Difference between conventional Eye-diagram voltage plot and "Eye-density" plot in ADS Keysight Simulation

Question

I know what is eye diagram. It is the sampled voltage plot at the clock frequency (Usually) of my circuit and superimposed. If the logic 0 corresponds 0V and logic 1 corresponds 1V then for the ideal case, the eye diagram Amplitude will also plot the superimposed samples of the signal which corresponds to 0V and 1V. For the lossy channel, the amplitude moves away from the 0V and 1V level for logic 0 and logic 1 respectively. And that movement depends on the attenuation and loss of the channel. Generally I have seen so far to be around 0.8V or 0.9V for logic 1. Like below.

This eye diagram is actually PAM4 level eye diagram. I generated in Cadence Virtuoso using its eye diagram tool using its embedded calculator. I am fine with that.

Now, I have experienced in ADS Keysight simulation that they have Eye density which also produces Eye diagram. but I see it always plot the eye diagram amplitude into half. Like my below eye diagram that I generated in ADS using its generic TX, RX driver and low pass filter like channel. I have driven the TX with V_low = -1V and V_high = 1V PRBS signal. So, the difference is 2V. At the receiver (in simulation), it produces the eye with V_high around 0.4V to 0.6V. This is the case most of the time I experienced in ADS. So I was confused that Eye density is not exactly the conventional Eye diagram plot (Maybe or my conception of Eye density is not clear).

Eye density Summary:

In the help of ADS it is written like below:

Density: A temperature view of the 3-dimensional eye histogram. The eye is automatically centered and displayed over 2UI on the horizontal axis. Color indicates the number of crossings of a segment in the time-voltage plane. Blue areas are cold, whereas red indicates comparatively more crossings.

In statistical mode, the density histogram is plotted as if 1e6 bits are run.

I am confused about what exactly then it produces if it is temperature view...because it is also showing the voltage amplitude of the signal. And the Amplitude (as it is showing in the summary is around 0.698V whereas my input signal difference is 2V. My channel was normal passive low pass filter-like channel.

So, is the Eye density plot in ADS different from conventional Eye diagram? If it is what is then? Because the definition in ADS help is not enough for me to understand density.

Answer 1

temperature view = RGB , Blue = least common , like a phosphor decay storage scope. But probably a colour histogram of all voltage samples for each sample of cycle time.

Your channel out/in = 0.7/2.0V is lossy and nothing much to say about this channel.

There is a conventional correlation between SNR and BER and eye pattern statistics. It is useful to predict error rate and improve margin statistics if possible with a short term measure instead of longer term comparing data for errors.

Symmetry, jitterRMS , width, height , & overshoot are all factors to consider.