To maximize the design margins of an oscilloscope, various key specifications must be considered when evaluating it. Each specification can and will affect the margins.
In evaluating oscilloscopes, vendors provide varying ideas on which specifications are the most important. The oscilloscope’s noise floor is considered one of the most important specifications when margin testing a digital signal. Oscilloscope noise impacts eye width while eroding eye height and rise times.
Oscilloscope vendors will present noise floor in two ways. First, they will set the oscilloscope at 50mV/div and disconnect the channels. They will then take the measurement after turning on the noise’s vertical histogram, then the measurement result is compared with other oscilloscopes with similar bandwidth.
The next method used is referred to as “Noise Percentage on Screen”. Measurement is taken in the same manner as the first method, but the noise number is further divided by the total possible mV on the screen.
Aside from noise floor, oscilloscope accuracy is also affected by its inherent jitter. As UIS continue to get smaller, oscilloscope jitter plays a significant influence on jitter measurements. Oscilloscope users should not just accept the listed jitter specification of an oscilloscope, since its intrinsic jitter measurement floor translates into random jitter. Some vendors will only indicate the absolute lowest jitter number (sample clock jitter), which does not accurately reflect the results of actual testing.
Depending on the level of sophistication required, measuring the oscilloscope’s jitter measurement floor is quite a simple task. Using a high accuracy sine wave source, the sine wave can be measured via the oscilloscope. The oscilloscope’s time interval error measurement should be turned on, once the sine wave is displayed.
Finally, another important element to measure accuracy of the oscilloscope is bandwidth. To measure an edge accurately, the oscilloscope must have sufficient bandwidth to deliver the edge’s truthful representation. A slower representation of the edge will generate higher jitter than what actually exist.
According to the book of Dr. Howard Johnson entitled High-speed Digital Design — A Handbook of Black Magic, a key frequency element is referred to as the “knee” frequency (fknee). He stated that all fast edges feature an unlimited spectrum of frequency components, and that there is a “knee” or inflection in the fast edge’s frequency spectrum where frequency components, which are higher than fknee, are trivial in determining the signal’s shape.
Once the fknee’s value is determined, it is paramount to understand the oscilloscope’s frequency response. Once an oscilloscope delivers a brickwall response or a response that rolls off very quickly, then the oscilloscope can represent faster edges accurately with limited bandwidth. However, if the oscilloscope provides a Gaussian response, then it will need more bandwidth to accurately represent faster edges.
Overall, noise floor, measurement jitter floor and bandwidth are three of the most important components of the oscilloscope that can affect margins.
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