It is critical for oscilloscope users to choose the oscilloscope that will meet all of their application needs. Oscilloscopes are grouped into two main types — the equivalent-time (ET) oscilloscopes and the real-time (RT) oscilloscopes. For every type of oscilloscope there are various specifications that users need to consider, and one of which is bandwidth. If the oscilloscope does not have sufficient bandwidth to meet the Nyquist criterion, users will encounter significant aliasing in their signal.
Although there is no simple way to determine how much bandwidth a user will need, oscilloscope vendors offers a “fifth harmonic” rule of thumb. Under this rule, oscilloscope users are advised to acquire an oscilloscope that has adequate bandwidth to capture the signal’s fifth harmonic. There are instances, however, that even if the oscilloscope features sufficient bandwidth to capture the fifth harmonic, it may not capture any fifth harmonic at all. Thus, it will be unwise to buy the oscilloscope, when it can only capture up to the third harmonic. Users could buy a lower-bandwidth oscilloscope that delivers similar harmonic content for reduced costs. This is also the reason why it is important for users to determine how much bandwidth they truly need as well as the required features.
The high-bandwidth oscilloscope’s higher price is not the only downside of purchasing more bandwidth than what is actually needed. Higher-bandwidth oscilloscopes also produce more noise, which in turn cause more distortion. The noise affects amplitude measurements as well as the accuracy of timing measurements, defeating the very purpose of the added bandwidth. The best solution is to use a measurement system that offers just enough bandwidth to accurately measure the signal while reducing the extra noise generated by the measurement system.
Generally, ET oscilloscopes deliver a lower noise floor compared to RT oscilloscopes. With proper research, however, users will discover that there are several RT oscilloscopes in the market that feature low noise floors.
Real-time oscilloscopes also provide various features that are not offered in ET oscilloscopes. The most important difference is that RT oscilloscope provide significantly higher sampling rates and memory depths than that offered by ET oscilloscopes.
With its capability to trigger itself, RT oscilloscopes also eliminate the need for an external trigger. In contrast, an ET oscilloscope needs an outside trigger while requiring a number of singlepoint acquisitions to display the data.
In addition, RT oscilloscopes commonly offer more automated compliance software applications, making it easier and faster for users to take measurements.
Meanwhile, ET oscilloscopes provide less noise and more bandwidth.
In fine, although most engineers employ the fifth harmonic to measure how much bandwidth is needed, RT oscilloscopes will not capture any fifth harmonic due to its high noise floor.
For it to be part of the measurement, users need to have an ET oscilloscope, which features low noise floor.
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