Oscilloscope Primer

An oscilloscope enables us to display the measured voltage with respect to time. Some of them can show multiple waveform signals. The waveforms can be different due to the voltage sources. The frequency or period and the amplitude of voltages can be read from the display. In 1897, the behavior of cathode ray indicated the varying voltage on the fluorescent screen in Germany. In 1934, the US company, DuMont commercialized the technologies as an oscilloscope.
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Basic Parts of an Oscilloscope


The display can be an analogue or a digital type. The analogue one uses Cathode Ray Tube (CRT), while the digital one uses Liquid Crystal Display (LCD). Although the display of a modern oscilloscope shows many of physical/engineering values, the basic style of the display is associated with horizontal and vertical lines, which is called the graticule. These lines are the references to measure times, voltages, currents, etc.


In order to measure the electrical signals, one has to have probes connected to the BNC (Bayonet Neill-Concelman) connector on an oscilloscope. Normally, the probe consists of two ends: One is a measuring clip colored sometimes in red; and the other is a grounding clip colored in black. The ground is used for the reference and not for measuring the values. In some cases, if the grounding probe touches a wrong terminal, the scope may display a wrong result.


Channels on oscilloscope are refered to as inputs. Each signal from each pair of probes is measured in each chennel. An oscilloscope can measure multiple values to compare them simultaneously. If there are two channels, it is called a two-channel oscilloscope. Each channel has its own set of control to adujust its scale of voltage and other values.

Controls and Indicators

The oscilloscope can manipulate various parameters, such as scales, X-Y coordinates, screen intensity, etc.. Some of them can use mathematical functions to transform the measured values. The control panel consists of dials, toggles, buttons, switches, and connectors.

Functioning of buttons and switches

The basic structure of an oscilloscope is shown in the figure below. According to the type of control, it can be divided into several sections. In this example, there are five sections; and each button, switch, and toggle is labeled by numbers to be explained:

Section A: Control for CRT

Section B: Control for Triggering

Section C: Control for power

Section D: Control for input/output from channels

Section E: Control for time calibration and related adjustment

How to measure with an oscilloscope

The basic procedure

The basic controls of an oscilloscope have been explained in the above section. Now, we will start measuring circuit signals as follows:
    oscilloscope and circuit
  1. Provide an electric circuit and make the probes (coaxial cable) connected to the BNC connector on an oscilloscope.
  2. Clip with the ground probe and the measuring probe across a circuit element as shown in the figure above.
  3. turn it on
  4. Turn on the oscilloscope and the circuit with a proper power supply.
  5. Check if the selection in the vertical control (D-5) matches the connected channel.
  6. trigger sweep
  7. Use the trigger level (B-1 and B-2) if the waveform keeps on moving in the display as shown above. This can stabilize the waveform.
  8. volts div
  9. Select an appropriate VOLTS/DIV control so you can have a proper waveform in the display. This should be as large as possible so you can measure the values accurately. The VOLTS/DIV control allows you to change how many volts are represented by each vertical increment of division overlay on the display. Essentially, this can zoom in and out the signal from the circuit along the y axis.
  10. Measure the number of divisions in the vertical direction from the top peak to the bottom peak. Then, multiply by the indicated value in VOLTS/DIV to calculate the peak to peak voltage. After dividing it by two, you can have the voltage amplitude of the waveform.
  11. time div
  12. Find the period by measuring the horizontal distance for one cycle of the waveform. Then, multiply the indicated value in TIME/DIV control to obtain the period. The TIME/DIV control can change the horizontal increment of the division overlay on the display.

Examples for voltage and period measurement

The procedure 6 to 8 above are the most important steps when finding the physical values of a circuit. After adjusting the waveform of the circuit signal, you are supposed to obtain the following display:
measuring voltage
As instructed, find the number of divisions in the vertical direction. In this example, the value becomes 2.00 div. + 0.80 div.(top fraction) + 0.80 div.(bottom fraction) = 3.60 divisions if you take it from the top peak to the bottom peak as shown. One small tick mark is one fifth of a division; namely, it is 0.2 div.

Then, according to the VOLTS/DIV control, it indicates "50 mV" as the factor of multiplication. Please watch out the units. "mV" means one thousandth of a volt.

Therefore, the peak-to-peak voltage is calculated as 3.60 div. × 50 × 0.001, which is equal to 0.18 volts.

However, this is NOT the voltage detected in the circuit yet. The amplitude of the waveform is the correspondent voltage, so 0.18 volts have to be divided by 2. The voltage amplitude is found out to be 0.090 volts or 90 millivolts.
measuring time
Now, let us find the time period of the signal. The horizontal axis represents time.

A waveform in general repeats one unit of a shape that is called a cycle. Then, the time for one cycle is defined as the period.

For example, you can see the period as the x-range with the red arrow in the figure. The number of division can be counted in the same way as 4.00 div. + 0.50 div(left fraction) + 0.60 div.(right fraction) = 5.10 div.

The TIME/DIV control indicates 50 micro seconds, so the period is computed as 5.10 div. × 50 × 10-6 (or 0.000001) which gives 2.55×10-4 seconds or 0.000255 seconds. (This could be rounded as 2.6×10-4 s.)

In addition, the frequency can be obtained by calculating 1 ÷ period. Thus, we have 1 ÷ 0.000255 = 3920 Hz ∼ 3900 Hz as the frequency of the signal.

Practice for the voltage and period measurements

Problem 1:

quiz for oscilloscope
What are the voltage amplitude, the time period, and the frequency? (Type only numbers, no comma, no symbol. Read it as accurately as possible.)
The voltage amplitude = V
The time period = s
The frequency = Hz

Problem 2:

quiz for oscilloscope
What are the voltage amplitude, the time period, and the frequency? (Type only numbers, no comma, no symbol. Read it as accurately as possible.)
The voltage amplitude = V
The time period = s
The frequency = Hz

Problem 3:

quiz for oscilloscope
What are the voltage amplitude, the time period, and the frequency? (Type only numbers, no comma, no symbol. Read it as accurately as possible.)
The voltage amplitude = V
The time period = s
The frequency = Hz

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