Lab-4 Oscilloscope and Function Generator:

Name:___________________________

Date:___________________________

Objectives:

To understand how to use an oscilloscope and a function generator to display and measure electrical waveforms

Equipment:

Digital Oscilloscope, Function Generator, Breadboard, One 100 Ohm Resistor, Power Supply, Bag of wires, BNC-to-BNC cable, Two Oscilloscope Probes, Pair of Banana to Alligator cables, BNC-to-Alligator Cable, T-connector

Overview:

  • Build a circuit and display outputs of circuit and power sources onto an oscilloscope

  • Measure delays and phase differences between signals

  • Measure rise and fall times, frequencies and periods

  • Measure voltage levels

  • Make screen shot (Print-Screen) of each display. Paste images into the Docx file at appropriate place.

  • Submit into Brightspace.

Preliminary Procedure:

  1. Power up the function generator.

  2. Push the Channel button

  3. Select output = High Z, ON

  4. Select Parameters

  5. Select Square Wave

  6. Select Frequency = 1KHz

  7. Select Amplitude = 5V peak-to-peak

  8. Select Offset = 2.5V so that signal oscillates between 0 and 5V.

  9. Connect a T-connector to output port of the function generator

  10. Connect a BNC-to-BNC cable to one end of the T-connector

  11. Connect other end of BNC cable to Channel 1 of the scope.

Before you start using the oscilloscope first:

  • Read the description below and solve the exercises
Overview of an Oscilloscope:

Introduction:

An oscilloscope is an electronic test instrument that displays electrical signals graphically, usually as a voltage (vertical or Y axis) versus time (horizontal or X axis) as shown in figure 1. The intensity or brightness of a waveform is sometimes considered the Z axis. There are some applications where other vertical axes such as current may be used, and other horizontal axes such as frequency or another voltage may be used.

Oscilloscopes are also used to measure electrical signals in response to physical stimuli, such as sound, mechanical stress, pressure, light, or heat. For example, a television technician can use an oscilloscope to measure signals from a television circuit board while a medical researcher can use an oscilloscope to measure brain waves.

Oscilloscopes are commonly used for measurement applications such as:

  • observing the wave shape of a signal

  • measuring the amplitude of a signal

  • measuring the frequency of a signal

  • measuring the time between two events

  • observing whether the signal is direct current (DC) or alternating current (AC)

  • observing noise on a signal

An oscilloscope contains various controls that assist in the analysis of waveforms displayed on a graphical grid called a graticule. The graticule, as shown in figure 1, is divided into divisions along both the horizontal and vertical axes. These divisions make it easier to determine key parameters about the waveform. In the case of the MSO/DPO2000 Series oscilloscope, there are 10 divisions horizontally and 8 divisions vertically.

A digital oscilloscope acquires a waveform by conditioning the input signal in the analog vertical amplifier, sampling the analog input signal, converting the samples to a digital representation with an analog-to-digital converter (ADC or A/D), storing the sampled digital data in its memory, and then reconstructing the waveform for viewing on the display.

Performance Terms and Considerations:

There are many ways to specify digital oscilloscope performance, but the most important are bandwidth, rise time, sample rate, and record length.

Bandwidth

Bandwidth is the first specification to consider. Bandwidth is the frequency range of the oscilloscope, usually measured in Megahertz (MHz). It is the frequency at which the amplitude of the displayed sine wave is attenuated to 70.7% of the original signal amplitude. When measuring high-frequency or fast rise-time signals, oscilloscope bandwidth is especially critical. Without adequate bandwidth, an oscilloscope will not be able to display and measure high-frequency changes. It is generally recommended that the oscilloscope’s bandwidth be at least 5 times the highest frequency that needs to be measured. This “5-times rule” allows for the display of the 5th harmonic of the signal and assures that measurement errors due to bandwidth are minimized.

\[oscilloscope \space bandwidth \space \ge 5th \space harmonic\space of \space signal\]

Example: If the signal of interest is 100 MHz, the oscilloscope would need a bandwidth of 500 MHz.

Rise Time

The edge speed (rise time) of a digital signal can carry more high-frequency content than its repetition rate might imply. An oscilloscope and probe must have a sufficiently fast rise time to capture the higher frequency components, and therefore show signal transitions accurately. Rise time is the time taken by a step or a pulse to rise from 10% to 90% of its amplitude level. There is another “5-times rule” that recommends that the oscilloscope’s rise time be at least 5 times faster than the rise time of the signal that needs to be measured.

\[oscilloscope \space rise \space time \space =\frac{signal \space rise \space time}{5}\]

Example: If the signal of interest has a rise time of 5 \(\mu sec\), then the oscilloscope rise time should be faster than 1 \(\mu sec\).

Sample Rate: Digital oscilloscopes sample the input signals at a frequency called the sample rate, measured in samples / second (S/sec). To properly reconstruct the signals, Nyquist sampling requires that the sample rate be at least twice the highest frequency being measured. That’s the theoretical minimum. In practice, sampling at least 5 times as fast is generally desirable.

\[sample \space rate \space \ge 5*f_{Highest}\]

Example: The correct sample rate for a 450 MHz signal would be ≥ 2.25 GS/sec.

Record Length:

Digital oscilloscopes capture a specific number of samples or data points, known as the record length, for each acquired waveform. The record length, measured in points or samples, divided by the sample rate (in Samples/second) specifies the total time (in seconds) that is acquired.

\[acquired \space length = \frac{record\space length}{sample\space rate}\]

Example:

With a record length of 1 Mpoints and a sample rate of 250 MS/sec, the oscilloscope will capture a signal 4 msec in length.

Initial Setup and Screen Explanation:

Creating a Stable Display:

  1. The following steps will describe how to automatically create a stable oscilloscope display using a 1 kHz, 5 Vpk-pk square wave.
  1. Power up the Series oscilloscope by pressing the power button on the lower left corner of the instrument.

  2. Press the front panel Default Setup button to set the oscilloscope to a known starting point.

  3. Press the front panel Autoscale button to cause the oscilloscope to automatically set the vertical, horizontal and trigger settings for a stable display of the Function Generator’s 1 kHz square wave.

Key Points to Remember:

  1. To return the oscilloscope to a known state, press the Default Setup button.

  2. The Autoscale button adjusts the vertical, horizontal and trigger settings such that four or five cycles of the waveform are displayed with the trigger near the middle of the screen.

Screen Explanation:

  1. Following is a review of the oscilloscope’s display.
  1. The channel 1 vertical axis button is yellow and most of the elements on the screen that relate to the channel 1 signal are yellow in color.

  2. On the display, the following items are yellow to indicate they are associated with channel 1:

  • Waveform

  • Waveform ground level indicator (center left of screen)

  • Vertical scale readout (bottom left of screen 2.00 V)

  1. The channel 2, 3, and 4 vertical axis buttons are blue, magenta and green respectively. The display uses the color coding of these channels just as it does for the yellow of channel 1.

  2. As can be seen on the oscilloscope screen, the square wave extends up about 2 ½ divisions on the display graticule from the ground level indicator. Since the vertical scale factor is 2 Volts/div, this indicates the signal’s positive peak is at about +5 V.

  3. One cycle of the waveform is about 2 ½ divisions wide. The time per horizontal division is indicated by the horizontal scale readout which in this case is 400 µsec/div (bottom center of the display). At 400 µsec/div, the period of the signal is about 1 msec and the frequency is about 1 kHz.

  4. Finally, the trigger frequency readout indicates the channel 1 signal has a frequency of about 1 kHz as shown in the bottom right corner of the display.

Key Points to Remember:

  1. The input channels are color coded. Onscreen channel information is in that channel’s color, including the waveform, ground indicator, and vertical scale factor (Volts/div).

  2. The amplitude of the signal can be determined by multiplying the number of vertical divisions the waveform spans times the vertical scale factor.

  3. The signal period can be determined by multiplying the number of horizontal divisions times the horizontal scale factor.

  4. Signal frequency is calculated by dividing 1 by the signal period.

Exercise:

Based on the display shown here, answer the following questions:

What is the peak-to-peak voltage of the signal?

What is the voltage of the signal’s positive peak? Negative peak?

What is the period and frequency of the signal?

Capture your actual ’scope waveform and paste the image here.

Instrument Controls:

The controls of a typical oscilloscope can be grouped into three major categories: vertical, horizontal, and trigger. These are the three main functions that are used to set up an oscilloscope. The use of these controls is described in the following sections of this lab.

Here are a few hints that will make using the oscilloscope controls easier:

  • Decide if the task is related to oscilloscope’s vertical axis (typically voltage), horizontal axis (typically time), trigger, or some other functions. This will make easier to find the correct controls and menu.

  • Pressing a front panel button will usually display a first-level menu at the bottom of the display. The menu items are logically prioritized from left-to-right. If they are selected in that order, the setup should be straightforward.

  • In most cases, pressing the button underneath a menu item at the bottom of the display results to-bottom.

  • Pressing the Menu Off button turns off one menu level at a time until all menus and readouts are removed.

Exercise:

The oscilloscope’s vertical axis controls are typically used to control which parameter?

Vertical Controls:

Introduction:

The vertical controls set or modify the vertical scale, position, and other signal conditioning for each of the analog input channels.

There is a set of vertical controls for each input channel. These controls are used to scale, position, and modify that channel’s input signal so it can be viewed appropriately on the oscilloscope display. In addition to the dedicated vertical controls for each channel, there are also buttons to access the math menu, reference menu and bus menus.

Vertical Position/Scale Controls:

  1. The following steps will explore the use of the vertical axis position and scale front panel controls.
  1. Use the channel 1 vertical Position knob to position the waveform near the bottom of the display and notice the ground level indicator also moves.

The vertical position control moves the waveform up and down. It is generally used to align the waveform with the vertical divisions on the graticule. Position is generally a graphical display function only and does not affect the acquired waveform data. b. Use the channel 1 vertical Scale knob to change the vertical scale from 2 V/div to 1 V/div.

The vertical scale (Volts/division) control adjusts the height of the waveform on the display. Generally, the vertical scale control changes the settings of the input amplifier and/or attenuator and does affect the acquired waveform data. Because the vertical scale controls the amplitude of the signal going into the ADC, the highest-resolution measurements are achieved when the signal almost fills the screen vertically without going off screen.

Key Points to Remember:

  1. The vertical position knob controls the position of the waveform on the vertical axis.
  2. The vertical scale knob controls the amount of voltage represented by a vertical division on the graticule.

Horizontal Controls:

Introduction:

The horizontal controls are used to scale and position the time axis of the oscilloscope display. There is a dedicated front panel control for setting the horizontal scale (time/division) of the display and another for setting the horizontal position of the displayed signals. The Acquire menu offers additional options for modifying the waveform display, as well as setting the record length.

Horizontal Position/Scale Controls:

  1. The following steps will explore the use of the horizontal axis scale front panel control. The horizontal scale control (also known as time/division or seconds/division) adjusts the amount of time displayed on the screen.
  1. Press the front panel Autoscale button to restore the oscilloscope to a known starting point and then set the vertical scale to 1 V/div.

  2. Use the vertical Position knob to center the waveform on the screen.

  3. Turn the horizontal Scale knob until the horizontal readout indicates 10μs/div (readout is shown in the bottom center of the display.)

Since there are 10 divisions horizontally, a scale factor of 10 µsec/div yields a 100 µsec time window. This setting shows the actual shape of the rising edge of the square wave.

  1. The horizontal Position control moves the waveform and its horizontal reference or trigger point (indicated by the orange icon at the top of the display) back and forth on the display. This is used to align the displayed waveform with the horizontal divisions on the display graticule.
  1. Turn the horizontal Position knob counter-clockwise to position the waveform’s falling edge at the center of the display.

Key Points to Remember:

  1. The horizontal scale control sets the time window displayed on the oscilloscope screen. Since there are 10 divisions horizontally, the time window is equal to:

time window = horizontal scale factor*10 divisions

  1. The horizontal position knob allows you to align the displayed waveform with the horizontal divisions of the display graticule or to view a different section of the displayed waveform.

Exercise:

If the horizontal scale factor were set to 1 μsec/div, the displayed time window would be:

Trigger Controls:

Introduction:

The trigger defines when a signal is acquired and stored in memory. For a repetitive signal, a trigger is required to stabilize the display.

There is a front panel control to set the trigger level and a button to force the oscilloscope to trigger. The Trigger menu offers different trigger types and allows you to set the conditions of the trigger.

Trigger Level Control:

  1. The following steps will explore the use of the front panel trigger level control.
  1. Use the Default Setup and Autoscale buttons to set the oscilloscope to a known starting point.
  2. Press the Menu Off button to turn off the menus. Set up the oscilloscope to match the display shown here.
  1. In the default trigger setting, the oscilloscope looks for a rising edge on the channel 1 input signal. The trigger level control is used to set the voltage at which the oscilloscope triggers. The waveform is displayed with the rising edge aligned with the trigger point (indicated by the orange T icon t the top of the display). The trigger voltage level is shown by a yellow arrow on the right side of the display. In this case, the arrow is slightly above the vertical axis midpoint.
  1. Turn the Trigger Level knob until the trigger level, as indicated by the arrow with a T on the left side of the screen, is above the top of the waveform (about 5.5 V) resulting in an un-triggered display.

Key Points to Remember:

  1. A trigger defines when a signal is acquired and stored in memory.

  2. The trigger level has to be within the signal range to properly trigger the oscilloscope.

  3. For a repetitive signal, a trigger is necessary to obtain a stable display.

Using the Trigger level control, move the trigger level in and out of the signal’s voltage range and note the effect this has on the displayed signal. Note how the text in the top left portion of the display (known as the trigger indicator) changes from Auto, to Trig?, to Trig’d depending on the position of the trigger voltage level.

(The Auto trigger indicator means the oscilloscope is in Auto trigger mode. This causes an acquisition to be made about once a second if no trigger event is found. This provides a display, but not a stable one, as illustrated here.)

Trigger Menu:

  1. During the following steps, a trigger will be set up to create a stable display.
  1. Press the Trigger Level knob (it doubles as a button) to force the trigger voltage setting to the 50% point of the signal. The oscilloscope display should now match the figure in the previous section.

  2. Change the horizontal scale factor to 100μsec to display one full cycle of the signal.

  3. Press the front panel Trigger Menu button.

The Trigger Menu allows you to specify the trigger event used to capture a waveform. Available trigger types include specific pulse widths and glitches, short digital “runt” pulses, rise time, fall time and several others.

  1. Press the Source bottom bezel button. The source menu allows you to select which signal to monitor for the trigger event.

  2. Use the Edge or Trigger control (depending on the type of scope) to select the channel that will be the source of the trigger. Select channels 2, 3 and 4 in sequence and note the effect this has on the triggered state of the display. When channel 1 is not selected, the display is not triggered because channels 2, 3 and 4 do not have an applied signal.

  3. Use the appropriate control to select channel 1 and ensure the display is triggered. Press the Menu Off front panel button.

  4. Select the trigger edge to select the falling edge of the signal as the trigger point.

This controls whether the trigger looks for a positive or negative edge on the trigger signal.

Key Points to Remember:

  1. Pressing the trigger level knob forces the trigger level to the 50% point of the applied signal.

  2. Use the trigger source menu to select which input channel to monitor for the trigger event.

  3. Use the trigger slope control to specify which edge (rising or falling) to trigger on.

Oscilloscope Measurements:

Introduction:

A digital oscilloscope can make a variety of measurements on electrical signals, such as peak-to-peak and RMS amplitude measurements and frequency, period, and pulse width timing measurements. The oscilloscope provides several ways to make these measurements. This section will review the three most common measurement methods:

  • Manual measurements

  • Cursor measurements

Manual Measurements. Manual measurements rely upon the graticule on the display and the vertical and horizontal scale settings to make measurements. A typical graticule has 8 divisions vertically and 10 divisions horizontally. For the highest accuracy, scale and position the waveform to fill the display vertically and horizontally and then visually measure the parameter in units of graticule divisions. Then multiply the number of divisions by the scale factor to get the final measurement value.

Cursor Measurements. Cursor measurements are made by manually aligning a pair of cursors to points on the waveform and then reading the measurement values from the display cursor readouts.

Manual Measurements:

  1. The following exercise will explore making manual waveform measurements.
  1. Reset the oscilloscope back to a known staring point and use the front- panel controls to create this display.

Normally, for greatest accuracy, the waveform is adjusted vertically to fill as much of the display as possible. For this exercise, leave the waveform as shown to the right.

Exercise:

  1. Determine the amplitude of the signal by counting the number of vertical divisions on the graticule and multiplying that by the vertical scale factor. Write the amplitude here:

  2. Calculate the period of the signal by counting the number of horizontal divisions on the graticule and multiplying that by the horizontal scale factor. Write signal period here:

  3. Calculate the frequency of the signal by performing the following calculation: Frequency = 1/(signal period). Write the frequency here:

Cursor Measurements:

  1. For greater measurement accuracy, the oscilloscope provides cursors that will be used in the next series of steps.
  1. Press the front panel Cursors button twice to turn on all the cursors.
  • The vertical cursors measure time along the horizontal axis.

  • When the cursors are inactive, they are dotted. Solid lines indicate the cursors are active.

  1. Press the Select button to cause the horizontal cursors to turn solid if they are not already. Use the Cursor knobs to position the horizontal cursors to the top and bottom of the waveform. Write the signal amplitude from the upper right corner of the display in the Exercise section below.

  2. Press the Select button to select the vertical cursors. Notice the a and b indicators move to the timing readout and the vertical cursors go from dotted to solid.

  3. Use the Cursor button to position the cursors at the start and end of one cycle of the signal (falling edge to falling edge) and then read out the time on the display.

Exercise:

Measurements with Resistors:

  1. Disconnect the Function generator cable from the scope’s channel 1 and the function generator.

  2. Take the breadboard and insert a resistor into it.

  3. Connect a T-connector to the function generator.

  4. Connect one end of the T-connector to power the breadboard and the other end to the oscilloscope channel.

  5. Measure the p-p voltage outputs for 100 Ω and 1 kΩ resistors and capture the displays to paste on your report and note down the effects and explain in words the output results.