MadSci Network: Engineering

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

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 

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 

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 

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.

To answer the second part of your question:

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 
3. The frequency measurement, however, will still be accurate. On the 
other hand, it may be very small to see clearly on the screen.

Your mad scientist,

-Aurelio R. Ramos

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