Oscilloscope Specifications: buying a scope

When buying or selecting an oscilloscope it is necessary to match the requirements to the specification

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Oscilloscopes are a very common form of test equipment - possibly the most important type of test equipment.

<>As a result it is often necessary to be able to choose one either from the test equipment store or as a rental or when buying an oscilloscope.

When selecting an oscilloscope, there are many different specifications and parameters to consider, each one related to the performance. When selecting an oscilloscope, what are the most important specifications and parameters and which ones will affect the performance of the scope in the particular application.

Types of oscilloscope

One of the major specifications associated with buying an oscilloscope is the actual type of oscilloscope that is required. Some types of scope will be able to perform other measurements better than others; some use current technology whereas others are older; and there may be cost implications as well.

Analogue, analogue storage, digital, digital storage, digital sampling, USB scopes and many more types are available.

. . . . . Read more about the different oscilloscope types.

Oscilloscope bandwidth specification

One important oscilloscope specification is related to the frequency or speed of the waveforms that can be measured. This is determined by the bandwidth of the oscilloscope and it is found that the capability of the oscilloscope to accurately display the waveform falls off with increasing frequency. The way in which this is specified can be seen in IEEE 1057 which defines electrical bandwidth as the point at which the amplitude of a sine wave input is reduced by 3 dB (i.e. attenuated to 70.7% of the true value of the signal - a fall of approximately 30%) relative to its level at a lower reference frequency.

The oscilloscope specification for bandwidth will typically be quoted in the format:   Bandwidth = -3dB at 1500 MHz. If the oscilloscope specification for the -3dB point is not sufficiently high it will be found that the edges of pulses and square waves will be slowed as a result of the reduction of the high frequency components. Also sine waves above the bandwidth frequency will be significantly attenuated - even those around the bandwidth frequency will suffer some attenuation.

In order to ensure that the oscilloscope specification is adequate it is necessary to ensure that the bandwidth of the scope is higher than the operating frequency. Often a Five Times Rule is used as a rule of thumb. Here the bandwidth of the oscilloscope should be five times the highest frequency component in the signal. Using this rule, the error due to the frequency limitations will be less than ±2%.

A type of oscilloscope as used in an electronics laboratory
A high performance digital oscilloscope type

Vertical DC gain accuracy

It is important when measuring the amplitude of signals, to know the accuracy of the measurement that is being made. As oscilloscopes are not intended to be used instead of digital multimeters, it is not anticipated that the voltage elements of the oscilloscope specification will be as accurate.

Vertical channel resolution

Digital oscilloscopes need to convert the incoming analogue signal into a digital signal. The vertical channel resolution determines the "granularity" of the signal. Most digital oscilloscopes have 8-bit resolution. Eight bits provides 256 digitizing levels (2 to the 8th power). Other oscilloscopes may have higher levels of resolution with a 10bit resolution giving 1024 different levels.

Rise time specification

Another important oscilloscope specification which needs to be accommodated is the rise time of the oscilloscope. This is a particularly important specification for any digital circuits where the edges on square waves and pulses are often of great importance. The oscilloscope must have a sufficiently fast rise time to capture the rapid transitions accurately, otherwise important information may not be displayed and the results could be misleading.

The rise time of the oscilloscope is defined as the time it takes for the image to rise from 10% to 90% of the final value.

Although the bandwidth of the scope must be sufficiently high, the rise time is also important. It can be seen as being akin to the slew rate on operational amplifiers where the rate of voltage change is the limiting factor. As a result the rise time of the scope must be sufficiently high to capture the required detail.

There is a relationship between bandwidth and rise time for a first order estimation. To estimate the rise time of an oscilloscope from its bandwidth, it is possible to use a simple formula:

BW   Tr = 0.45

    BW = 3dB bandwidth of the scope
    Tr = rise time.

This relationship for the oscilloscope rise time is used for most high end oscilloscopes. Historically, older types tended to use 0.35 rather than 0.45 as the constant. This corresponds to the roll off of a 1 or 2 pole filter.

Oscilloscope sample rate

With the proportion of digital oscilloscopes rising, the sample rate oscilloscope specification is becoming a more widespread and important specification. The sample rate is specified in samples per second (S/s). The faster the oscilloscope samples the waveform, the greater the resolution of the detail on the waveform and with greater sample rates the less the likelihood that any critical information will be lost. While the maximum sample rate tends to be the headline rate, the minimum sample rate may also be important. This is occurs when looking at slowly changing signals over longer periods of time. It is also important to note that the displayed sample rate changes with changes made to the horizontal scale control. This is to maintain a constant number of waveform points in the displayed waveform display.

For most applications it is necessary to define the minimum number of samples that are required and this should be used when looking at the overall oscilloscope specification. The oscilloscope takes in the waveform from the voltage input and then digitizes it, after which it is processed. For the display it is necessary to construct the waveform. To avoid aliasing, the Nyquist theorem dictates that the sampling frequency should be twice that of the highest frequency components to be displayed. However this makes some assumptions about repetitive waveforms, anomalous events such as glitches and interpolation methods. In reality it is better to assume that when using sin(x)/x interpolation (a common option). As a result the industry has adopted a rule of thumb:

Sample rate = 2.5 Highest frequency

If linear interpolation had been used, then a sample rate should be at least ten times the highest frequency signal component.

Memory depth

This is the memory for storing signals. The greater the memory depth the more signal it is possible to capture at the highest sample rate.

Memory depth = (Acquisition Time Window)       (Sample Rate)

With a 1MSa per channel an oscilloscope can capture 1 ms or time with a 1GSa/s sample rate. Thus sufficient memory must be available to capture this amount of data.

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Oscilloscope rental or purchase

The required oscilloscope specifications may be governed by decisions about how the test equipment will be obtained. There are several options: buying the oscilloscope new, as a used test equipment purchase, or test equipment rental. If used test equipment is required then this can provide a good option, especially if it is to be reconditioned test equipment then this can provide an excellent option. Reconditioned test equipment may be relatively new and can be obtained at significant reductions.

When considering the choice between new, used reconditioned test equipment or test equipment rental, this may alter the oscilloscope specification that is required. For the test equipment rental option, the period when the equipment is on site should be relatively short, and therefore the equipment can be matched to the particular requirements in hand. For either buying new or reconditioned test equipment, consideration should be given to other applications that the equipment may be used for. The oscilloscope specification should not only match the current application but, dependent upon cost, it should include some future-proofing. For specifications such as bandwidth and rise time, especially, some margin to allow for future faster higher speed developments may be applicable. Other specifications should also be viewed with a view to whether they would accommodate the development or measurement of future products.

There are very many specifications that will be seen in the datasheets for the different oscilloscopes. These datasheets can generally be obtained via the Internet. The scope specifications can then be compared and prices obtained to select the most suitable type and model to select, buy or rent for any particular situation.


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