Eye Diagram from S-Parameters
Upload a Touchstone (.s2p) file and generate a PRBS eye diagram. Shows the channel insertion loss and impulse response, then renders the eye with its height, width and jitter measured over the whole bit sequence.
How It Works
An eye diagram superimposes many unit intervals (UI) of a digital waveform to visualise signal integrity. The "eye opening" shows how much voltage and timing margin exist for a receiver to correctly sample the data.
This tool constructs the eye from S-parameter data. It reads the channel's transmission (S21) from a Touchstone file and passes one full period of a standard PRBS (ITU-T O.150 polynomials) through the channel in the frequency domain, so the whole impulse response is kept however long the channel's delay. Above the file's highest frequency the response is extended with a 1/f roll-off. The received waveform is aligned to the transmitted bits and sliced into UI-length segments.
Metrics are measured over every bit of the sequence, not over the traces drawn: eye height is the worst-case opening between the received ones and zeros at the sampling instant, eye width the horizontal opening at the decision threshold, and jitter the spread of the threshold crossings. When the received waveform has no crossings at all, width and jitter are reported as not measured, with the reason, rather than given a default.
PRBS-7 is suitable for quick checks; PRBS-15 covers most realistic ISI patterns; PRBS-31 (Pro) stresses the channel with longer run lengths and is required by standards like IEEE 802.3. Its period is too long to simulate whole, so 32,768 bits are simulated after the channel has settled.
Related Calculators
Read the Guide
FAQ
What Touchstone file format do I need?+
Upload a 2-port .s2p file where S21 represents the channel insertion loss. The file should cover frequencies from DC (or near-DC) up to at least 1.5x the Nyquist frequency of your data rate. 4-port .s4p files are also supported for differential channels.
Why is my eye completely closed?+
A closed eye typically means the channel loss is too high at the Nyquist frequency for the specified data rate. Check that your S21 insertion loss is reasonable (e.g. < 20 dB at Nyquist). Also verify the frequency range covers the full bandwidth needed.
What is the difference between PRBS-7 and PRBS-15?+
PRBS-7 generates a 127-bit sequence; PRBS-15 generates 32,767 bits. Longer sequences contain more bit patterns, producing more realistic inter-symbol interference (ISI). PRBS-15 is the standard choice for most signal integrity analysis.
How does the tool handle differential channels?+
For .s4p files, the tool extracts the differential insertion loss (SDD21) using mixed-mode conversion. Make sure your port numbering follows the convention: ports 1-2 on the near end, ports 3-4 on the far end.
Why are eye width and jitter shown as "not measured"?+
Width and jitter are measured at the points where the received waveform crosses the decision threshold. A channel that blocks the signal, or one whose output never changes sign, has no crossings to measure, so the tool says so instead of reporting a placeholder width.