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Issue #197 December 2006
ARM Scope
Build an Analog Oscilloscope Companion
by Greg Cloutier
Greg’s ARM Scope improves upon ordinary analog oscilloscopes that don’t provide suitable results at low speeds. The LPC2138-based system features a timed PWM output for the more precise calibration of the oscilloscope as well as an analog input and an analog output with a graphical display. As a result, the displayed signal is more accurate at high or low speeds.
Start | Hardware Overview | Embedded Code | User Interface | Functional Code | System Development | Sources & PDF
At work, I have access to a host of high-end instrumentation. Having access to such equipment spoiled me after a while, and I felt like something was missing when I was working on projects at my house. Although my digital multimeter worked well for most projects, I really wanted to be able to see signals as well. I couldn’t justify spending a lot of money on new test equipment, so it looked like I would need to settle for an older analog oscilloscope. After spending some time searching the Internet for an affordable system, I ended up with the winning bid on an auction site for a Hewlett-Packard HP1742A oscilloscope. Spending $40 on a 100-MHz oscilloscope was worth it for me. The oscilloscope worked out great and has been a welcome addition to the small lab in my house.
Once I had the oscilloscope hooked up, I thought that the void was filled. But a problem with an old analog oscilloscope is that it can’t provide much useful information at low speed. The display is not retained, and you are left guessing at what the signal looked like. Another problem is that I am never too sure about the calibration throughout the timebase range. The on-board calibration pin is for voltage, while the frequency is not specified. Even if the frequency were specified, it wouldn’t necessarily be useful throughout the full timebase range.
Recognizing these issues, I designed an analog oscilloscope companion to help the system achieve complete functionality (see Photo 1). The ARM Scope does not duplicate what the old analog oscilloscope does well. It simply adds some functionality where it’s needed most.
The companion requires three functions to make a nice addition. The first function is a pulse-width modulator (PWM) source that can be used to calibrate or verify the oscilloscope’s timebase throughout its entire range. The second function is a low-speed capture and display that enables me to visualize the slower signals that the oscilloscope can’t retain. While I was at it, I decided that it would be nice to be able to source an arbitrary analog signal. This can be used to test a filter or drive a sensor. Signal generation is also useful for test and measurement applications.
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