Understanding Oscilloscope Specifications / Specs

- summary, tutorial or overview providing information for understanding oscilloscope specifications / scope specs for buying an oscilloscope as new or used test equipment or for 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.

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.

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.

More oscilloscope specifications to follow .... .... .... ....

Summary

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.