How ICL 8038 Works Part II: Detailed Discussion on Duty Cycle, Sine Distortion, and Frequency Sweep Adjustment of ICL8038

In my previous Instructables article, I introduced the ICL8038 and explained the basic setup using fixed resistors. In this article, I explain how to develop better adjustable waveform applications using potentiometers

Link to my previous article: How ICL8038 Works ?

In this article I explains adjustment circuits via Proteus Simulations.

The Intersil datasheet presents two example circuits related to duty cycle adjustment.

The first circuit (Figure 3A) suggests keeping the timing potentiometers RA and RB separate for precise adjustments.

The second example (Figure 3B) provides a more convenient connection for adjusting the duty cycle around 50%. To ensure a precise 50% duty cycle, it is recommended to use a 2kΩ or 5kΩ potentiometer instead of a 1kΩ, which may not allow for adequate adjustments across all devices. The frequency can be calculated using the formula f = 0.33 / RC when RA = RB = R.

I preferred to study the second example circuit in Proteus. I couldn't successfully perform a valid simulation with a 1 kΩ potentiometer, but I achieved a successful simulation using a 3 kΩ potentiometer.

The ICL8038 datasheet outlines three methods for minimizing sine wave distortion, providing detailed instructions for each:

Using a Fixed Resistor: In the basic test circuit, a fixed 82kΩ resistor is used between pins 11 and 12. This setup provides a baseline for sine wave distortion.

Using a Variable Resistor: It suggests that replacing the fixed 82kΩ resistor with a variable resistor allows for distortion to be reduced to less than 1%. In Figure 3B a 100K potantiometer is used.

Using Two Potentiometers: For even further improvement, it recommends connecting two potentiometers, as illustrated in Figure 4 of the datasheet. This configuration can lower sine wave distortion to approximately 0.5%.

Simulation results for 100K potantiometer is presented for 100 K, 50 K and 1K

The configuration in Figure 4 of the datasheet can lower sine wave distortion to approximately 0.5%. Additionally, there is a 10kΩ resistor along with the 100kΩ potentiometers. I would like to explain the role of these resistors.

First 10kΩ Resistor (Between Pin 12 and VCC):

This resistor is connected between pin 12 and the VCC (positive supply voltage). In this configuration, the resistor plays a crucial role in establishing a specific voltage level for the sine wave generation process. It helps stabilize the voltage levels at pin 12, ensuring that the ICL8038 operates within its optimal parameters. By providing a consistent voltage reference, this resistor minimizes fluctuations and contributes to a more reliable output waveform. This stabilization is essential for reducing distortion in the generated sine wave, resulting in a cleaner and more accurate signal.

Second 10kΩ Resistor (Between Pin 1 and VEE):

This resistor is positioned between pin 1 and VEE (negative supply voltage). In this arrangement, the second 10kΩ resistor serves as a reference point for the output signal. By connecting pin 1 to VEE through this resistor, it helps set the baseline for the waveform generated by the ICL8038. This reference level is critical for ensuring that the output signal is accurately produced and maintains its integrity. Properly configuring this resistor ensures that the sine wave is generated correctly, further enhancing the performance of the circuit by contributing to a more stable output.

In the context of the ICL8038 function generator, FM Bias and Frequency Sweep are related to how the output frequency of the signal can be altered.

FM bias refers to a DC voltage that is applied to establish a baseline or reference frequency for frequency modulation. In the ICL8038. Voltage applied to pin 7 affects the basic frequency of the oscillator.

A Frequency Sweep refers to a gradual change in the frequency of the output signal over a set range. In the ICL8038, a frequency sweep is typically achieved by varying the voltage applied to the frequency control input pin 8. As this control voltage changes, the output frequency will smoothly "sweep" across a specified range.

In the ICL8038 datasheet, two sample configurations for frequency adjustments are provided.

In the first setup, pin 7 is directly connected to pin 8. This configuration is illustrated in various sample circuits throughout the datasheet.

In the second setup, depicted in Figure 8, a 10K potentiometer is connected to pin 8. Additionally, a coupling capacitor is connected between VCC and pin 8 to facilitate stable frequency adjustments.

On the simulation you can see the frequency output for 100K, 50K and 1K values.

In the attached figure, you can observe the final circuit, which includes settings for duty cycle, sine distortion, and frequency sweep adjustments.