The digital signal’s unwanted phase modulation, jitter is one of the most important specifications for measuring the quality of a device. In fact, a high jitter measurement can significantly affect a product, forcing it to be redesigned or from being shipped altogether.
Jitter can be accurately measured using an oscilloscope. Although most Windows-based oscilloscopes provide analysis software that helps users measure jitter, such are limited by the instruments’ jitter measurement floor, which can cause devices to unnecessarily fail tests while eroding crucial design margins. The jitter of the DUT (device under test) can not be measured if it is lower than the jitter measurement floor of the oscilloscope. Thus, it is important for engineers to understand the jitter specifications of an oscilloscope prior to making any purchase.
The most important jitter specification in an oscilloscope is the jitter measurement floor, which is a combination of the sample clock jitter and noise floor of the oscilloscope. This can result into random jitter in the oscilloscope, effectively affecting its accuracy.
It has to be noted, however, the oscilloscope vendors specify the jitter measurement floor in various ways. Hence, in order that engineers can make an educated decision when purchasing an oscilloscope, it is important that they understand the meaning of each specification, which include intrinsic jitter, jitter measurement floor as well as long-term jitter.
The intrinsic or sample clock jitter of an oscilloscope refers to the amount of jitter transmitted using an internal timing. For a clearer understanding of this definition, take for instance a real-time oscilloscope, which sample data at a relatively fast rate, up to 120 GSa/s. With this, ensuring that data points are aligned can be achieved in one of two ways — either by utilizing a chip or time-base system, which offers the tight-time correlation needed between the sampled input signals delivered to the internal clock and the A-to-D (analog-to-digital) converter. Much like the oscilloscope, the internal clock also has its own jitter specification and is characterized by how it can align the sample points of the oscilloscope through its time base. Consequently, sample clock jitter refers to the jitter specification of the whole oscilloscope time base.
Meanwhile, the jitter measurement floor of an oscilloscope is the combination of the slew rate’s noise influence and the sample clock jitter. It is also influenced by the oscilloscope’s noise floor. Generally, slower rise time (slew rate), implies greater influence of the noise floor. Taking into account the noise floor of the oscilloscope, this specification is critical since noise is one of the biggest contributors to oscilloscope jitter.
The last jitter specification of an oscilloscope is the long-term jitter. This specification measures the maximum change in the output transition of the clock from its ideal in a large number of data points.
In sum, an oscilloscope needs a low jitter measurement floor to obtain an accurate representation of what a design really looks like. However, reading the data sheet measurement is not enough in order that users can determine if the requirement is satisfied. Engineers should have a good understanding of what the oscilloscope vendor is specifying.
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