Generate noisy sine waves with a sound card

Texas Instruments LM386

José M Miguel

EDN

Testing audio-noise-reduction circuits, PLLs (phase-locked loops), and audio-frequency filters may require a noisy sine wave, one that is summed with white noise. Using a typical computer sound card, free software, and an external amplifier circuit, you can create a noisy sine wave.

The Generatosaur's user interface is a dialogue-box-style control panel.
Figure 1. The Generatosaur's user interface is a dialogue-box-style control panel.

Free Generatosaur software from Wavosaur [1] turns your sound card into a low-frequency wave generator. It lets you independently choose amplitude, frequency, and waveform for the left and the right channels. The Generatosaur’s user interface is a dialogue-box-style control panel (see Figure 1). If you select a sine wave for the left channel and a white noise for the right channel, you then need only to use an amplifier to add the signals. Figure 2 shows the complete circuit.

Generatosaur is free software that turns your sound card into a low-frequency wave generator.
Figure 2. Generatosaur is free software that turns your sound card into a low-frequency wave generator.

The differential amplifier employs a LM386 audio power amplifier with a supply voltage of 15 V. The output of the LM386 has a self-centered quiescent voltage that is half the power-supply voltage and that requires a blocking capacitor, C3. Resistor R5 sets the output impedance to 50 Ω.

You need the voltage dividers R1/R2 and R3/R4 because the output-voltage range for a standard sound card is 0 to 2 V. Taking into account that the voltage gain of the LM386 amplifier is internally set to 20 and that its output voltage range is 7 V, you need an attenuation factor, K, of 7/(2×20) in each amplifier input. The circuit also includes a selectable 20-dB attenuator that you can invoke with the two DPDT (double-pole/double-throw) switches.

If you need to hear the generated noisy signal, connect a loudspeaker to the output of IC LM386.

References

  1. Generatosaur – low frequency signal tone generator

Materials on the topic

  1. Datasheet Texas Instruments LM386

EDN

You may have to register before you can post comments and get full access to forum.
User Name