GB885459A - Improvements relating to electronic sawtooth generating circuits - Google Patents

Abstract

885,459. Semi-conductor sawtooth generating circuits. DAWE INSTRUMENTS Ltd. April 25, 1960 [April 24, 1959], No. 14171/59. Class 40(6). A sawtooth generator comprises a capacitor in parallel with a four-layer diode and in series with a constant current device. In Fig. 1, capacitor 2 is charged linearly from transistor 3 until the critical voltage of the fourlayer diode 1 is reached when the diode breaks down, discharging capacitor 2. The applied voltage between terminals 10, 11 may be larger than the diode critical voltage so that the circuit is free-running. The circuit may be synchronized by pulses of the required frequency or of a sub-multiple of that frequency applied to terminal 9. If the applied voltage between terminals 10, 11 is less than the critical voltage of the diode 1, each sweep can be terminated by a trigger pulse fed to terminal 9. The rate of charge and hence the frequency of a free-running circuit is controlled by variable resistor 4 calibrated in terms of frequency. The output may alternatively be frequency modulated by a sine wave at terminal 8. Resistor 5 is made very much larger than the input resistance. In Fig. 2, the base/emitter supply is provided by an inert cell 12 or a Zener diode fed by resistor 13. A diode 14 is provided to increase the impedance to synchronizing signals at terminal 9. Diode 14 can be biased to allow the circuit to operate from a lower supply voltage. A step-up transformer may be connected in series with diode 1 to provide high voltage pulses. Fig. 3 (not shown) relates to a similar circuit in which the charging current is controlled by variation of a resistor forming part of a potential divider across a D.C. source. The frequency is then proportional to the value of the variable resistor.