What should you think about when specifying a USB oscilloscope?

EasysyncUSB-based oscilloscopes are becoming increasingly popular. Performance enhancements and new levels of functionality mean these instruments are becoming more attractive to test engineers.

They can be used in a range of applications from OEM production lines right through to academic research.

In the following article, the basic structure of a USB scope will be detailed, along with its various operational parameters. It will then go on to cover the main criteria, both technical and economical in nature, that need to be met when attempting to specify one.

For a long while USB oscilloscopes were not given the respect they deserved, and have sometimes been regarded by the industry as little better than toys. This has changed, following the emergence of more sophisticated models with enlarged feature sets and raised performance levels.

Also the more cost-sensitive environment in which we now find ourselves has meant that the purchase of conventional bench-top oscilloscopes harder to justify.

Basic USB scope setup

Figure 1 shows the main constituent elements found in a USB scope test system. It is very similar in structure to a standard digital storage oscilloscope, with analogue signals being passed through data conversion circuitry.

The waveform produced is then kept in a digital form, making it easier to work with. However, whereas the data in a digital storage oscilloscope normally has to rely on the unit’s own processing mechanisms, a USB scope can make use of the additional processing capabilities provided by the connected computer.

22jan14EasySYNCFig1The signals acquired by the oscilloscope from the test circuit will be subjected to pre-conditioning processes. This can include amplification or attenuation, adjustment of the DC offset, and filtering, in order to ensure that the signal is in the optimal range for the subsequent stages.

Some aspects of the pre-conditioning circuit are typically adjustable using controls on the computer software application, allowing the test operative to configure parameters such as the gain and offset to get the optimal display of the waveform.

The acquired analogue signals will then be transformed into digital data using an analogue-to-digital converter (ADC) device. The resolution and the speed of the ADC will each have a great deal of impact on the performance of the oscilloscope.

Because USB oscilloscopes derive many of their key functions from the laptop/desktop computer to which they are connected, they present a much lower financial outlay than those of conventional oscilloscopes – thereby making them a more attractive prospect, generally speaking.

By using the connected computer as the user interface through which data is studied, the test operative can benefit from a broader array of analysis software solutions (though often the scope manufacturer will have its own related software which can be uploaded onto the computer).

When setting up a USB oscilloscope test system it will normally be necessary to install device drivers onto your computer so that communication between the computer and the oscilloscope can be achieved. Once the drivers are in place, the supporting software can be installed too.

Specifying a USB oscilloscope

Benefitting from the widespread adoption of the USB interconnect over what is a period of almost two decades, USB oscilloscope are, as we have seen, less expensive than even low end bench-top oscilloscope alternatives.

There is now a choice of different USB oscilloscopes available, each offering their own particular set of functions and performance levels, as well as corresponding price tags.

Ensuring that the model eventually specified is as close a match as possible to the demands of the test procedure, while also keeping to an acceptable budget, is paramount. It is advisable to thoroughly check the manufacturer’s datasheet, to as to ensure that there is no confusion about what the model is actually capable of.

If this is not uncovered until after purchase, then additional costs will be accrued in having to upgrade to a higher spec model. It should be ascertained whether:

Multi-channel operation is likely to be needed.

Measurements will be varying, repetitive or just single shot.

High sampling rates will be called for.

The performance of the model must be examined. The greater the resolution it supports, the more accurate the acquired data will be.

Selecting a 10-bit or 12-bit model, as opposed to one that only has 8-bit resolution, may prove beneficial – as the waveforms can be analysed more meticulously, so there is less chance of errors emerging.

The bandwidth supported is another important parameter – the frequency of the specified scope must be above the maximum frequency that will be measured in the test procedure, with ideally some additional provision.

For test procedures, where analysis of 2nd or 3rd order harmonics is required, the bandwidth will need to several times more than the measured signal. The sample rate of the scope must also be several times the frequency of the signal being measured to ensure accurate reproduction on the screen.

The supporting equipment must also be given thought. It is pointless specifying a relatively high value instrument that satisfies all your technical demands only to find out afterwards that the probes that are supplied with it are not going to be up to the job.

Cheaper, poor performance probes can potentially distort the signal being captured and result in the data derived from the test process being inaccurate and of little real value.

Use of x10 probes is recommended as this will enable the input range covered to be significantly extended and consequently allow analysis of a broader spectrum of different applications and test scenarios.

Likewise the oscilloscope’s accompanying software should be both simple to install and intuitive to use. Its graphic user interface needs to be designed in such a way that it is easy to navigate through all the available function options.

Though in the past USB oscilloscopes struggled to be taken seriously, their utilisation of has increased substantially over the last few years. This is thanks to the widespread prevalence of USB interconnect technology, as well as the price performance advantages and overall convenience that these items of equipment offer.

By following the tips outlined in this article and scrutinizing product data sheets, USB oscilloscope purchase can be made with a far greater degree of authority.

As a result the product will be better suited to the test/analysis tasks for which it is to be employed and the costs involved will be kept to a minimum.

Susan Maxwell is general manager at USB instrument supplier EasySYNC




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