Five key tips for using an oscilloscope

The oscilloscope is one of the most widely used pieces of test equipment. It is used my many engineers almost every day and as a result it sometimes helps to recap some simple but important tips that enable the best to be gained from this type of test equipment.

When making measurements using an oscilloscope there are a number of pitfalls that result in measurements not being as accurate or useful as they might otherwise be. Taking a few simple precautions when using an oscilloscope, it is possible to ensure that the measurements taken are as accurate as possible and that the measurements are as useful as possible.

Some of the precautions result from knowing the limitations of using an oscilloscope. However other precautions can be reflected in the way the oscilloscope is used, and the equipment, including probes that are used with the test equipment.

Beware of oscilloscope bandwidth limitations
As with any item of test equipment used for making measurements of signals and waveforms the bandwidth of the oscilloscope is particularly important. If the oscilloscope is to be able to reproduce the image of the waveform accurately then it must have sufficient bandwidth to accommodate the frequencies within the signal.

For sinusoidal type waveforms, measurements should not be made near the 3dB bandwidth as operating in this area will give a 30% reduction in the signal level. For digital measurements, the expected rise time of the signal can be used to determine the bandwidth requirements of the oscilloscope.

Ensure the correct triggering
In order that any waveform can be seen clearly when using an oscilloscope, it is necessary to ensure that the oscilloscope triggers correctly. If it does not trigger properly then the waveform will not be seen clearly. In view of the fact that it is necessary to view many complex waveforms on an oscilloscope, it is not always possible to enable it to trigger correctly when using the automatic trigger facility taken from the input channels. When looking at the best methods it is necessary to consider using the external trigger, and using a pulse of other suitable waveform from another point on the circuit under test. In this way it is often possible to gain a much better signal so that the oscilloscope can display the optimum image of the waveform.

Use the right oscilloscope probe
In the same way that the performance of any piece of test equipment can have its performance limited by some of the peripheral equipment, the same is particularly true when using an oscilloscope. While it is common to focus on the specification of the oscilloscope as this is the item into which the major investment is made. However the performance of the oscilloscope probe is just as important. If a poor oscilloscope probe is used then the performance of the whole test equipment will be impaired.

The oscilloscope probe should provide a simple way of presenting the signal on the board or item under test to the input of the oscilloscope, and ideally being totally transparent. A typical probe will consist of three major elements: probe tip; length of shielded wire; and a compensation network.

The most common types of oscilloscope probe are the passive probes. Of these there are two major types that are used namely a X1 and a X10. The X1 probe presents the signal as it is to the oscilloscope. Normally the oscilloscope input impedance is 1 Mohm. However this can load the device under test and distort the waveform. In addition to this tip capacitance of a x! oscilloscope probe can be as high as 100 pF. To overcome some of these problems and load the circuit under test less, a X10 probe can be used. Providing an input impedance of 10 Mohms and a capacitance which may be typically around 10pF, it will distort the waveform much less. Where even greater levels of performance are required, active oscilloscope probes may be sued. Having the active element very close to the tip of the oscilloscope probe, these have much higher levels of input impedance and lower levels of capacitance.

Remember to calibrate the oscilloscope probe
When using an oscilloscope, it is very easy to plug the oscilloscope probe in and start to make measurements. Unfortunately oscilloscope probes need to be calibrated before they are sued to ensure that their response is flat. There is a built in calibrator on virtually every oscilloscope for this purpose. It provides a square wave output, and there is a small preset adjustor on the probe. With the oscilloscope probe connected to the output of the calibrator the shape of the waveform displayed on the screen should be adjusted until it is perfectly square. If the high frequency response of the probe is down then the edges of the square wave will be rounded. If it is up then the square wave edges will show overshoot.

Although a simple adjustment, it is essential that it is undertaken to ensure that the performance of the probe is correct.

Beware using ground clips for high speed measurements
Oscilloscope probes normally come with a clip that is removable, and an earth or grounding clip that provides the earth return to the circuit under test. The clip can normally be taken to a convenient earth test point on the board. While this is perfectly adequate for most low frequency measurements and tests, when high speed tests are to be made, it is not satisfactory. The wire to the clip introduces inductance and this can introduce ringing into the circuit and this affects the measurements.

The only way to overcome this problem is to significantly reduce the length of any return path, and also reduce the length of wire to the actual probe tip. This can be done by removing the ground clip and its associated wire. In addition to this the clip on the centre of the probe can also be removed. This leaves a short exposed coaxial style connection. The centre connection or pin is for the signal and the surrounding connection is for a direct connection to an earth plane. While this may present some physical problems in making connections, it provides a far better level of electrical performance.

Using the oscilloscope
Oscilloscopes are easy to use, and with the developments in recent years oscilloscopes, along with many items of electronic test equipment have become even more versatile. It is possible to use them to see waveforms that might not have been at all easy even a few years ago. However the basic knowledge of using oscilloscopes is only gained from years of using them, but it is hoped that these few tips will enable them to be used in a better way and to be able to gain more from them.

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