A Soundcard-Oscilloscope

Most of today's personal computers are equipped with quite advanced soundcards. These soundcards often contain powerful analog/digital converters, able of up to 100 ksamples per second at 16 bit or more. Why not use this capability and build a digital storage oscilloscope around it...?

1. History
2. Function
3. Operating

1. Introduction  

When using the soundcard as an oscilloscope, the low input impedance is a large disadvantage. Not only would it influence the measured circuit and possibly shortcircuit low input signals. It also would not allow the usage of commercial oscilloscope probes which normally are designed to work with oscilloscope inputs with a typical impedance of 1 Mohm.
Using a 10:1 oscilloscope probe at a typical soundcard input with an impedance in the order of 1  kOhm would give an attenuation of 10000:1 instead and since the impedance of most soundcards is not even known to the user, a complicated calibration would be necessary.
The bandwidth of the oscilloscope is limited to at max half the sample frequency, i.e. to audio signals below approx. 20kHz for a standard sound card. Since the signals are ac coupled, the lower frequency limit of this adapter is limited to about 16mHz.

2. Function  

The circuit consists of a large bandwidth voltage follower built around a CMOS operational amplifier CA3140. The circuit is not sensitive to the type of the used opamp as long as the input resistance is sufficiently high. In order to simplify the design, a single-voltage operational amplifier should be preferred.
The input and output signals to the circuit are ac coupled through sufficiently high capacitors to guarante a low value for the lower cutoff frequency. Normally the input signals are also ac coupled on the soundcard itself, suppressing dc components and thus limiting the bandwidth towards lower frequencies.
The input impedance for audio range frequencies which make up the usable frequency range of the oscilloscope is determined by the two 2 Mohm resistors which are parallely coupled for ac signals, hence resulting in the standard 1 Mohm impedance. The output impedance is determined by the output impedance of the used opamp and thus easily low enough to adapt to the input of the soundcard.

Schematic of the input adapter for the soundcard oscilloscope.
(click on the image to view it in full size)

3. Operating  

Of course you could now start to write your own software for this project, or you could use the Windows sound recorder (just kidding...). Instead I can recommend a look around on the web were you'll find different solutions both for Linux and Windows platforms.
At this point I can really recommend the use of the AudioTester package containing a great stand-alone digital oscilloscope software. The software is written by Ulrich Müller and you can download a test version at:
http://www.sumuller.de/audiotester

The user interface of the digital storage oscilloscope "osci" showing noise input on both channels.
(click on the image to view it in full size)

Responsible for these pages: U. Zimmermann