Understanding Oscilloscope Specifications / Specs

- a summary or tutorial for understanding the key oscilloscope specifications / scope specs for selecting, buying or test equipment rental.

Oscilloscopes are one of the most versatile items of test equipment. Oscilloscopes provide a graphical view of the waveforms within a circuit and this gives a particularly useful view of what is happening within a circuit. This makes them an essential item of test equipment for use within electronics design, production test and also for use within service organizations.

Oscilloscopes are relatively complicated items of test equipment. As a result they are more expensive than items such as digital multimeters and this means that choosing the correct oscilloscope is important.

In order to choose the right scope for purchase new or as used test equipment, or to obtain as test equipment rental, it is necessary to understand the specifications used. As oscilloscopes are fairly complicated, the oscilloscope specifications can also be fairly involved. These can normally be obtained either from the manufacturer, the used test equipment supplier, of the test equipment rental company. Once obtained they can be viewed and it can be relatively easy to shed some light on them so that the oscilloscope specifications can be understood.

When evaluating an oscilloscope for purchase, either new or as used test equipment or for use as test equipment rental it is necessary to ensure that the unit is suitable for the given application. Often banner specifications are used as promotional material to give a brief overview of the oscilloscope. While these top level specifications are useful in determining the broad level of performance, it is essential to look beyond them to those specifications that are deeper in the literature and give a more exact indication of the performance of the scope and its actual usefulness.

Types of oscilloscope

One of the major elements of an oscilloscope specification describes what type of oscilloscope the particular unit is. There are a number of different types, and each one is suitable for different applications:

  • Analogue oscilloscope:   The analogue oscilloscope is the traditional form of oscilloscope that has been used in laboratories for many years. It relies on analogue techniques and takes in the vertical and sometimes horizontal signals, amplifying them in an analogue format and displaying them on a cathode ray tube.
  • Digital storage oscilloscope (DSO):   The digital storage oscilloscope (DSO) is the conventional form of digital oscilloscope. It uses a raster type screen like that used on a computer monitor or television and in this way displays an image that fills the screen and may include other elements in addition to the waveform. These additional items may include text on the screen and the like.
  • Digital phosphor oscilloscope (DPO):   The digital phosphor oscilloscope (DPO) is a highly versatile form of oscilloscope that uses a parallel processing architecture to enable it to capture and display signals under circumstances that may not be possible using a standard DSO. The key element of a DPO is that it uses a dedicated processor to acquire waveform images. In this way it is possible to capture transient events that occur in digital systems more easily. These may include spurious pulses, glitches and transition errors. It also emulates the display attributes of an analogue oscilloscope, displaying the signal in three dimensions: time, amplitude and the distribution of amplitude over time, all in real time.
  • Sampling oscilloscope :   These oscilloscopes are used for analyzing very high frequency signals. They are used for looking at repetitive signals which are higher than the sample rate of the scope. They collect the samples by assembling samples from several successive waveforms, and by assembling them during the processing, they are able to build up a picture of the waveform. The oscilloscope specifications for these items may detail a frequency capability or bandwidth sometimes as high as 50 GHz. However these scopes are very expensive.

Beyond the basic type of oscilloscope there are many elements to the oscilloscope specification that need to be analysed to judge whether the performance meets the needs for the particular requirement in mind.

Image of an oscilloscope used for wide variety of test applications

Bandwidth specification

One important oscilloscope specification is related to the 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.

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%.

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 DC gain resolution

Today, many oscilloscope use all digital techniques, converting the incoming vertical or X axis voltage to a digital format. It is therefore important to know what the resolution of this scale is. Resolution and dynamic range determine, respectively, the "granularity" of measurements and the largest measurement that can be made without clipping the waveform. Most digital oscilloscopes have 8-bit resolution. Eight bits provides 256 digitizing levels (2 to the 8th power) over the dynamic range.

Checking the resolution within the oscilloscope specification will ensure that the instrument will offer the correct dynamic range and resolution.

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.

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.

In order to be able to capture the detail sufficiently a similar equation to that used for the bandwidth calculations is used:

Fastest signal rise time   ³   Oscilloscope rise time   x   5

This equation is only a guideline and enables signals to be viewed as accurately as required. Although in some instances where very fast rise times are required this may not be feasible.

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), the oscilloscope should have a sample rate at least 2.5 times that of the highest frequency component within the signal. If linear interpolation is used, then it is often accepted that the sample rate should be at least ten times the highest frequency signal component.

Test equipment 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.

The oscilloscope specifications described above are necessary when choosing a particular oscilloscope for a particular application. It is necessary to look in detail at the specification to see whether the instrument meets its requirements. This is particularly important when buying an oscilloscope either new or as used test equipment, or when looking to hire an oscilloscope from a test equipment rental company. By fully checking the specification it will be possible to determine whether the oscilloscope will be able to meet the requirement before placing an order. In this way it is possible to prevent considerable sums of money from being wasted.

By Ian Poole

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