Basic Instrumentation and Measurement - Oscilloscope

An oscilloscope is a type of electronic test instrument that allows signal voltages to be viewed, usually as a two-dimensional graph of one or more electrical potential differences (vertical axis) plotted as a function of time or of some other voltage (horizontal axis). It is the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next.

Although an oscilloscope displays voltage on its vertical axis, any other quantity that can be converted to a voltage can be displayed as well. In most instances, oscilloscopes show events that repeat with either no change, or change slowly. The oscilloscope is one of the most versatile and widely-used electronic instruments.

All oscilloscopes  although they are different model but still the functions are the same.

Oscilloscope must be operated systematically if not it could be difficult to get a right signal.

The function of oscilloscope is V/t  graph ( Voltage against time ) , voltage on the vertical axis  (X- axis) and time on the horizontal axis (Y-axis).  On the screen of oscilloscope there are 8 squares or divisions in vertical axis and 10 squares or divisions on the horizontal axis. These squares usually are 1cm in each direction.

The controls of the scope allow us to change the vertical and horizontal scales of the V/t graph. With this we can display the clear picture of the signal that we want to investigate. Dual trace scope displays 2 V/t graphs at the same time .

Before we use an oscilloscope we have to check that all controls are in their normal positions

·         All push button switches are in OUT position

·         All slide switches are in the UP position

·         All rotating controls are centered

·         The central TIME/DIV and VOLTS/DIV and the HOLD OFF controls are in the calibrated or CAL position.

Set both VOLTS/DIV controls to 1V/DIV and the TIME/DIV control to 2 s/DIV its slowest setting :

Switch ON .

Find the Y  -POS 1 control .

When you turn the knob the spot will move upward and downward the screen. Now for the time being the trace have to be adjusted horizontally across the center of the screen.

Now we have to check the INTENSITY and the FOCUS control. If the spot is correctly set, the spot is bright but not glaring. And as sharply focused as possible.

The TIME/DIV control determines the horizontal scale of the graph which appears on the screen. With 10 squares across the screen and the spot moving at 0.2 s/DIV, how long does it take for the spot to cross the screen? The answer is 0.2 x 10 = 2 s. Count seconds. Does the spot take 2 seconds to cross the screen?

Now rotate the TIME/DIV control clockwise. With the spot moving at 0.1 s/DIV, it will take 1 second to cross the screen.

Continue to rotate TIME/DIV clockwise. With each new setting, the spot moves faster. At around 10 ms/DIV, the spot is no longer separately visible. Instead, there is a bright line across the screen. This happens because the screen remains bright for a short time after the spot has passed, an effect which is known as the persistence of the screen. It is useful to think of the spot as still there, just moving too fast to be seen.

Keep rotating TIME/DIV. At faster settings, the line becomes fainter because the spot is moving very quickly indeed. At a setting of 10 µs/DIV how long does it take for the spot to cross the screen?

The VOLTS/DIV controls determine the vertical scale of the graph drawn on the oscilloscope screen.

Check that VOLTS/DIV 1 is set at 1 V/DIV and that the adjacent controls are set correctly:

A connection to the input of channel 1, CH 1, of the oscilloscope can be made using a special connector called a BNC plug.

Check on the effect of Y-POS 1 and X-POS:

Y-POS 1 moves the whole trace vertically up and down on the screen, while X-POS moves the whole trace from side to side on the screen. These control are useful because the trace can be moved so that more of the picture appears on the screen, or to make measurements easier using the grid which covers the screen.

You have now learned about and used the most important controls on the oscilloscope.

You know that the function of an oscilloscope is to draw a V/t graph. You know how to put all the controls into their 'normal' positions, so that a trace should appear when the oscilloscope is switched on. You know how the change the horizontal scale of the V/t graph, how to change the vertical scale, and how to connect and display a signal.

What is needed now is practice so that all of these controls become familiar.

Digital Oscilloscope

Functional Check

Perform this quick functional check to verify that oscilloscope is operating correctly:

1. Wait until the display shows that all the power-on test passed. Push the DEFAULT SETUP button. The default probe option attenuation setting is 10x.

2. Set the switch to 10x on the P2200 probe and connect the probe to channel 1 on the oscilloscope. To do this , align the slot in the probe connector with the key on the CH1 BNC, push to connect and twist to the right to lock the probe in place. 

Connect the probe tip and reference lead to the PROBE COMP connectors.

3. Push the AUTOSET button. Within a few seconds , we should see a square wave in the display of about 5V peak-to-peak at 1kHz.

Push the CH1 Menu button twice to remove channel 1. push the CH2 MENU button to display channel 2, repeat steps 2 and 3. For 4 channel models , repeat for CH3 and CH4.

Probe Safety

A guard around the probe body provides a finger barrier for protection from electric schock.

Connect the probe to the oscilloscope and connect the ground terminal to ground before take any measurements.

Probe Check Wizard

The probe check wizard is used to quick verify that the probe is operating properly.

1.      Push the PROBE CHECK button. If the probe is connected properly, compensated properly and the Probe entry in the oscilloscope will display a PASSED message at the bottom of the screen. Otherwise, the oscilloscope will display directions on the screen to guides in order to solve these problems.

2.      Probe check is useful for 1X, 10X and 100X probes; it does not work with the EXT TRIG front-panel BNC.

To compensate a probe connected to the EXT TRIG front-panel BNC, follow these steps:

1.      Connect the probe to any channel BNC, such as to CH 1.

2.      Push the PROBE CHECK button and follow the directions on the screen.

3.      After verify that the probe function properly and is compensated, connect the probe to the EXT TRIG BNC.

Manual Probe Compensation

As an alternative method to Probe Check, it can manually perform this adjustment to match the probe to the input channel.

1.      Set the Probe option attenuation in the channel menu to 10X. Set the switch to 10X on the P2200 probe and connect the probe to channel 1 on the oscilloscope. If use the probe hook-tip, ensure a proper connection by firmly inserting the tip onto the probe.

2.      Attach the probe tip to the PROBE COMP ~5V connector and the reference lead to the PROBE COMP Ground connector. Display the channel and then push the AUTOSET button.

3.   Check the shape of the display waveform.

4.   If necessary, adjust the probe.

UNDERSTANDING OSCILLOSCOPE FUNCTIONS

To use the oscilloscope effectively, we need to learn about the following oscilloscope functions:

•       Setting up the oscilloscope
  
•       Triggering
  
•       Acquiring signals (waveform)
  
•       Scaling and positioning waveform
  
•       Measuring waveforms
  

Setting Up the Oscilloscope

1.      Using Autoset : The Autoset function obtains a stable waveform display. It automatically adjusts the vertical scale, horizontal scale and trigger settings. Autoset also displays several automatic measurements in the graticule area, depending on the signal type.

2.      Saving a Setup : The oscilloscope saves the current setup if we wait five seconds after the last change before we power off the oscilloscope. The oscilloscope recalls this setup the next time we apply power.

We can use the SAVE/RECALL Menu to permanently save up to ten different setups.

3.      Recalling a Setup : The oscilloscope can recall the last setup before power off, any of your saved setups or the default setup.

Triggering

The trigger determines when the oscilloscope starts to acquire data and display a waveform. When a trigger is set up properly, the oscilloscope converts unstable displays or blank screen into meaningful waveforms.

When push the RUN/STOP or SINGLE SEQ buttons to start an acquisition, the oscilloscope goes through the following steps:

1.      Acquires enough data to fill the portion of the waveform record to the left of the trigger point. This is also called the pretrigger

2.      Continues to acquire data while waiting for the trigger condition to occur.

3.      Detect the trigger condition.

4.      Continues to acquire data until the waveform record is full.

5.      Display the newly-acquired waveform.

Source : We can use the Trigger Source option to select the signal that the oscilloscope uses as a trigger. The source can be any signal connected to a channel BNC, to the EXT TRIG BNC or the AC power line (available only with Edge triggers).

Type : The oscilloscope provides three types of triggers: Edge, Video and Pulse Width.

Modes : We can select a Trigger Mode to define hoe the oscilloscope acquires data when it does not detect a trigger condition. The modes are Auto and Normal. To perform a single sequence acquisition, push the SINGLE SEQ button.

Coupling : We can use the Trigger Coupling option to determine which part of the signal will pass to the trigger circuit. This can help us attain a stable display of the waveform.To use trigger coupling, push the TRIG MENU button, select an Edge or Pulse trigger and select a Coupling option.To view the conditioned signal being passed to the trigger circuit, push and hold down the TRIG View button.

Slope and Level : The Slope and Level controls help to define the trigger. The Slopes option (Edge trigger type only) determines whether the oscilloscope finds the trigger point on the rising or the falling edge of a signal. The TRIGGER LEVEL knob controls where on the edge the trigger point occurs.

Acquiring Signal

When we acquire a signal, the oscilloscope converts it into a digital form and displays a waveform. The acquisition mode defines how the signal is digitized and the time base setting affects the time span and level of details in the acquisition. There are three acquisition modes: Sample, Peak detect and Average.

Sample : In this acquisition mode, the oscilloscope samples the signal in evenly spaced intervals to construct the waveform. This mode accurately represents signals most of the time. However, this mode does not acquire rapid variations in the signal that may occur between samples. This can result in aliasing (figure below) and may course narrow pulses to be missed. In these cases, we should use the Peak Detect mode to acquire data.

Peak Detect : In this acquisition mode, the oscilloscope acquires several waveform, averages them and display the resulting waveform.

Time Base : The oscilloscope digitized waveform by acquiring the value of an input signal at discrete points. The time base allows to control how often the value are digitized.To adjust the time base to a horizontal scale that suits with purpose, use the SEC/DIV knob.

Taking Measurement

There are several ways to take measurement which is can use graticule, the cursors or an automated measurement.

Graticule : This method allows us to make a quick, visual estimate. For example we might look at a waveform amplitude and determine that it is a little more than 100mV. Taking a simple measurement by counting the major and minor graticule divisions involved and multiplying by the scale factor.For example, if we counted five major vertical graticule divisions between the minimum and maximum values of a waveform and knew we had a scale factor of 100mV/division, then we could easily calculated the peak-to-peak voltage as follows:

5 divisions x 100mV?division = 500mV

Cursors : This method allows us to take measurement by moving the cursors, which always appear in pairs and reading their numeric values from the display readouts. There are two types of cursors : Voltage and Time.When use the cursors, be sure to set the source to the waveform on the display that we want to measure. To use cursors, push the CURSORS button.Voltage Cursors appear as horizontal lines on the display and measure the vertical parameters.Time Cursors appear as vertical lines on the display and measure the horizontal parameters.Automatic : The MEASURE Menu can take up to five automatic measurements. When we take automatic measurement, the oscilloscope does all the calculating for you. Because the measurements use the waveform record points, they are more accurate than the graticule or cursors measurements.Automatic measurements use readouts to show measurement results. The readouts are updated periodically as the oscilloscope acquires new data.

OPERATING BASICS

FRONT PANEL

Display Area

In addition to display waveform, the display is filled with many details about the waveform and the oscilloscope control settings.

1.      Icon display shows acquisition mode.

•      Sample mode
  
•      Peak detect mode
  
•      Average mode

2.      Trigger status indicates the following:

•       Armed. The oscilloscope is acquiring data. All triggers are ignored in this state.
  
•       Ready. All pretrigger data has been acquired and the oscilloscope is ready to accept a trigger.
  
•       Trig’d. The oscilloscope has seen a trigger and is acquiring the posttriger data.
  
•       Stop. The oscilloscope has stopped acquiring waveform data.
  
•       Acq.Complete. The oscilloscope has completed a Single Sequence acquisition.
  
•       Auto. The oscilloscope is in auto mode and is acquiring waveform in the absence of triggers.
  
•       Scan. The oscilloscope is acquiring and display waveform data continuously in scan mode.
  

3.   Marker shows horizontal trigger position. Turn the HORIZONTAL POSITION knob to adjust the position of the marker.

4.      Readout shows the time at the center graticule. The trigger time is zero.

5.      Marker shows Edge or Pulse width trigger level.

6.      On-screen marker shows the ground reference points of the displayed waveforms. If there is no marker, the channel is not displayed.

7.      An arrow icon indicates that the waveform is inverted.

8.      Readout shows the vertical scale factors of the channels.

9.      A Bw icon indicates that the channel is bandwidth limited.

10.      Readout shows main time base setting.

11.      Readout shows window time base setting if it is in use.

      12. Readout shows trigger source used for triggering.

 13.      Icon shows selected trigger type as follows:

•      Edge trigger for the rising edge.
•      Edge trigger for the falling edge.
•      Video trigger for line sync.
•      Video trigger for field sync.
•      Pulse Width trigger, positive polarity.
•      Pulse width trigger, negative polarity.

14.      Readout shows Edge or Pulse Width trigger level.

15.      Display area shows helpful messages; some messages display for only three seconds.

If we recall a saved waveform, readout shows information about the reference waveform.

16.      Readout shows trigger frequency.

Using Menu System

The oscilloscope uses four methods to display menu options:

1.   Page (submenu) Selection: For some menus, we can use the top option button to choose two or three submenus. Each time we push the top button, the option change. Foe example, when we push the top button in the SAVE/REC Menu, the oscilloscope cycles through the Setups and Waveform submenus.

           2.  Circular List: The oscilloscope sets the parameter to a different value each time  we push the option button. For example, we can push the CH 1 MENU button and then push the top option button to cycle through the Vertical (channel) Coupling options.

3.     Action: The oscilloscope displays the type of action that will immediately occur when we push an Action option button. For example when we push the DISPLAY Menu button and then push the Contrast Increase option button, the oscilloscope changes the contrast immediately.

      4.  Radio : The oscilloscope uses a different button for each option. The currently-selected option is highlighted. For example, the oscilloscope displays various acquisition mode option when we push the ACQUIRE Menu button. To select an option, push the corresponding button. 

Vertical Controls

      1.  CH 1, CH 2, CH 3, CH 4, CURSOR 1 and CURSOR 2 POSITION. Positions the waveform vertically. When we display and use cursors, an LED lights to indicate the alternative function of the knobs to move the cursors.

      2.  CH 1, CH 2, CH 3 & CH 4 MENU. Displays the vertical menu selections and toggles the display of the channel waveform on and off.

       3. VOLTS/DIV (CH 1, CH 2, CH3 & CH 4). Selects calibrated scale factors.

      4.  MATH MENU. Display waveform math operations menu and can also be used to toggle the math waveform on and off.

Horizontal Controls

     1.  POSITION. Adjust the horizontal position of all channel and math waveforms. The resolution of this control varies with the time base setting

Note : To make a large adjustment to horizontal position, turn the SEC/DIV knob to a larger value, change the horizontal position and then turn the SEC/DIV knob back to the previous value.

     2. HORIZ MENU. Displays the Horizontal Menu.

     3.  SET TO ZERO. Sets the horizontal position to zero.

      4. SEC/DIV. Selects the horizontal time/div (scale factor) for the main or the windows time base. When window zone is enabled, it changes the width of the window zone by changing the window time base.

Triger Controls

      1.  LEVEL and USER SELECT. The primary function of the LEVEL knob is to set the amplitude level the signal must cross to cause an acquisition. The knob can also be use to perform USER SELECT alternative functions. The LED lights below the knob to indicate an alternative function.

USER SELECT	Description
Holdoff	Sets the amount of time before another trigger event can be accepted
Video line number	Sets the oscilloscope to a specific line number when the Trigger Type option is set to video and the Sync option is to Line Number
Pulse width	Sets the width of the pulse when the Trigger Type option is sets to Pulse and you select the Set Pulse Width option

     2.  TRIG MENU. Display the Trigger Menu.

     3.  SET TO 50%. The trigger level is set to the vertical midpoint between the peaks of the trigger signal.

     4.  FORCE TRIG. Completes an acquisition regardless of an adequate trigger signal. This button has no effect if the acquisition is already stopped.

     5. TRIG VIEW. Display the trigger waveform in place of the channel waveform while the TRIG VIEW button is held down.

Menu and Control Buttons

     1. SAVE/RECALL. Display the Save/Recall Menu for setups and waveforms.

     2. MEASURE. Displays the automated measurements menu.

     3. ACQUIRE. Display the Acquire Menu.

     4. DISPLAY. Display the Display Menu.

     5. CURSOR. Display the Cursor Menu. Vertical Position controls adjust cursors position while displaying the Cursor Menu and the cursors are activated. Cursors remain displayed (unless the Type option is set to Off) after leaving the Cursor Menu but are not adjustable.

    6. UTILITY. Display the Utility Menu.

     7. HELP. Display the Help Menu.

     8. DEFAULT SETUP. Recalls the factory setup.

     9.  AUTOSET. Automatically sets the oscilloscope controls to produce a usable display of the input signals.

     10. SINGLE SEQ. Acquires a single waveform and then stops.

     11. RUN/STOP: Continuously acquire waveform or stops the acquisition.

     12. PRINT. Starts print operations. 

Connectors

 PROBE COMP. Voltage probe compensation output and ground. (refer to Manual Probe Compensation)