Buying an oscilloscope

- six points to determine when buying an oscilloscope or 'scope'

An oscilloscope or scope, is an essential element of test equipment for the electronics design, test and maintenance engineer. The oscilloscope displays amplitude on the vertical scale against time on the horizontal scale and as a result it provides a very good insight into what is happening in a circuit. While the oscilloscope is an essential tool for the engineer, it can be a costly item, and it is therefore necessary to specify the scope correctly when buying one. Requesting features on the oscilloscope that are not required will add cost to it and bring no benefit. Similarly not including features that are needed on the oscilloscope will limit its use, and again it will not provide the most cost effective solution.

When buying an oscilloscope, it will be quickly realized that there is a huge choice of available. Not only are there many different oscilloscope manufacturers, but in addition to this each manufacturer generally has a wide range of scopes, and furthermore there are options that can be included to increase their functionality - and their cost.

In view of the choice of oscilloscopes that is available, it can be difficult to determine which scope is going to offer the best value for money, and will perform its function best. One way is to simplify the decision making process by answering some simple questions and determining exactly what is required from the oscilloscope.

To assist in the choice of an oscilloscope, six questions are presented below. Each one discusses a different aspect of the performance of an oscilloscope and its aim is to help determine the level of functionality required for the oscilloscope in this area. It should be remembered that these are only offered as suggestions and no responsibility is or can be accepted for any decisions that result from them.

How many channels are needed?

One of the primary decisions to be made when choosing an oscilloscope is the number of channels that are required. For many scopes it is simply the number of channels that need displaying. This may be typically be two or four. This decision is quite easy.

For many digital oscilloscopes, the decision is not quite as easy because traditional two or four channel scopes do not always provide the required number of channels on which to trigger. Oscilloscopes known as 'Mixed Signal Oscilloscopes' MSOs are available and these provide additional logic timing channels in addition to the two or four scope channels. Assuming a four channel oscilloscope with 16 logic timing channels, this provides s scope with a total of 20 channels that can be used for triggering, acquisition and viewing.

What bandwidth is needed?

The bandwidth of an oscilloscope is one of the most important of its specifications. There are a variety of ways of looking at the required bandwidth dependent upon the signals that are likely to be viewed. A signal with sharp edges, like those produced by the logic circuits will contain many high frequency elements - harmonics of the fundamental signal. Removing these high frequencies by using a low frequency oscilloscope will result in the edges of a square wave becoming rounded. This may impair measurements such as the length of a pulse as the time between the rise and fall may be altered. On occasions the amplitude of the signal may be reduced if the fundamental frequency of the waveform is close to the limit of the oscilloscope.

In order to enable the correct bandwidth to be chose there are a few simple rules of thumb that can be applied:

It is often assumed that to fully capture a digital signal, the bandwidth should be around ten times the fundamental repetition rate of the particular signal.

Additionally for other analogue signals not containing harmonics in the same way, the oscilloscope bandwidth should be around twice the signal bandwidth. This ensures that the signal frequency is sufficiently far away from the roll off point of the oscilloscope to maintain its accuracy.

A further rule of thumb is that the sample rate for a digital oscilloscope should be four times the bandwidth.

What triggering capabilities are required?

Traditional oscilloscopes can be set to trigger on a particular channel and with in a positive or negative going edge. The triggering voltage can also be set. This provides amply triggering facilities for many applications, particularly on the analogue region. However for many digital applications where multi-channel trigger channels are available it may be helpful to trigger when a specific pattern arises. This can be very useful in debugging complex logic circuits. When considering the purchase of an oscilloscope, the test scenarios in which it may be used should be considered.

What sample rate is required?

The sample rate specification of an oscilloscope needs to be examined carefully. Typically the sample rate should be four times the bandwidth of the scope. Another method can be to determine the sample rate by determining the required resolution between points on the acquired waveform. The sample rate is simply then the inverse of the resolution.

This requirement is for each channel. Beware of the specifications as most oscilloscopes use interleaving where the sample rates can be increased when two or more channels couple their A2D converters to provide a maximized sample rate on only one or two channels of a four channel scope. It is often this highest specification that is used as the headline specification and reading the full specification will reveal the exact details.

What memory is required?

As most oscilloscopes in use today are digital, the amount of memory required to store the digitised waveform for processing is of great importance. Once a waveform has been captured, the digitised version can be processed a variety of ways and perform many functions including zooming in on an area for further investigation.

The sampling speed is specified as a number of samples per second. It can be seen that when a short time base period is chosen, a comparatively small number of samples are taken and a corresponding small amount of memory is used. However if a slow time base time is selected, the oscilloscope has to store very much more data if the same sampling speed is used.

In view of the fact that adding large amounts of memory to an piece of equipment adds cost, the amount of memory used in a scope must be kept within reasonable limits. It is clearly nonsensical to operate a scope at its maximum sampling rate for very slow time bases, especially if it is operating at sampling speeds of a Gigasample per second for example. It is a simple matter to calculate the memory required for a sample as it is simply the sample speed multiplied by the time of the overall sample (i.e. the time for the time base to traverse the screen).

Rather than sampling at the maximum speed all the time, the sampling speed is normally linked to the time base period. For most typical applications the manufacturers choice of linking will be adequate, but for more specialised applications it is necessary to check that the memory size will hold samples of the required definition.

What is the capability required for the display?

The display technology used in an oscilloscope is of great importance. Nowadays, the basic display is not the limiting factor in the way it used to be. Instead the display tends to be governed by the level of signal processing used in the oscilloscope. While the display should be checked to ensure that it is satisfactory, it is likely that all will be quite satisfactory.

In terms of deciding between the display functions it is necessary to compare the different offerings and estimate which ones are more likely to be used, and whether they are essential.

Additional requirements

Beyond these basic functional requirements for the oscilloscope it will also be necessary to put aspects such as the level of service and support that is required. These need to be assessed on the requirements of the company purchasing the oscilloscope. It is obviously necessary to be able to calibrate and maintain the equipment to ensure that it is always available when needed. However the level of support that is required will depend on the arrangements that are already in place. In some instances there may already be a repair and calibration contract in place and decisions need to be made, looking at the complete picture.

Technical support can be particularly useful. With all types of electronic equipment becoming more complicated there will be times when technical support is needed. This may be less for the more straightforward scopes, but for some high end ones this may be more of an issue.

The choice of test equipment is not always easy. By looking at the requirements and the options that are available it should be possible to gain a good match between them taking into account the price considerations. With such a comprehensive choice these days there should be little problem in finding a good solution.

By Ian Poole

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