Oscilloscope basics - an overview or tutorial about what an oscilloscope, or scope is, how to use it and how it works. This oscilloscope tutorial is split into several pages: [1] The basics of an oscilloscope [2] Oscilloscope probes [3] How to use an oscilloscope [4] Video of using 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: 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. 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. 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. Oscilloscope probes Oscilloscope probes normally comprise a BNC connector, the coaxial cable (typically around a metre in length) and what may be termed the probe itself. This comprises a mechanical clip arrangement so that the probe can be attached to the appropriate test point, and an earth or ground clip to be attached to the appropriate ground point on the circuit under test. Care should be taken when using oscilloscope probes as they can break. Although they are robustly manufactured, any electronics laboratory will consider oscilloscope probes almost as "life'd" items that can be disposed of after a while when they are broken. Unfortunately the fact that they are clipped on to leads of equipment puts a tremendous strain on the mechanical clip arrangement. This is ultimately the part which breaks. 1X and 10X oscilloscope probes The 1X probes are suitable for many low frequency applications. They typically offer the same input impedance of the oscilloscope which is normally 1 M Ohm. However for applications where better accuracy is needed and as frequencies start to rise, other test probes are needed. To enable better accuracy to be achieved higher levels of impedance are required. To achieve this attenuators are built into the end of the probe that connects with the circuit under test. The most common type of probe with a built in attenuator gives an attenuation of ten, and it is known as a 10X oscilloscope probe. The attenuation enables the impedance presented to the circuit under test to be increased by a factor of ten, and this enables more accurate measurements to be made. As the 10X probe attenuates the signal by a factor of ten, the signal entering the scope itself will be reduced. This has to be taken into account. Some oscilloscopes automatically adjust the scales according to the probe present, although not all are able to do this. It is worth checking before making a reading. Adjusting 10X oscilloscope probes 10X oscilloscope probes also allow some compensation for frequency variations present. A small variable capacitor enables adjustment to be made. Most oscilloscopes have a small square wave oscillator output. By attaching the oscilloscope probe to this a quick adjustment can be made. As the square wave requires all the harmonics to be present in the correct proportions to provide a "square" wave, the probe can be quickly adjusted accordingly. If the leading edges of square wave, when viewed on the screen has rounded corners, then the high frequency response of the probe is low and an adjustment can be made. However if the leading edges have spikes and rise too high, falling back to the required level, then the high frequency response has been enhanced and this needs to be adjusted. Only when the square wave is truly square is the frequency response correct. Other types of probe Although they are not as common, 100X probes are also available. These oscilloscope probes tend to be used where very low levels of circuit loading are required, and where the high frequencies are present. The difficulty using the is the fact that the signal is attenuated by a factor of 100. Although 10X probes are widely used because of their superior response, they are not able to provide all the performance that may be needed for some applications. By using active electronic circuits in the remote end of the oscilloscope probe it is possible to offer very high levels of performance. However active probes are very costly and are normally only used in specialised professional applications Summary Oscilloscope probes are an essential addition to any oscilloscope. In most case 10X passive scope probes may be used, but other types of test probe need to be considered dependent upon the applications envisaged. Basic oscilloscope controls In view of the flexibility and level of control required to use an oscilloscope, there are a large number of controls that are present. These need to be set correctly if the required view of the signal is to be obtained. Fortunately it is quite easy to get used to operating an oscilloscope and using the controls to be able to view the waveform correctly. A summary of the main controls on an oscilloscope is given below: Nevertheless, a short overview of some of the controls is given below: Vertical gain: This control on the oscilloscope alters the gain of the amplifier that controls the size of the signal in the vertical axis. It is generally calibrated in terms of a certain number of volts per centimetre. Therefore by setting the vertical gain switch so that a lower number of volts per centimetre is selected, then the vertical gain is increased and the amplitude of the visible waveform on the screen is increased. When using the oscilloscope, the vertical gain is normally set so that the waveform fills the vertical plane as best as possible, i.e. as large as possible without going outside the visible or calibrated area. Vertical position: This control on the oscilloscope governs the position of trace when no signal is present. It is normally set to a convenient line on the graticule so that measurements above and below the "zero" position can be measured easily. It also has an equivalent horizontal position control that sets the horizontal position. Again this one should be set to a convenient position for making any timing measurements. Timebase: The timebase control sets the speed at which the screen is scanned. It is calibrated in terms of a certain a certain time for each centimetre calibration on the screen. From this the period of a waveform can be calculated. This if a full cycle of a waveform too 10 microseconds to complete, this means that its period is 10 microseconds, and the frequency is the reciprocal of the time period, i.e. 1 / 10 microseconds = 100 kHz. Normally the timebase is adjusted so that the waveform or a particular point on the waveform under investigation can be seen at its best. Trigger: The trigger control on the oscilloscope sets the point at which the scan on the waveform starts. On analogue oscilloscopes, only when a certain voltage level had been reached by the waveform would the scan start. This would enable the scan on the waveform to start at the same time on each cycle, enabling a steady waveform to be displayed. By altering the trigger voltage, the scan can be made to start at a different point on the waveform. It is also possible to choose whether to trigger the oscilloscope on a positive, or a negative going part of the waveform. This may be provided by a separate switch marked with + and - signs. Trigger hold-off: This is another important control associated with the trigger function. Known as the "hold-off" function it adds a delay to the trigger to prevent it triggering too soon after the completion of the previous scan. This function is sometimes required because there are several points on a waveform on which the oscilloscope can trigger. By adjusting the hold-off function a stable display can be achieved. Beam finder: Some oscilloscopes possess a beam finder function. This can be particularly useful as it is possible that sometimes the trace may not be visible. Pressing the beam finder button enables the beam to be found and adjusted so that it is in the centre of the screen. Although there are many other controls, these are the main ones to understand when learning how to use an oscilloscope. Nevertheless it is very useful to understand the other controls on an oscilloscope, but some will vary from one type to another. First steps in using an oscilloscope Using an oscilloscope is quite easy once one has been used and it is possible to become familiar with the use of the controls. The first stage comes when turning on an oscilloscope and this is where knowing a few steps about how to use an oscilloscope can be very useful. Turn power on: This may appear obvious but is the first step. Usually the switch will be labelled "Power" or "Line". Once the power is on, it is normal for a power indicator or line indicator light to come on. This shows that power has been applied. Wait for oscilloscope display to appear: Although many oscilloscopes these days have semiconductor based displays, many of the older ones still use cathode ray tubes (crts), and these take a short while to warm up before the display appears. Even modern semiconductor ones often need time for their electronics to "boot-up". It is therefore often necessary to wait a minute or so before the oscilloscope can be used. Find the trace: Once the oscilloscope is ready it is necessary to find the trace. Often it will be visible, but before any other waveforms can be seen, this is the first stage. Typically the trigger can be set to the centre and the hold-off turned fully counter-clockwise. Also set the horizontal and vertical position controls to the centre, if they are not already there. Usually the trace will become visible. If not the "beamfinder" button can be pressed and this will locate the trace. Set the gain control: The next stage is to set the horizontal gain control. This should be set so that the expected trace will nearly fill the vertical screen. If the waveform is expected to be 8 volts peak to peak, and the calibrated section of the screen is 10 centimetres high, then set the gain so that it is 1 volt / centimetre. This way the waveform will occupy 8 centimetres, almost filling the screen. Set the timebase speed: It is also necessary to set the timebase speed on the oscilloscope. The actual setting will depend on what needs to be seen. Typically if a waveform has a period of 10 ms and the screen has a width of 12 centimetres, then a timebase speed of 1 ms per centimetre or division would be chosen. Apply the signal: With the controls set approximately correctly the signal can be applied and an image should be seen. Adjust the trigger: At this stage it is necessary to adjust the trigger level and whether it triggers on the positive or negative going edge. The trigger level control will be able to control where on the waveform the timebase is triggered and hence the trace starts on the waveform. The choice of whether it triggers on the positive or negative going edge may also be important. These should be adjusted to give the required image. Adjust the controls for the best image: With a stable waveform in place, the vertical gain and timebase controls can be re-adjusted to give the required image. Summary One a few measurements have been made, it becomes much easier knowing how to use an oscilloscope. As oscilloscopes are one of the mainstay pieces of equipment, it is important for anyone involved in electronics to know how to use an oscilloscope and how to make the best use of them. The oscilloscope is a particularly useful item of test equipment that can be used for testing and fault-finding a variety of electronics circuits from logic circuits through analogue circuits to radio circuits. It is necessary to know how to use an oscilloscope properly to be able to make the best use of it. By knowing the basics of using an oscilloscope it is possible to fault find circuits more effectively and more swiftly as well as gaining a better understanding of how they the circuits work. Summary One a few measurements have been made, it becomes much easier knowing how to use an oscilloscope. As oscilloscopes are one of the mainstay pieces of equipment, it is important for anyone involved in electronics to know how to use an oscilloscope and how to make the best use of them.