Oscilloscopes: Function, Features, Applications & Selection Guide

The Function of an Oscilloscope

An oscilloscope is an electronic instrument that presents a graphical display of its input voltage as a function of time. It is important to note that oscilloscopes only measure voltages not currents or resistances however it is possible to convert quantities such as current, strain, acceleration, pressure and so forth into voltages that can then be used by the oscilloscope. For example, to convert a current into a voltage, a resistor of known value is used; the current is measured indirectly by measuring the voltage drop across the resistor and then applying Ohm’s law. An oscilloscope is an important tool for electronic engineers and scientists; using the oscilloscope, more details of a complex signal source like frequency, duty cycle, peak-to-peak amplitude, overshoot, rise time, fall time, etc. can be completely described.

An oscilloscope is basically a xy plotter capable of plotting an input signal versus time or versus another input signal. As a signal is supplied to the input of a scope, a luminous spot appears on the screen. As changes in the input voltage occur, the luminous spot responds by moving up or down or left or right. In many applications, the oscilloscope’s vertical-axis (y axis) input receives the voltage part of an incoming signal and then moves the spot up or down depending on the value of the voltage at a particular instant in time. The horizontal axis (x axis) is typically used as time axis, where internally generated linear ramp voltage is used to move the spot across the screen from left to right at a rate that can be controlled by the operator.

Basic Features of the Oscilloscope

An oscilloscope contains four basic circuit blocks, namely:

• Vertical amplifier
• Time base
• Trigger
• Display

These basic circuit blocks are illustrated in the figure below:

Vertical Amplifier

The vertical amplifier conditions the input signal so that it can be displayed on the CRT. The vertical amplifier provides controls of volts per division, position and coupling, allowing the user to achieve the desired display. This amplifier must have a high sufficient bandwidth to ensure that all of the significant frequency components of the input signal reach the CRT.

Trigger

The trigger is responsible for starting the display at the same point on the input signal every time the display is refreshed. It is the stable display of a complex waveform that allows the operator of an oscilloscope to make interpretation of the waveform and how it affects the operation of the device under test.

Time Base

This circuit block also referred to as the horizontal system causes the input signal to be displayed as a function of time. The circuitry in this block causes the CRT beam to be deflected from left to right as the input signal is being applied to the vertical deflection section of the CRT. Controls for time-per-division and position (or delay) allow the operator of the oscilloscope to adjust the display for the most useful display of the input signal.

Analog vs. Digital Oscilloscopes

An analog oscilloscope is built around a cathode ray tube (CRT); since CRT display depends on the production of visible light from the phosphor being excited by an electron beam, the display must be refreshed frequently; this makes analog oscilloscope a low-dead-time display system that can follow rapidly changing signals, additionally, there is little lag time in front panel control settings however this also presents a downside as it implies that analog scopes do not have any waveform storage. Digital oscilloscopes, widely used in modern measurement and analysis applications are basically computers with fast analog-to-digital converters (ADCs) and an LCD or OLED display. The waveform data in a digital scope can have correction factors applied to remove errors in the scope’s acquisition system and then stored, measured and/or displayed. Digital scopes offer many advantages over the analog scopes, for example: they have better accuracy than analog scopes because the microprocessor can apply correction factors to the data to correct for errors in the calibration of the scope’s vertical system. The timing accuracy of a digital oscilloscope is an order of magnitude better than that of an analog scope. Additionally, digital scopes can store the waveform data for comparison to other test results or uploading to a computer for analysis or project documentation.

Applications of Oscilloscopes

An oscilloscope can make a sinusoidal wave appear to stand still; this makes a scope a helpful tool for analyzing time-varying voltages.

The ability of an oscilloscope to “freeze” a high-frequency waveform makes it an excellent instrument for testing electronic components and circuits whose response curves, transient characteristics, phase relationships and timing relationships are of essential importance. For instance, oscilloscopes are used to study the shape of specific waveforms such as squarewave, sawtooth, etc.

Oscilloscopes are used to measure static noise (current variation caused by poor connections between components), pulse delays, impedances, digital signals, and so forth.

The Oscilloscope Selection Guide

Analog or Digital Scope

Various factors need to be considered when choosing between an analog scope and a digital scope. For example, when it comes to accuracy, digital scopes are more accurate than analog ones, analog scopes do not have continuous displays.

Display dead-time (the time than an oscilloscope is blind to the input signal has an effect on the scope’s ability to display rapidly changing signals). While display dead-time affects both analog and digital scopes, it is low in analog scopes.  This is a very important consideration in the operation of a digital scope because it determines the scope’s ability to respond to front-panel commands and to follow changing waveforms.

How much Bandwidth?

You only need to purchase scope with the bandwidth required to make the measurement. The bandwidth (BW) of the scope’s vertical system can affect the scope ability to correctly display narrow pulses and to make interval measurements. Because of the scope’s Gaussian frequency response, one can determine its ability to correctly display a transient event in terms of risetime (tr) with the equation:

tr = 0.35/BW

Hence, a 100 MHz scope will have a rise time of 3.5 ns.

The scope’s risetime should be at least no more than 1/5 of the shortest time interval to be measured. For time interval measurements, this should be > 1/10.

How many Channels?

Presently, most oscilloscopes are dual-channel models. We also have models described as being 2+2 and 4 channels.

The 2+2 models have limited features on two of their channels and cost less than 4-channel models. Both of these four channels classes are useful for applications involving the testing and development of digital based systems where the relationship of several signals must be observed.

Sampling Speed

Scope’s sampling speed is a function of memory depth and full-scale time base setting. Establish the sampling speed at the sweep speeds that your application is most likely to require before selecting a scope. For example, if waveforms are mostly repetitive, you can save on costs by choosing a scope that offers equivalent time or random repetitive sampling.

Triggering

Triggering is useful troubleshooting tool for electronic components to establish whether a suspected condition exists or not. Extra triggering features add complexity to the cope’s user interface; therefore try them out to ensure that they can be easily applied in the field.

Memory Size

As aforementioned, memory depth and sampling speed are interrelated. The memory depth needed depends on the time span required to measure and the time resolution required. The longer the time span to be captured and the finer the resolution required, the more memory is required.

Analysis Functions

Since digital scopes are microprocessor-based, they have the ability to perform mathematical operations that can give additional insight into waveforms. These operations include: addition, subtraction, multiplication, integration, and differentiation.

Computer Input/output

Most digital scopes available from different manufacturers can be interfaced to a PC. Trace images can be incorporated into documents as either PCX or TIF files. Waveform data can be transferred to spreadsheet applications for additional analysis.

How Reliable is the Display?

Since all digital scopes operate on sampled data, they are subject to aliasing i.e. the false reconstruction of the signal caused by under-sampling the original. An alias will often be displayed as a lower frequency than the actual signal. Some manufacturers use proprietary methods to minimize the likelihood of this problem occurring. Therefore test the scope before purchasing to establish if it produces the correct or aliased display.

Related articles:

• Key Instruments Used By Electronics Engineers
• Radio Frequency (RF) Signal Generator – Features & Applications
• Pulse Generator: Function, Operation & Applications
• Function Generator: Features & Applications
• Radio Frequency Spectrum Analyser
• RF Scalar and Vector Network Analyzers
• Radio Frequency (RF) and Microwave Spectral Analysis
• RF Power and Voltage Measurement