Rectangular/Triangular Waveform Converter

Description: Many function generators utilize a rectangular waveform generator that comprises a Schmitt trigger and an integrator. The triangular signal generated by the integrator is subsequently transformed into a sinusoidal signal using a diode network. The converter described here operates inversely, converting the output of a high-quality sine-wave oscillator into both rectangular and triangular signals. The sinusoidal signal is converted to a rectangular signal by IC2A. Since the output of this gate fluctuates between -15 V and +15 V, it is adjusted to a suitable level for integration using a potential divider formed by resistors R3 and R4. This adjusted signal is then integrated by a transconductance amplifier, IC1A, along with capacitor C2. The amplifier's current output is regulated by the current flowing through pin 1, causing the output to behave like a resistance, which allows for manipulation of the integration time. The voltage across C2 is available in buffered form at the output of impedance inverter IC2B, representing the triangular signal. The amplitude of this triangular signal is compared with a voltage set by potentiometer P2, and the difference between these voltages, which is the output of IC2C, is fed to the current source at the output of IC1 through resistor R5. This configuration ensures that the output voltage remains largely unaffected by the frequency of the rectangular signal or the sinusoidal input. A challenge in precision integration is the influence of offset voltages and bias currents. The feedback loop consisting of R6 and C1 ensures accurate tracking of the potential across R4. However, minor deviations may arise from the bias current in circuit IC1, which does not exceed 8 µA at 70°C. The time constant formed by R6 and C1 is intentionally large to guarantee that the triangular signal, even at low frequencies, does not distort the integrated signal, maintaining the rectangular shape. The converter is capable of processing signals with frequencies ranging from 6 Hz, where amplitude remains unaffected, to 60 kHz, where the amplitude is reduced to 10%. Due to the extended time constants, the recovery time for the amplitude of the triangular signal at frequencies above 1 kHz is relatively prolonged. The peak value of this signal should be set to 1 V. Diode D1 is a stabistor, which consists of three diodes in a single package, and it may be substituted with three discrete 1N4148 diodes. The current consumed by the converter is approximately 9 mA.

The described circuit operates as a signal converter, transforming sinusoidal waveforms into rectangular and triangular outputs. The primary component, IC2A, functions as a comparator, converting the sinusoidal input into a rectangular waveform. The potential divider R3/R4 plays a critical role in scaling the output voltage from IC2A to a range compatible with the subsequent integration stage. The integration process is carried out by the transconductance amplifier IC1A, which outputs a current proportional to the input voltage. The capacitor C2 integrates this current, creating a triangular waveform that is buffered by IC2B to ensure a stable output.

The feedback mechanism involving R6 and C1 is essential for maintaining accuracy in the presence of offset voltages and bias currents. The large time constant is a design choice that mitigates the impact of low-frequency signals on the rectangular waveform, preserving its integrity during the integration process. The circuit's operational range from 6 Hz to 60 kHz indicates its versatility in handling various signal frequencies, while the specification that the amplitude is reduced by only 10% at the upper frequency limit signifies its effectiveness.

The choice of using a stabistor diode, which can be replaced by discrete diodes, adds flexibility to the design, allowing for easy sourcing of components. The current consumption of 9 mA suggests that the circuit is efficient while still providing the necessary performance for signal processing applications. Overall, this converter design provides a robust solution for generating triangular and rectangular signals from a sinusoidal input, suitable for various electronic applications. Many function generators are based on a rectangular waveform generator that consists of a Schmitt trigger and integrator. The triangular signal produced by the integrator is then used to form a sinusoidal signal with the aid of a diode network. The converter presented here works the other way around. It converts the output of a good-quality sine-wave oscillator into a rectangular and a triangular signal.

The sinusoidal signal is converted into a rectangular signal by IC2A. Because the output of this gate varies between -15 V and +15 V, it is reduced to a value that is suitable for integration by potential divider R3/R4. It is then integrated by transconductance amplifier IC1A and C2. The amplifier has a current output that is controlled by the current through pin 1. The output therefore behaves as a resistance, with which it is possible to influence the integration time.

The voltage across C2 is available in buffered form at the output of impedance inverter IC2B; this is the triangular signal. The amplitude of this signal is compared with a voltage set by P2 and the difference between these voltages, which is the output of IC2C, is applied to the current source at the output of IC1 via R5.

This arrangement ensures that the level of the output voltage is virtually independent of the frequency of the rectangular signal or of the sinusoidal input. One problem with a precision integration is its being affected by offset voltages and bias currents. Feedback loop R6/C1 ensures that the output follows the potential across R4 accurately. However, tiny deviations might be caused by the bias current in circuit IC1, which is not greater than 8 at 70°C.

The time constant R6/C1 is large for a purpose: to ensure that the triangular signal, even at low frequencies, cannot affect the waveform of the signal to be integrated—the rectangular shape must be retained. The converter can process signals at frequencies from 6 Hz (where the amplitude is not affected) to 60 kHz (where the amplitude is reached by 10%).

Because of the long time constants, the time taken for the recovery of the amplitude of the triangular signal at frequencies above 1 kHz is rather long. The peak value of this signal should be set to 1 V. Diode D1 is a so-called stabistor—three diodes in one package. It might be replaced by three discrete type 1N4148 diodes. The current drawn by the converter is of the order of 9 mA.