Oscilloscope using PC Sound card

Any serious hobbyist-grade two-channel digital oscilloscope is worth a few hundred bucks, with professional lab models reaching into the few thousands. One way to avoid spending too much on such a versatile equipment is to use a personal desktop or laptop computer as a digital oscilloscope. Commercially available plug-and-play probing interfaces and compatible software and drivers are still quite an expensive option. Alternatively, here is a way to turn any GNU/Linux computer with an integrated sound board into a free digital oscilloscope using free software and custom-built probes. Just follow this step-by-step visual guide.

Requirements

All you need is a GNU/Linux system (I recommend the popular and easy-to-install Ubuntu distribution), which has to support your sound hardware. If you can record stereo music at 44100 Hz (CD-quality) through the microphone or aux input of your sound board under GNU/Linux, you’re all set-up. Just build the special probing cable as instructed hereunder (this easy do-it-yourself project should take less than one hour), install the free Xoscope software package, and fire up your brand new real time desktop oscilloscope!

Do it yourself cable/probe building instructions

The Xoscope software will use your soundboard’s input port (or mic entry) as an interface. We need to build a special measuring cable that will connect to this port. On most sound cards, the mic entry is through a 3.5 mm stereo “mini jack”. We thus need to build a dual probing cable that connects to this mini-jack port through a mini TRS 1/8″ plug. You can either buy a bare TRS stereo mini jack plug, or strip one from an audio/helmet cable that you don’t use.

Quality laboratory equipment probes always use coaxial cables. Coax cables are shielded against electromagnetic interferences, and are designed to carry weak signals over long distance with less distortion than regular electric cords. This is why I recommend using coaxial cords for this project. Those can be found in TV, antenna, or cable box connections. Some (but far from all) high-end audio cables are also coaxial. You can recognize a coaxial cable by cutting it and looking at the way the cable wires and threads are arranged: coaxial cables have a central “core” copper wire that carries the signal, wrapped into a plastic insulation that is itself wrapped inside a threaded outside shielding wire net (or “mesh”). Look at the photograph if you are unsure. I used an old TV/VCR connection cable (you can easily find a lot of those used cables around the house now that most TV sets, satellite, cable boxes, and VCRs went digital… The new digital connections usually do not use coaxial cables, so this is a good project to recycle a few feet of otherwise obsolete cords.)

Due to the precise and weak electronic signals that our probing cords will carry, I advise to keep the length as short as practically usable. 1 meter seems an appropriate maximum length to avoid too much power loss due to cable electric resistance. My cable has two 1 m long probing cords and performs great, carrying sharp signals while being long enough to comfortably reach away from the laptop.

We need two lengths of coax cable to assemble the two oscope probes (remember, we’re aiming for a dual channel scope!). Strip about an inch of the cable at each end using a sharp cutter knife (see pictures). Then carefully proceed to soldering the cable cores to the “tip” and “ring” terminals of the mini-jack plug. The shielding wire meshes should be twisted together and soldered to the “sleeve”

terminal (usually the longest connecting pin on the plug). All parts should be pre-tinned before actual soldering. While soldering, attach a metal clip to the mini-plug to help dissipate heat from the iron (otherwise, plastic parts of the plug will melt). Be careful while soldering to avoid any possible electric contact between the signal cores and the outside shielding thread. This is rather tricky to achieve given the super-small size of the mini-jack plug. You might have to try more than once, and check with an ohmmeter that there is no electrical continuity between each signal core, and between the core and the shielding.

While you are at it, check the continuity between the mini-jack plug tip and the other end of one cable, and between the plug ring and the end of the other cable. Do this before proceeding with wrapping your soldering work in heat-shrinking cable sleeves (unfortunately, the original mini-jack casing cannot accommodate for a pair of thick coax cables). The special shrink-wrap sleeves retract to fit tightly around the plug assembly when slightly heated with a small butane cigarette lighter.

I connected the probing ends of the cable to crocodile clips, and again wrapped the connection in heat-shrinking sleeves for a perfect look. An alternative would be to use probing tips such as the one provided with multimeters, or just a bare copper wire tip. Be careful that the shielding wire net thread should not come into contact with the tip or clips. It should be trimmed and insulated in a way to avoid contact. The shielding mesh is not connected to anything on the probing end of the

cables. It should not come into contact with any metal parts, or you risk turning your cable into some kind of antenna!

Look closely at the photos to get a precise idea of the step-by-step construction of the cable. You can click twice on any photograph to enlarge it.

Installing the Xoscope software

If you’re under Debian or Ubuntu Linux, just type:

sudo apt-get install xoscope

to automatically install the software package and any needed dependencies. The software launcher icon will then be found in the Applications > Sciences menu of Ubuntu.

Plug you probing cable into the audio input jack of your sound card, and start experimenting in your own basement with your new, free digital dual-channel real time oscilloscope!

If you are using another distribution of GNU/Linux or possibly another OS, check out the Xoscope sourceforge site for other binary distribution, or just download the open sources and compile them to your system.