### Re: How do you read an anologue oscilliscope?

Date: Mon Mar 18 14:40:01 2002
Posted By: Aurelio Ramos, Grad student, Computer Engineering, Not a member of any institution
Area of science: Engineering
ID: 1015615254.Eg
Message:
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You question has two parts, let's take care of the first part first.

It is unclear what you mean when you say the oscilloscope does not have a
read out. I would assume it has to at least have a CRT display to be of
any use. Analog oscilloscopes normally do not have frequency or amplitude
readouts, but you can obtain those values by looking at the CRT display
and using the right settings.

Analog oscilloscopes have two knobs for both vertical and horizontal
scale. Assuming you are measuring amplitude and frequency of a periodic
signal (like a sinewave or square wave) you can use the CRT (display) and
the current possition of the scale knobs to determine both amplitude and
frequency.

Oscilloscopes also have calibration settings for horizontal and vertical
scale and displacement, and your measurement will only be good if the
oscilloscope was previously calibrated. Because the calibration settings
are changed by knobs in the front panel, this is something you should
check.

Assuming you have a well calibrated analog scope, you can measure
amplitude by measuring the height of the waveform from the tip of a bottom
crest to the tip of a top crest, then divide by two. This height will be a
number of divisions. You have to "eyeball" this measurement. A division is
the distance between one line and the next line, given by a grid on the
CRT display (about one centimeter), so this measurement may be fractional
(1.5 divisions, etc).

Once you have the number of divisions, you convert divisions to volts by
looking at the vertical scale knob.

For example: If the vertical scale knob is set to 5 V/div (volts per
division), and your measurement happens to be 4.5 divisions you have a
wave of amplitude:

5 V/div * 4.5 div = 22.5 V

For the frequency a similar procedure is used. Using your eyes, estimate
the WIDTH of the waveform, from the beginning of an upward crest to the
end of a downward crest. This is the period of one cycle, measured in
divisions.

After you have measured this period, you can convert the measurement from
divisions to time using the scale set in the horizontal scale knob.

For example: if your horizontal scale knob is set to 50 ms/div
(milliseconds per division) and the width of the wave appears to be 6
divisions you have a period of:

10 ms/div * 6 div = 60 ms

You can easily convert the period to the frequency because the frequency
is the inverse of the period.

F = 1 / 60 ms = 16.67 Hz

Points to remember: changing either scale knob will result in a "zoom in"
or "zoom out" in that dimension, and it may be helpful, so that you can
see clearly the number of divisions for height and width. Set those knobs
so you can see all of the wave (at least one cycle) filling as much of the
screen as possible (nice and big).

In order to calibrate the scope, feed a signal of known frequency and
amplitude (say a 1kHz sine wave of 5 volts amplitude), work the math I
have provided "backwards" so you obtain the desired number of divisions,
(you simply DIVIDE the volts or the period by the scale setting) then
rotate the fine tune knobs on the scope so that the wave fills exactly the
right number of divisions vertically and horizontally.

If an oscilloscope is rated at a certain maximum frequency, that means you
cannot use it for higher frequencies than specified. You may try feeding a
higher frequency signal, however, the amplitude measurement will quickly
become inaccurate as the frequency is increased past the maximum.
Eventually, the signal will disappear. The horizontal resolution may be
insufficient as well to provide a good look at the wave.

If you attempted the same experiment with a square wave, the square wave
would quickly loose its "sharp" edges, and become more and more rounded,
until it looked similar to a sine wave, again, loosing some of the visual
information you need.

However, depending on the particular scope you are using, it is possible
you could go very high in frequency before the signal disappeared. So:

1. Yes you can display a higher frequency signal, up to a point, by
decreasing the vertical scale (zooming in) *but*,
2. The amplitude measurement and the wave shape will both be distorted and
inaccurate.
3. The frequency measurement, however, will still be accurate. On the
other hand, it may be very small to see clearly on the screen.

-Aurelio R. Ramos

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