IPI in High-Speed SerDes Channels

• IPI is defined as the time-domain skew between P and N conductors in high-speed SerDes channels, significantly impacting signal integrity at >200 Gb/s.
• Reciprocal metrics such as SILD and FOM_SILD enable precise quantification of intra-pair skew by analyzing de-skewed mixed-mode S-parameters over frequency.
• Accurate IPI diagnosis supports actionable design practices, including symmetric trace routing and controlled cable setup, to minimize BER and optimize channel performance.

Intra-Pair Instability (IPI) in high-speed differential serializer-deserializer (SerDes) channels refers to the time-domain mismatch ("skew") between the P and N conductors of a differential pair. As data rates exceed 200 Gb/s, the typical unit interval (UI) becomes shorter than ~5 ps. Within such constraints, even minor intra-pair skews on the order of 1–3 ps become significant fractions of the UI, leading to degradation of differential eye amplitude and a corresponding increase in bit-error-rate (BER). The precise quantification, diagnosis, and channel engineering around IPI is essential for achieving robust signal integrity in next-generation interconnect systems (Nozadze et al., 3 Nov 2025).

1. Definition and Physical Impact of IPI

Intra-Pair Instability is characterized by the relative timing difference with which differential signals (P/N) traverse a channel segment. The primary result is the desynchronization of the two halves of a differential symbol, reducing eye amplitude and impairing recoverable signal quality at the SerDes receiver. For signal rates greater than 200 Gb/s, intra-pair skew as small as 1–3 ps can introduce impairments comparable in magnitude to the UI itself, resulting in marked BER elevation and signal amplitude closure. This phenomenon is particularly acute in strongly coupled transmission media (e.g., twinax cables), where velocity mismatches due to even- and odd-mode propagation lead directly to time-varying and amplitude-varying skew.

2. Limitations of Conventional Skew Measurement Approaches

Conventional frameworks for quantifying skew include both time-domain and frequency-domain measurements, each with critical deficiencies in the context of IPI:

• Time-Domain Transmission (TDT): TDT is effective for loosely coupled structures such as PCB striplines, where P/N rising edges maintain parallelism. In twinax and other coupled environments, however, velocity mismatches cause even- and odd-mode transients, yielding skew that is threshold- and waveform-dependent, frustrating unambiguous time-domain quantification.
• Frequency-Domain S-Parameter Skew: Frequency-based extraction defines skew as $[\angle(S_{sd,21}) - \angle(S_{sd,41})] / (2\pi f)$, but the result varies with frequency and critically fails the requirement of reciprocity: the same physical channel, when measured in both directions, can yield different skew values, thereby violating the SerDes system assumption of bidirectional BER symmetry.
• Mode Conversion Approaches: Use of differential-to-common-mode insertion loss or “Effective Intra-Pair Skew” (EIPS) is confounded by non-reciprocal mode conversion effects, leading to ambiguous results not reliable for robust channel qualification.

Differential insertion loss is inherently reciprocal, which is fundamental for any meaningful skew metric in SerDes application environments (Nozadze et al., 3 Nov 2025).

3. Reciprocal Metrics: SILD and FOM_SILD

To address the deficiencies above, two physically grounded, reciprocal metrics are introduced:

• Skew-Induced Insertion Loss Deviation (SILD):

$$\mathrm{SILD}_{1(2)} = \left|S_{dd12(21)}\right| - \left|S^0_{dd12(21)}\right|$$ Here $S_{dd12}$ and $S_{dd21}$ are the measured insertion loss magnitudes for left-to-right and right-to-left propagation, respectively, and $S^0_{dd12}$, $S^0_{dd21}$ are the corresponding responses after de-skewing the S-parameters. SILD is reciprocal as ensured by the de-skewing process.

• Figure of Merit for SILD (FOM_SILD):

$$\mathrm{FOM_SILD}_{1(2)} = \frac{1}{N} \sum_{i=1}^{N} W_i\cdot \mathrm{SILD}_{1(2)}(f_i)^2$$ The weighting function $W_i$ follows an IEEE 802.3-style profile: $$W_i = \mathrm{sinc}^2(f_i / f_b) \cdot \frac{1}{1+(f_i/f_r)^8} \cdot \frac{1}{1+(f_i/f_t)^4}$$ Where $N$ is the count of frequencies up to the Nyquist rate $f_b$ (half the signaling rate), with $f_r$ and $f_t$ as the 3-dB bandwidths of the receiver and transmitter, and $f_i$ as each frequency point.

These metrics directly formalize the link between channel-induced skew effects and insertion loss distortion in a fully reciprocal manner (Nozadze et al., 3 Nov 2025).

4. Measurement Methodology and De-Skewing Procedure

The quantitative extraction of SILD and FOM_SILD proceeds as follows:

• De-Skewing S-Parameters: Phase delays between THRU/FEXT and differential/single-ended S-parameters are determined. The nonlinear system governing the phase delays is solved so that the intra-pair skew is zeroed ($t^{0}_{skew,12} = t^{0}_{skew,21} = 0$). The de-skewed S-parameters $S^0_{ij}$ are then computed.
• Metric Extraction: Full mixed-mode S-parameters are measured for the SerDes-to-SerDes channel, including fixtures, connectors, and twinax cable. Following de-skewing, SILD is calculated as the differential magnitude deviation across frequency, and FOM_SILD is derived via numerical integration according to the prescription above.
• BER Measurement Setup: BER testing is performed with 224 Gb/s PAM-4 SerDes IP at both ends. Controlled frequency-independent skew is injected using programmable phase-shifter modules; frequency-dependent skew is introduced via mechanical manipulation (twisting, bending) of 1.5 m twinax cables. Full two-port VNA calibration ensures S-parameter reciprocity at or above 99% as required by IEEE P370.

5. Empirical Characterization and Channel Statistics

A comprehensive set of measurement results demonstrates a robust correlation between FOM_SILD and BER:

• BER Stability Threshold: With frequency-independent skew (phase shifters), BER remains constant up to FOM_SILD ≈ 0.3 dB, after which it rises sharply. Frequency-dependent manipulations exhibit analogous trends, with higher FOM_SILD corresponding to greater BER.
• Practical BER-Linked Thresholds: Channels exhibiting FOM_SILD below 0.2–0.3 dB show negligible BER degradation; error rates rise significantly for FOM_SILD values above ~0.3 dB, with error floors exceeding $10^{-12}$.
• Statistical Analysis: Among >3,000 twinax cables, FOM_SILD spans 0 to ~0.49 dB; the majority of samples are below 0.1 dB, and 90% lie under 0.13 dB.

Channel Property	Typical Metric	Observed Range
FOM_SILD (dB)	<0.1 (majority)	0 – 0.49
Threshold for BER Floor	~0.3
Reciprocal Enforcement	≥ 99% (S-param)

6. Design Guidelines, Channel Qualification, and Best Practices

The precise quantification of IPI via SILD and FOM_SILD enables actionable qualification and engineering practices:

• Channel Qualification:
  • Specify maximum allowable SILD within the SerDes bandwidth (SILD_max per segment).
  • Impose a global FOM_SILD threshold (0.2–0.3 dB) for system-level BER compliance.
• IPI Minimization:
  • Route P/N traces symmetrically to prevent glass-weave-induced dielectric constant mismatch.
  • Employ tightly centered twinax conductors and control extrusion geometry to reduce inherent skew.
  • Minimize cable bends, twists, and mechanical stresses along the interconnect.
  • Enforce measured S-parameter reciprocity (≥99%) for reliable metric extraction.
  • Allocate the aggregate skew budget across PCB, connectors, and cables, using de-skewing to isolate segment contributions.

Quantifying IPI through reciprocal, insertion-loss–based metrics—SILD and FOM_SILD—provides a robust linkage between skew, channel response distortion, and end-to-end BER. This framework underpins more precise channel qualification and enables more resilient high-speed interconnect design (Nozadze et al., 3 Nov 2025).