Pulse Duration Modulators:
A fundamental aspect of pulse duration modulators involves generating a perfect triangular signal characterized by absolute symmetry and straight sides, as any deviation from this ideal form leads to distortion. The conventional design employs a rectangular-triangular oscillator, where a Schmitt trigger transforms a triangular wave into a rectangular one. This trigger, formed by XOR gates IC1a and IC1b configured as inverters, facilitates continuous charging and discharging of a capacitor through positive and negative current sources (T1 and Tr, respectively). Unused gates within the IC remain available for potential applications, such as a pulse duration modulator. The hysteresis of the converter is determined by resistors R1 and R2.
Signal Switching and Voltage Regulation:
The alternating activation of transistors T1 and T2 is contingent upon the output of the trigger. When the output of IC1b is high, T2 conducts, and when it is low, T1 is switched on. The consistency of the base voltage of T1 and consequently the potential across emitter resistor R7 is maintained at a constant level when the output of IC1b is low, thanks to resistors R3 and R4. This stability ensures that capacitor C1 is charged via T1 with a steady current. The rapid switch-off of T1 is facilitated by diode D1. Transistor T2 operates similarly, with its activation tied to the high output of IC1b. It is crucial to note that the amplitude of the triangular signal must not exceed the base voltage of T1 and T2. For instance, assuming a 10 V supply voltage, the amplitude of the triangular signal should not surpass 2.5 Vpp.
Opamp 1C2 as Signal Buffer:
Utilized as a buffer between the output and input of the Schmitt trigger, Opamp 1C2 plays a crucial role in maintaining signal integrity. As a high-quality, fast type with a rate of 200 V/μs, it has minimal impact on the triangular signal’s shape and quality due to its superior performance. In situations where it is necessary, slight adjustments to the symmetry of the triangular signal can be made by connecting a low-value (11a preset) in series with either R3 or R6. The chosen resistor’s value should be reduced by half the preset value for optimal correction.
Frequency Generation and Circuit Characteristics:
Capable of generating signals with frequencies reaching up to 300 kHz, the circuit, configured with the provided values, yields an output frequency of 38 kHz. Modifying the value of C1 allows for adjustments in the output frequency. The circuit itself draws a current of approximately 8 mA, with 5 mA directed through IC2. These characteristics collectively contribute to the circuit’s versatility and adaptability in generating signals across a range of frequencies while maintaining a moderate current draw.