Pulse duration modulators 19%; a perfect triangular signal, that is, one that is absolutely symmetrical with straight sides. Any deviation from this results in distortion. The design is the usual rectangular-triangular oscillator. A Schmitt trigger converts a triangular wave into a rectangular one. which is used to continuously charge and discharge a capacitor via current sources. Positive current source T1 arranges the charging and negative current source Tr, the discharging. e Schmitt trigger is formed by XOR gates IC1a and 1-1b, which are connected as inverters. The remaining; gates in the IC are not used here, but are available, for example, for a pulse duration modulator. Resistors R1 and R2 determine the hysteresis of the convertor.
The output of the trigger switches on T1 and T2 alternately. If the output of ICH is high. T2 conducts; if it is low, T1 is switched on. Resistors R3 and R4 ensure that the base voltage of T1, and thus the potential across emitter resistor R7, is constant if the output of ICib is low. This ensures that C1 is charged via T1 with a constant current. Diode D1 arranges T1 to be switched off rapidly. Transistor T2 operates similarly, but when the output of IC lb is high.
The amplitude of the triangular signal must not be greater than the base voltage of T1 and T0. Assuming a supply voltage of 10 V. This means that the amplitude of the triangular signal is 2.5 Vpp.
Opamp 1C2 is a good-quality, fast type (200 V Us-1) that functions as a buffer between the output and the input of the Schmitt trigger. Since it is a good-quality type, it has virtually no effect on the shape and quality of the triangular signal. If required, the symmetry of the triangular signal may be corrected to a small degree by connecting a low-value (1 la preset in series with R3 or R6. The value of the chosen resistor must be reduced by half the value of the preset.
The circuit can generate signals at frequencies up to 300 kHz. With values as shown. the output frequency is 38 kHz. The frequency may be altered by changing
the value of C1.
The circuit draws a current of about 8 mA, of which 5 mA flow through IC2.