Eye Diagram Analysis for 10 Gbps SerDes: Validating Your Channel Before You Spin
A PCB designer routes a 10 Gbps SerDes lane across a 20 cm FR-4 trace with two connectors. Learn how to use S-parameter data and an eye diagram simulation to catch a failing channel before committing to fab.
Contents
The Problem With Trusting Your Layout Tool
You've routed a 10 Gbps SerDes lane — a PCIe Gen 3 or XAUI link — across a 20 cm FR-4 trace with two edge-mount SMA connectors. The DRC is green, the impedance is 100 Ω differential on paper, and the trace is straight with minimal vias. Should be fine, right?
Maybe. Maybe not. At 10 Gbps, your Nyquist frequency is 5 GHz, and FR-4 loses roughly 0.5–1 dB/cm at that frequency depending on the specific laminate grade. A 20 cm run is 10–20 dB of insertion loss before you've even touched the connectors. Add two connectors at 1–2 dB each and you're looking at a channel with 12–24 dB total loss at Nyquist — potentially enough to collapse the eye entirely.
The only way to know for sure, without spinning a board, is to simulate the eye diagram from measured S-parameters.
What You Need: A 2-Port S-Parameter File
Before opening the eye diagram tool, capture your channel response with a VNA. The file should be:
- Format: Touchstone .s2p (2-port)
- Frequency range: 10 MHz to at least 15 GHz (3× the data rate is a good rule)
- Points: 1001 or more, log-spaced or linear-spaced both work
- Port reference impedance: 50 Ω single-ended (100 Ω differential for a differential pair requires a 4-port .s4p, or a 2-port measurement of the mixed-mode S21)
| S-parameter | What It Tells You | Typical Limit (10 Gbps) |
|---|---|---|
| S21 magnitude at 5 GHz | Insertion loss at Nyquist | Better than −15 dB |
| S11 magnitude at DC–5 GHz | Return loss / impedance mismatch | Better than −10 dB |
| Group delay variation | Inter-symbol interference risk | Less than 50 ps pk-pk |
Setting Up the Eye Diagram Tool
Upload your .s2p file to the Eye Diagram tool and configure these parameters:
| Parameter | Value | Reason |
|---|---|---|
| Data Rate | 10e9 bps (10 Gbps) | Matches SerDes link rate |
| PRBS Length | PRBS-15 | Standard for BER testing; long enough to stress ISI |
| Samples per UI | 64 | Good time resolution without excessive compute |
| Input voltage swing | 800 mVpp differential | Typical SERDES TX swing |
| Rise/fall time | 35 ps (10–90%) | Typical for a 10G TX driver |
Reading the Results: Open vs. Closed
A healthy eye at 10 Gbps should show:
The tool reports these numbers directly. As a rough guide:
| Eye Height | Eye Width | Verdict |
|---|---|---|
| > 200 mV | > 0.5 UI | Pass — comfortable margin |
| 100–200 mV | 0.35–0.5 UI | Marginal — use equalization |
| < 100 mV | < 0.35 UI | Fail — channel too lossy |
What To Do When the Eye Is Closed
Option 1: Reduce trace length. The simplest fix. If you can reroute to 12 cm instead of 20 cm, you recover approximately 4–8 dB of insertion loss. Re-simulate to confirm. Option 2: Switch to a lower-loss laminate. Moving from standard FR-4 to a mid-loss laminate like Isola 370HR or Panasonic Megtron 6 cuts loss at 5 GHz by 30–50%. The Controlled Impedance calculator can help you verify the new stack-up dimensions hold your 100 Ω target. Option 3: Add a CTLE or DFE equalizer. Most 10G SerDes PHYs have a continuous-time linear equalizer (CTLE) with adjustable peaking. A CTLE with 6 dB of peaking at 5 GHz can rescue channels with up to −22 dB insertion loss. Run the simulation again with the CTLE transfer function applied to see the equalized eye. Option 4: De-embed the connectors. If your VNA measurement includes fixture launches or connector pads you're not using in the final design, de-embed them. Even 1 dB of artificial loss recovery can move a marginal eye into the pass zone.A Note on Via Stub Resonance
One failure mode that S-parameter simulation catches but layout checks miss entirely: via stub resonance. A through-hole via on a 1.6 mm board with a 0.8 mm stub resonates at approximately:
That's well above 5 GHz, so a standard via is fine. But a 3.2 mm stub (common if you route in the middle of a thick backplane) resonates near 12 GHz — adding a notch that chops the eye. The Via Stub Resonance calculator will flag this before you even capture the S-parameters.
Before You Send the Files to Fab
The eye diagram tool turns a gut-feel layout check into a quantitative pass/fail decision. Upload the measured .s2p, enter your link parameters, and look at two numbers: eye height and eye width. If both are in the green zone, commit. If not, you know exactly which knob to turn before you spend money on a board spin.
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