Patent GB-1020343-A (GB)
Title: Improvements relating to control circuits for signal generators especially time base generators of cathode ray oscilloscopes
IPC: G01R13/24
Filed: 1962-11-09
Abstract: 1,020,343. Transistor circuits; cathode-ray tube circuits. TEKTRONIX Inc. Nov. 9, 1962, [March 23, 1962], No. 42537/62. Headings H3T and H4T. A circuit for generating a time base or other waveform is arranged to be self-oscillating in the absence of incoming trigger pulses but is provided with a switching circuit to inhibit selfoscillation when trigger pulses are received The circuit shown is a time base generator for a cathode-ray oscilloscope having three modes of operation selected by switch 68. Free running operation.-Switch 68 is set to FREE RUN. The sawtooth generator 14 utilizes a Miller circuit comprising an amplifying transistor 114 having feed-back via a Miller capacitor 106 and a cathode follower 112. The sweep is fed via an emitter follower 118 and a diode 120 to the base of transistor 84 so that when the waveform reaches a predetermined value, transistor 84 reduces the current in a bi-stable tunnel diode 80 so as to switch the diode to its low voltage state. The voltage change at the anode of tunnel diode 80 is inverted by the NPN transistor 96, cutting off diode 98 and allowing capacitor 106 to discharge via diodes 102, 104 and transistor 100, thus terminating the sweep. Capacitor 122 lengthens the flyback of the waveform at the base of transistor 84 so as to prevent the tunnel diode 80 being re-triggered by any pulses via diode 42 until the Miller capacitor 106 has fully discharged. Triggered operation.-When switch 68 is set to NORM, the current supplied by transistor 84 to the tunnel diode 80 is such that it can only be switched to its high voltage state by a trigger pulse via diode 42. Such trigger pulses are derived from the vertical deflection waveform at 16 after limiting by transistor 18 and tunnel diode 24. The limited waveform is differentiated by an auto-transformer 38, 40 and the resulting positive pulses passed via diode 42 to trigger the bi-stable tunnel diode 80 and hence initiate a sweep waveform in circuit 14. Automatic changeover from free running to triggered operation.-When switch 68 is set to AUTO, the current in transistor 84 is such as to allow the circuit 14 to be self-oscillating until a vertical deflection waveform is received at 16, when the circuit acts as a triggered time base. The waveform at 16 is limited at 18, 24, integrated at 34 and differentiated at 45, giving a short positive pulse to switch a bi-stable tunnel diode 48 to its high voltage state. Transistor 56 is thus turned on, discharging capacitor 71 until diode 78 becomes conductive, when the tunnel diode 48 is restored to its low voltage state, turning off transistor 56. In the absence of a further signal at 16, capacitor 74 re-charges until transistor 70 becomes conductive, increasing the current in transistor 84 and switching the tunnel diode 80 to its high voltage state to allow the sweep circuit 14 to resume free running operation. If another signal is received at 16 before capacitor 74 is recharged, tunnel diode 48 will again be switched to its high voltage state, discharging capacitor 74 and preventing circuit 14 from running freely. A C.R.T. blanking pulse is available at 97.