Another DIY Oscilloscope

Some time ago I wrote about DIY test equipment.  I recently found another DIY scope that has freely available plans.  It has a single channel with 40 MHz sample rate, 5 MHz bandwidth, and adjustable trigger delay.  It’s based on an ATMega162 microcontroller with an external A/D chip, and a CPLD that generates the sample clock and places the data into a FIFO.  The display is a 240×128 LCD panel.  The Eagle schematics and PCB information are available, and the layout is single sided so you can make the PCB using the laser printer/iron on method.  All but two of the parts used are through hole.

That’s the good part.  The bad part is that it has 10K ohm input impedance and only DC coupling mode.  It also does triggering logic on the microcontroller meaning that it can miss glitches — samples that are taken while the ATMega is processing data or updating the screen are thrown away.

With a BOM cost of around $50, not including the display or any enclosure, and considering the time required to assemble it, I think it’s a great project for someone learning about electronics.  If I were looking for a piece of test equipment for my bench I would, as I noted in my previous articles, put the $50 towards a used scope from ebay.

Improvements?
A good follow on project for the student who built this project would be to improve its specifications.  First up would be to increase the input impedance.  I think an interesting improvement would be to move the trigger logic into the sampling domain, possibly by using an analog comparator circuit that takes its reference from the microcontroller via a DAC or PWM output.  A faster microcontroller like an mbed could bit bang a VGA interface, allowing you to connect to readily available CRTs or LCD panels.  A touchscreen would also be a nice addition.

Other projects

eOsystems also has a stand alone monitor test pattern generator that could have some interesting applications.

Steve Ravet

EDN Gadget Freak