Chapter 21 Electrical Test and Measurement Equipment 411 Vertical and Horizontal Adjustments Many digital oscilloscopes have several knobs to adjust various parameters of the device. Th e most important are the V/div knobs, which set the voltage scale. Refer to Figure 21-22. Most oscilloscopes have two or four input channels. Each channel has a voltage scale (V/div) adjustment. Th e math button allows you to perform math functions between the two channels. Unfortunately, unknowledgeable technicians press this button and receive a display that they are unable to interpret. Th is is one of the most common mistakes an operator can make. If you are unsure of the results of a setting or adjustment, do not use it. Turning the vertical position or vertical off set knobs moves the trace higher or lower on the screen. For exam- ple, you may want to view channel 1 on the top half of the screen and channel 2 on the bottom half. On some oscil- loscopes, pressing this knob centers the trace on the screen. Horizontal controls include a time/division adjust- ment knob used to set the horizontal scale. A horizontal position adjustment knob allows you to shift the trace from side to side. Grid lines Y axis (voltage) X axis (time) Positive voltage Negative voltage oscilloscope set these values. Th e V/div (volts per division) control sets the value of a division on the Y axis, and the t/div (time per division) control sets the value of each divi- sion on the X axis. Th ese controls are adjusted until the waveform can be conveniently measured. For the waveform shown in Figure 21-21, the scales are set to 1 V/div and 200 μs/div. Count the number of divisions vertically from the positive peak of the wave- form to the negative peak to arrive at the peak-to-peak value of 5 V. Count the number of divisions between any point on a cycle, such as where the wave begins to go positive starting from the center line, to the same point on the next cycle, to fi nd 5 divisions. Multiply the num- ber of divisions by the time/div (5 div × 200 μs/div) to calculate 1000 μs or 1 ms. Goodheart-Willcox Publisher Figure 21-20. A modern digital oscilloscope with probes. The probe for channel 1 is connected to the scope calibrator output. The scope screen displays the waveform output of the scope calibrator. Goodheart-Willcox Publisher Figure 21-21. A square waveform as shown on the oscilloscope screen. Move on-screen trace up or down Adjust trace position from side-to-side Time scale adjustment Voltage scale adjustment for channel 2 Voltage scale adjustment for channel 1 Turn channels on or off and set parameters Perform math functions between channels Access menu of horizontal scale parameters Triggering controls Goodheart-Willcox Publisher Figure 21-22. Typical controls of a digital oscilloscope. Triggering Triggering allows the waveform shown on the screen to remain stationary without moving horizontally. Without triggering, the waveform drifts or rolls either right or left. Triggering is the point in time at which the oscillo- scope begins sweeping the displayed waveform from left to right. You can set the scope to trigger at a specifi c voltage. You can also specify if the trigger happens on the leading or trailing (rising or falling) edge of the waveform. Copyright Goodheart-Willcox Co., Inc.