Skills Mapping

07/10/2025

Schedule:

Thursday, .

Further Instrumentation Techniques

Introduction

This series of practical experiments gives knowledge and experience in using electronic instrumentation in alternative ways, for example, to capture and display and examine single-shot, transient waveforms produced by contact bounce that is normally obtained with mechanical switches, to display and measure waveforms of differing form in diode circuits when it is useful to superimpose more than 2 waveforms on a 2 channel oscilloscope, and to explore downloading the oscilloscope display or data directly to a computer for a more professional publication finish than is possible by simply taking photo-shots of the oscilloscope display with a smart phone.

Aims and Objectives

The aim is to extend your knowledge and experience of using the more advanced features of common electronic lab instruments. At the end of this lab, you should be able to:

appreciate how to store and download oscilloscope-display images or capture waveform data for reports or post processing,
set up an oscilloscope to reliably capture single-shot transient waveforms,
trigger the oscilloscope to capture some more complex waveforms,
display for comparison a greater variety and number of waveforms simultaneously.

Saving Oscilloscope display images in reports

Connect the waveform generator to the Waveform generator set to give a 1kHz, 2V pk-pk output. Set the oscilloscope to display two cycles of the waveform with maximum and minimum amplitude levels just within the display. Press Run/Stop to capture the waveform for download into the results Word document.

Enter Keysight Benchvue into the bottom left Start field.

Close the User Guide window.

The Benchvue software should link the computer with the oscilloscope if the USB A to USB B cable attaching computer and oscilloscope is in place. When the link has been made an oscilloscope icon showing the correct model details should be visible at the top right of the Benchvue window.

Double click the oscilloscope icon to open the oscilloscope control window. Then click current screen.

The simplest way to copy the oscilloscope display is just to click the top save icon which saves the display image as a xxx.png file.

Alternatively, the Export All option may be selected to give a choice of display image or data export options.

Exercise 1

Exercise 1 Save a copy of the sinewave display in your results Word file using the following 3 methods:

using the Save button and importing the resulting xxx.png file,
using the Export All button and selecting the Word option,
taking a photoshoot image of the display with your smartphone and pasting the image into your Word document.

Which capture option gives the best result if you just wish to import an image into a report document and achieve a professional report finish suitable for publication?

When capturing oscilloscope screen copies for reports, it is generally better to insert a downloaded scope png file in a report (left image below). The alternative of using or a phone-camera-shot (right image below) often results in poor clarity of some of the essential scope set-up data as illustrated in .

Imported oscilloscope display (left) photoshoot of display (right) for comparison of image quality.

Capturing Single-shot or transient waveforms

Connect the $47 \Omega$ resistor and switch assembly across the +25 V output of the power supply.

Select the +25V output and switch it on using the Output On/Off key. Adjust the voltage to 10V.

Connect the oscilloscope probe BNC connector to Ch1, and the tip and crocodile clip directly across the power resistor using the resistor terminals, or wire close to them, for connection. Make sure the probe crocodile clip is connected to the power-supply COM side

Position the Ch 1 zero-level symbol to 1 div from the base of the screen using the Ch 1 position knob, and set the vertical scale to $2 V/div$ and the timebase to $200 \mu s/div$.

Turn on and off the switch connected to the resistor to identify which position is on and which is off. In the on position, the scope trace should jump up 5 divisions to indicate that the probe is measuring a DC level of 10V.

Set the trigger level to 5 V within the Waveform section. Make sure that the Mode/Coupling key is set to Normal within the Mode menu. Press Acquire and set the oscilloscope to Normal mode.

Turn the resistor-assembly switch on and off and you should capture a switching transient each time the switch is toggled which is something like one of the waveforms shown in .

Warning: you must use a proper oscilloscope probe to capture the waveform most accurately since using just a coaxial cable with crocodile clips adds too much capacitance. You might also have to use the oscilloscope probe set at x10, which similarly minimises the impact of connecting a probe on the circuit being investigated since it effectively maximises the oscilloscope channel input impedance.

If you have difficulty obtaining traces like those shown , you may have to press the Single key in the Run Control section prior to each change in switch state.

The switch contact bounce is most evident in the turn-on switching transition in . The pattern of pulses tends to vary each time the switch is thrown.

Note: The switch contact bounce is most evident in the turn-on switching transition in Figure 2. The pattern of pulses tends to vary each time the switch is thrown.

Change the timebase setting to 100 ns/div and change the trigger edge to the falling edge in the Edge menu.

Switch contact-bounce transients during switch on (left) and switch off (right).

Exercise 2

Capture the switching transients as shown in on the oscilloscope by pressing the Single (for single-shot capture) before each throw of the switch. Save good turn-on and turn-off switching waveform examples, as in Fig.2, in your lab results Word document.

Change the timebase setting to $100 ns/div$ and change the trigger edge to the falling edge in the Edge menu.

Exercise 3

Capture the bounce falling transient as shown in on the oscilloscope by pressing the Single (for single-shot capture) before each throw of the switch. Save a good example, as in , in your lab results Word document.

Use the oscilloscope cursors to measure the frequency of the damped ringing.

Record the value obtained.

Oscillating voltage decay obtained across the switch when breaking current.

Connect a $0.1 \mu F$ ceramic disc capacitor directly across the switch contacts connected to the resistor, using the grey terminal block. The capacitor leads should be trimmed to 1 cm.

Set the oscilloscope to trigger on a falling edge and the timebase to $5 \mu s /div$.

Press the Run/Stop key in the Run Control section and observe the new switching transient as you increase the timebase from $5 \mu s/div$ to $200 \mu s/div$. Save the $200 \mu s/div$ waveform results like those in in your Word document.

Exercise 4

Capture the new switching transients as shown in Fig.4 on the oscilloscope by pressing the Single (for single-shot capture) before each throw of the switch. Save good turn-on and turn-off switching waveform examples as in Fig.4 in your lab results Word document.

Use the oscilloscope cursors to measure the frequency of the damped ringing. Record the value obtained.

Why are the capacitors that are often connected in parallel with switch contacts in practical applications termed suppressor capacitors?

Switch contact-bounce transients for switch on (left) and off (right) with a suppressor capacitor fitted.

Disconnect the scope probe on Ch1 and put it aside.

Waveform capture using alternative trigger features

Connect the function generator output to Ch 1 of the scope.

Set the function generator to produce an amplitude-modulated (AM) waveform, as follows.

Press the sinewave key and set the Freq to 10 kHz and Ampl to 1V pp.

Press Shift followed by the sinewave select key.

Note: Shift followed by sinewave selects an AM waveform because AM is marked above the sinewave select key.

Press Auto-Scale on the scope and adjust the timebase to 2 ms/div. A jittery amplitude-modulated sinewave should be obtained.

Exercise 5

Why is it difficult to stabilise the AM waveform in the oscilloscope display using Ch1 as the trigger source? How might you overcome the problem to capture on the oscilloscope display an AM waveform FM waveform?

Connect the SYNC output of the function generator to the scope Ch 2 input.

Use Ch 2 as the scope trigger source. Adjust the trigger level to a slightly positive voltage, if required.

A stable amplitude-modulated display should now be obtained.

Press the Ch2 select key to display the Ch2 waveform.

Change the timebase to $10 \mu s/div$ to view the Ch 2 waveform and record its form.

Exercise 6

Why have you been able to stabilise the displayed waveform with this signal? Save a copy of your displayed waveform in your results.

Turn off the Ch2 display and connect the function generator to the scope Ext trigger input.

Select Ext as the trigger source in the Edge menu. A stable waveform should once again be obtained.

Exercise 7

Can you think of one advantage and one disadvantage of using the Ext Trigger input rather than the Ch2 input to stabilise the display in this way?

Exercise 8

Set the waveform generator to display a relatively deeply modulated FM signal. Attempt to achieve a stable trace within the oscilloscope display. Pres Stop to fix the display and save a copy of the oscilloscope display in your results and explain how you have achieved a stable image. What trigger controls/features have you used?