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• Hardware
Overview •
Embedded Code •
User Interface •
Functional Code •
System Development
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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.
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(Click
here to enlarge)
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Photo
1—Based on the user interface’s setting, the
processor generates a signal that is being
read back in and displayed. At the same time,
the PWM signal is generated and used to verify
my analog oscilloscope timebase. |
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.
