Jitter Noise Floor indicators are often misunderstood and misused. What is the jitter noise base? The jitter noise base is the overall noise contributed by the oscilloscope itself to the jitter measurement results. It is often calibrated as a statistical result and is represented by the effective value or standard deviation value (when the average value is 0, the standard deviation value and the effective value are the same). If the jitter noise base is much smaller than the signal jitter to be measured, then the oscilloscope is the correct instrument to measure jitter. On the contrary, if the jitter noise base is much higher than the jitter to be measured, then the oscilloscope is not a suitable instrument.
Timebase Jitter is a part of the jitter noise base, which originates from the jitter of the oscilloscope’s clock, that is, the jitter of the input signal at the sampled time. Another part of the jitter noise base comes from the vertical noise of the oscilloscope. Jitter measurement is based on the comparison between the time when the signal crosses a threshold and the time when it should ideally cross a threshold. Vertical noise brings measurement error, which seems to add more jitter. The size of the error depends on the size of the noise and the conversion rate (slope) of the signal at the threshold position. More noise means more jitter, and faster slope means less jitter.
The jitter noise base depends on the vertical noise of the oscilloscope, the slope of the input signal and the time base jitter of the oscilloscope. When comparing the jitter performance of oscilloscopes, it is very important to understand whether the jitter noise base or time base jitter has been correctly calibrated. If the jitter noise base is calibrated, the vertical noise and the corresponding signal slope should also be known.