After reading about the DIY logic analyzer kits and designs that are out there, I was curious to find out what other sorts of DIY test equipment are available out there in the cyber-inter-web-net-thing. In this article I’ll look at some DIY/open source type oscilloscopes.
How does free sound?
The easiest and cheapest way to get a scope on your bench is to load software onto your PC that uses your sound card to digitize waveforms and display them. Sound card scope software typically supports two channels, deep storage buffers, advanced waveform analysis such as A+B, A-B, real time spectrum analysis, and others. Some also have built in signal generation. However these scopes also have limitations, most notably bandwidth. The common denominator of sound cards is CD quality sampling at 44.1 kHz, giving these scopes a claimed bandwidth of 20 kHz. They have a very limited input voltage range, around 1 volt or less, and are usually A/C coupled. Nevertheless, if you are working on audio circuits these programs may give you all you need.
xoscope is an open source Linux sound card scope that can also connect to a commercial USB mixed signal oscilloscope. Interestingly xoscope has some support for being an MSO by itself, using the parallel port to sample digital data while using the sound card for analog data.
Multi Instrument is a commercial package that is very complete: extensive spectrum analysis options, many signal generation options, data logging, filtering, and much more. It also supports a couple different DSO boxes that give real world sampling rates and input gain settings. There is also a pocket PC version available.
DIY hardware scopes
You can throw a little dedicated hardware at the sampling problem and end up with much better results for still very little money.
The Digital Storage Oscilloscope is available assembled or in kit form from Seeed Studio and SparkFun. This credit card sized scope has an LCD display,1 MHz analog bandwidth, 8 bit x 256 sample depth, and can transfer the screen to a PC as a bitmap
ScopeOnPC is a down and dirty PC based scope that uses an ADC0820 chip and a 74HC257 multiplexer on a solderless breadboard, attached to a parallel port, to provide a 100 KHz sample rate. It’s not a lot better than the sound card oscilloscope, but the parts cost is only about $5 and the software is free.
There are lots of Atmel and PIC based projects that use the built in A/D converter of the microcontroller to do data collection. These converters usually have variable conversion times and low bandwidth so aren’t well suited to this application
Finally there is the retro scope shown in the photo. What it lacks in accuracy it more than makes up in character.
Used equipment
As noted in part one, used test equipment is readily available on ebay, organized and searchable by brand or features. While not DIY, this is a good option for decent quality test equipment at reasonable prices.
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