Oscilloscopes - Vertical Scale, Horizontal Scale, and Trigger

This post became public before I finished it, so it was finished in a hurry. Let me know if you spot an error.

Introduction

I've been working on these tutorials for some time. I originally started with the idea to use only videos, but the goal of the reader should be to learn. Having said that, I think a combination of text, picture, and video is best. To inform the reader, this tutorial will be about as basic as it gets. I'll assume you have some mathematical background and understand time-varying voltage and current.

This tutorial will feature the TEKTRONIX TBS1202B-EDU OSCILLOSCOPETEKTRONIX TBS1202B-EDU OSCILLOSCOPE, which was graciously given to me by Element14 to create these tutorials.

Figure 1: TEKTRONIX TBS1202B-EDU OSCILLOSCOPE (Newark)

There are three basic controls all oscilloscopes will have. These controls will be:

1. The horizontal scale adjustment
2. The vertical scale adjustment
3. The trigger system.

This tutorial is going to discuss these three functions and hopefully place them in context when using an oscilloscope.

Background Material

There is a good chance you already know this theory, but a quick reminder is always nice, Let's explore the basic equation of a sine wave:

where g(t) is our function,

A is the amplitude of the sine wave,

f is the frequency of the sine wave,

and t is time.

Figure 2 shows a sine wave with amplitude 3.3 Volts and frequency 2300 Hz that I plotted in Excel.

Figure 2: 3.3V, 2300 Hz sine wave.

The frequency defines the repetition rate of the signal and has units of [cycles/second]. Following along with Fig 2, at 0 microseconds (1 microsecond = 1 μs = 1 millionth of a second) the signal amplitude is 0 Volts. The signal amplitude rises to 3.3 Volts just after 100 μs. Just after 200 μs the amplitude is back to 0 Volts. The signal goes through a negative half cycle and ends up back at 0 Volts at approximately 433 μs. The frequency is therefore:

We're going to use square waves to discuss the scaling systems. For a sine wave the amplitude is considered half the entire span of the function. This is because the amplitude comes from the mathematical representation. For a square wave, the amplitude is the entire distance from peak to valley (highest voltage to lowest voltage). Square waves are typically characterized by amplitude and offset.

The Vertical Scale

The vertical scale is controlled by a rotary knob on the unit and adjusts the number of Volts per division on the instrument. The knob for the TBS1202B-EDU is reproduced in Figure 3. The easiest way to explain something, sometimes, is to just give an example. In Figure 3 I have reproduced a square wave captured by my TBS1202B-EDU. On the right side of the figure I have shown the vertical scaling as 1 Volt per division. For every division on the vertical scale (up and down) 1 Volt is represented. Looking at the left side I have shown that there are 5 divisions in the amplitude of this waveform, and it, therefore, spans 5 Volts. I.e. 5 divisions at 1 Volt per division is 5 Volts. The channel 1 marker on the left side shows where the 0 Volt level or ground is located.

Figure 3: Square Wave Captured on the TBS1202B-EDU with 1 Volt per Division.

For the exact same waveform I have adjusted the vertical scale so that it is 2 Volts per division. The waveform captured is shown in Figure 4. Note, the waveform has NOT changed. It's still a 5 Volt amplitude square wave. We've simply changed the settings of the TBS1202B-EDU. We can determine the amplitude once again by counting the number of divisions and multiplying by the scale. That is, 2.5 Divisions times 2 Volts per division gives 5 Volts. It's the same waveform.

Figure 4: Square Wave Captured on the TBS1202B-EDU with 2 Volts per Division.

Once again, the vertical scale has been adjusted. This time the scale is 5 Volts per division. The captured waveform is shown in Figure 5. Notice the waveform is 1 division at 5 Volts per division, which results in an amplitude of 5 Volts.

Figure 5: Square Wave Captured on the TBS1202B-EDU with 5 Volts per Division.

The Horizontal Scale

The horizontal scale is used to change the timescale of the instrument. It is, like the vertical scale, controlled by a rotary knob as shown in Figure 6. In Figures 3, 4, and 5 the timescale or horizontal scale was not changed; it was set at 250 μs per division. Figure 6 shows the same waveform with a horizontal scale of 100 μs per division. The vertical scale has been returned to 1 Volt per division, so you can compare Figure 3 to Figure 5 to see the effect of changing the horizontal scale.

Figure 6: Square Wave Captured on the TBS1202B-EDU with 100 μs per Division.

Finally, the horizontal scale has been set to 500 μs per division. You can see the effect on the captured waveform in Figure 7.

Figure 7: Square Wave Captured on the TBS1202B-EDU with 500 μs per Division.

Triggering System

The triggering system essentially tells the oscilloscope when to start drawing the waveform on the screen. It is best shown through examples, so I have included it in the video below. The video also shows the vertical and horizontal scale adjustments in action.

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Test Your Knowledge

Test your skills in the next figure. I've provided a time-base and voltage scale at the bottom of the Figure 8 (50 μs/division and 5 Volts/division). For this example, The center hashed line is 0 Volts or ground.

Find:

1. The peak voltage (same as amplitude in the sine wave equation)
2. The peak-to-peal voltage (twice the amplitude in the sine wave equation)
3. The frequency of the signal.

The answers are provided below, so don't scroll too far.

Figure 8: Testing skills waveform.

Note, the vertical and horizontal knob adjustment portions of the figures were initially taken from the TBS1202B-EDU manual and edited for my purposes.

Answers

Figure 8:

1. 10 Volts
2. 20 Volts
3. 10,000 Hz or 10 kHz.