived from the phase control unit 13 varies the impedance of a valve 32 which is effectively in parallel with resistor 30 of the potential divider, thus varying the bias and in this way the P.R.F. of the pulse generator. In the line pulse generator 15a which is otherwise similar to unit 15 the D.C. potential from the phase control unit 14 controls the P.R.F. of the generator by varying the impedance of a valve 32a which is effectively in shunt with resistor 27a of the resistor-capacitor circuit in the cathode of the oscillator valve.