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Wideband Waveform Generator Schematic Circuit Diagram

This circuit is designed to provide a wideband digital sine wave signal source. Its main feature is that because it synthesizes the signal in 32 steps, no low-pass filter is required to suppress the odd harmonics.

Wideband Waveform Generator Schematic Circuit Diagram

Synthesizing Sine Waves: Traditional Approach and Limitation

A common technique for generating a controlled sine wave involves applying a low-pass filter to a square wave of the same frequency, encompassing not only the fundamental frequency but also odd harmonics. By filtering out these components, a clean sine wave at the desired frequency remains. However, this method is constrained by the corner frequency of the lowpass filter, limiting the range of usable frequencies.

Innovative Approach: Increasing Voltage Levels for Smoother Sine Wave

The solution depicted in Figure 1 eliminates the need for low-pass filtering by employing more than just ‘high’ and ‘low’ voltage levels. Instead, it incorporates 16 distinct voltage levels that follow one another across a sequence of 32 samples. The voltage steps are controlled by outputs Q0-Q3 of counter IC1, with Q4 altering the output polarity in the second half of the period. While this approach doesn’t entirely eradicate odd harmonics, it significantly diminishes their presence. Although the signal still contains steps, they are greatly attenuated, resulting in a much smoother output.

Wideband Waveform Generator Schematic Circuit Diagram 2

Generating 16 Voltage Levels for Operational Amplifier

Resistors R1-R4 collaborate to furnish operational amplifier IC3 with 16 distinct voltage levels. R5 and R6 maintain the non-inverting input of IC3 (pin 3) at half the supply voltage, causing the operational amplifier to function as an inverting amplifier. Feedback resistor R7 ensures this operation. For precise symmetry in the output signal, employing a potentiometer for fine adjustment of this path is advisable. Before measuring distortion, fine adjustment was performed, resulting in a Total Harmonic Distortion plus Noise (THD+N) figure of less than 10% (over a 22 kHz bandwidth) and less than 13% (over a 500 kHz bandwidth) at an input frequency of 32 kHz, producing an output frequency of 1 kHz, as displayed in Figure 2.

Flexible Output Waveform Determined by Resistors

The output waveform’s shape, such as a sine wave in this case, is dictated by the ratios among resistors R1 to R4, allowing ample room for experimentation. It’s crucial to ensure that the clock frequency input to the counter is consistently 32 times the intended output frequency for proper functioning.

Addressing DC Offset and Operating Range

The operational amplifier’s output carries a DC offset equal to half the supply voltage. If this offset interferes with the driven circuit, a coupling capacitor C4 must be incorporated. The value of this capacitor should increase with decreasing operating frequency and lower load impedance, tailoring it to the specific circuit requirements.

Power Supply Range and Current Consumption

The circuit operates within a supply voltage range of +5 V to +15 V, determining the required input clock signal’s amplitude to drive counter IC1. The output signal’s amplitude can be adjusted using resistor R7, and this adjustment remains independent of the output waveform’s characteristics. The waveform generator’s current consumption is approximately 3 mA.

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