Home | <Signal Processing Front-End (soundcard-Oscilloscope) | OSignal processing Back-End (FPGA-Signal Generator)>
Consumer sound cards are not intended to supply a lot of power. They serve as signal source or sink for external audio systems and operate on voltages of a few volts. Line level for consumer cards is defined as -10 dB at a reference voltage of 1 VRMS, which equals 447 mVpeak for sine waves. As manufacturers push the limits to advertise the loudness of their devices, many devices actually provide more, including all devices the user application was tested on. Measurements yielded peak voltages between 1-2Vpeak, which exceeds the nominal line level by far. Still, for a signal generator higher amplitudes are desirable. Therefore an analog output stage was developed. The output stage amplifies the signal and provides variable DC offsets. It is connected by a 3.5mm TRS audio jack to the sound card. This ensures a maximum of compatibility. A USB connector provides the necessary power. In the this diagram, the functionality is shown schematically. More details about individual parts can be found in the following sections.
Just to control a DAC, a microcontroller seems to contradict the low-cost goal, but since the here designed hardware is subject for later integration on a final printed circuit board (PCB) with the sound card oscilloscope of the OpenLab project, it is possible to share some resources. Hence a microcontroller, namely an ATmega16U2 from Atmel , could be utilized without increasing the final price too much.
The content of his chapter is taken from the master thesis "Development of a Sound Card Based Arbitrary Waveform Generator" by Jakob Kriegbaum, MSc, May 2017.
ATMEL: 8-bit Microcontroller with 8/16/32K Bytes of ISP Flash and USB Controller, 2012