Oscilloscope basics

- an overview or tutorial about what an oscilloscope, or scope is, how to use it and how it works.

Oscilloscope Tutorial Includes:

Oscilloscope basics     Oscilloscope probes     How to use an oscilloscope    

Oscilloscopes, or scopes are an important tool in the armoury of the electronics engineer or tester. An oscilloscope enables waveforms to be seen and in this way makes it very much easier to see any problems occurring in an electronics circuit. In view of the advantages which they posses, oscilloscopes are an essential tool for any electronics laboratory or area testing electronics hardware.

The name oscilloscope, comes from the fact that it enables oscillations to be viewed. Sometimes the name cathode-ray oscilloscope, or CRO is used. The reason for this is that cathode ray tubes (CRT) used to be used to enable the waveforms to be displayed. Nowadays, LCDs, or plasma displays are used as they are smaller, and more convenient to use, especially as the do not require the very high voltages of the old CRTs.

Function of an oscilloscope

The function of an oscilloscope is to be able to display waveforms on some form of display. In the normal mode of operation time is displayed along the X-axis (horizontal axis) and amplitude is displayed along the Y axis (vertical axis). In this way it is possible to see an electronic waveform on an oscilloscope as it may be envisaged. The waveform could be likened to that of the ripples on travelling along the surface of a pond when a stone is dropped into it.

By seeing a waveform in this manner it is possible to see analyse the operation of the circuit and discover why any problems may exist.

Oscilloscope exterior

An oscilloscope will normally have a large array of items on the exterior of the case. The font panel will typically have a number of items on it:

  1. Display   The first things that is noticed on an oscilloscope is the large display that is used for displaying the waveform. This typically may take around a quarter of the space on the front panel or even a little more. It is often good to have a reasonably large display then it is easier to see the various elements of the waveform.
  2. Connectors   There is a variety of different connectors on the front panel. Typically there is an input for each of the channels to be displayed - often an oscilloscope will have more than one channel. Many oscilloscopes are dual channel and can therefore display two signals at the same time, allowing waveforms to be compared. Other inputs may include a trigger input that will enable the trace on the oscilloscope to be triggered according to this signal.
  3. Controls   There is a variety of controls on the oscilloscope:

    • Vertical gain / signal input sensitivity: This is generally calibrated in V/cm, i.e. each vertical division on the scale represents a given number of volts.
    • Timebase: This alters the speed at which the trace crosses the screen horizontally on the oscilloscope. It is calibrated in terms of time / division, e.g. 1ms / cm, assuming the divisions are at one centimetre intervals.
    • Trigger: The controls that are associated with the trigger enable the timebase of the oscilloscope to be triggered in various ways. This enables a still or stable picture to be obtained on the screen of the oscilloscope.

In order to be able to operate the oscilloscope correctly it is necessary to connect the right signals into the inputs, and also to use the controls correctly.

Operating an oscilloscope

Like any other piece of complicated test equipment, an oscilloscope can take a few minutes to get used to if one has not been used before. However once familiar with it, the controls soon become second nature and it becomes very easy to use.

It is obviously necessary to turn the oscilloscope power on, and then once it is running it may be necessary to adjust the intensity of the trace so that it is easily visible - often oscilloscopes will free run when no signal is present. If the oscilloscope does not free run, then no trace will be seen yet.

Then the next control to set is the vertical gain control. Set is so that the anticipated waveform will fill a reasonable amount of the screen. Leave some margin so that if it is bigger than expected, it will not go wildly off the screen.

Next set the timebase of the oscilloscope. This is often set so that a period of the waveform will fill most of the horizontal axis of the screen. If it is initially set to this then it can be adjusted to suit later.

Connect the signal to be viewed. The oscilloscope will posses a connector for the input - this is normally a BNC connector. In most cases where a connection to a circuit board is required, a scope probe will be used, so that they are easy to connect to pins or connection points on the board.

With the signal now present it is necessary to adjust the trigger control to gain a stable trace of the signal.

With a trace of the signal now visible, the vertical gain and timebase controls can be re-adjusted to produce the best picture of the signal.

Although these instructions do not give an exhaustive description of how to use an oscilloscope, the exact number of controls and operation will depend upon the particular scope in use. However they should enable the scope to be used in a basic but reasonable manner.

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More Test Topics:
Analogue Multimeter     Digital Multimeter     Oscilloscope     Spectrum analyzer     Frequency counter     Dip meter, GDO     Logic probe    
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