Eye Height Result
The Jitter mode Eye Height measurement measures the height of each eye. The results are placed in the following results table:
- Amplitude table for NRZ signals.
- Eye table for PAM4 signals.
Use the Advanced tab of the Jitter Mode Measurement Setup dialog to specify that the measurement be based on probability or Symbol Error Ratio (SER). The eye height measurements specified in the latest PAM4 standards are made at probabilities instead of SER.
When measuring PAM4 waveforms, this measurement is the recommended and fastest Eye Height measurement when compared to Eye Mode's Eye Height measurement. Eye mode's Eye Height measurement on PAM4 waveforms can be used as a quick comparative check between signals. For example, when tuning pre-emphasis on a transmitter. In the following situations, you may want to use Eye Mode's measurement:
- For longer patterns, such as PRBS31Q, as Eye mode does not require pattern lock.
- When data is random or when measuring multiple waveforms at the same time.
Probability vs. SER (PAM4 signals)
To make an eye height measurement at a probability, we make a Cumulative Distribution Function (CDF) for the top and bottom eye edges at our sampling threshold. These CDFs are the probability that a level passes through the sampling threshold anywhere between this "y" location and the eye center. The probabilities range from 0 at the eye center to 1 outside the eye. However each level only occurs a fraction of the time (the symbol density, or ¼ in this example), so the CDFs need to be scaled by ¼ to calculate the eye height at an SER. In the general case, the probability on each side of the eye (top/bottom) must be scaled by the symbol density on that side.
For example, if we are interested in the eye height at a probability of 10-6:
- The equivalent SER to search for at the top is SDtop * 10-6 (0.25 * 10-6 in this case).
- The equivalent SER to search for at the bottom is SDbottom * 10-6 (0.25 * 10-6 in this case).
The top and bottom SERs at a given probability can be different from each other.