Timing is critical in circuit design, and skew (delay difference) is a key factor in signal integrity. Excessive skew causes timing violations and product failure. This article introduces two methods to calculate skew without PCB design tools: using PLTS and ADS.
Timing is critical in circuit design, and skew (delay difference) is a key factor in signal integrity. Excessive skew causes timing violations and product failure. This article introduces two methods to calculate skew without PCB design tools: using PLTS and ADS.
Overview
Physical equal length does not equal equal time delay. Skew can be calculated from S‑parameters, which fully characterize passive interconnects (PCB, connectors, cables, packages).
Method 1: Measure Skew Using PLTS (Physical Layer Test System)
Step-by-Step Operation
- Import S‑parameters into PLTS.
- Select Time Domain analysis.
- Choose port data (e.g., T21, T43).
- Right‑click → Measure → Skew.
- Enable Skew Measurement to read skew directly.
Result: Skew(T21, T43) = 20.02 ps
PLTS software interface – S‑parameter import settings
PLTS time‑domain waveform showing T21 and T43 traces
PLTS skew measurement window displaying 20.02 ps
Method 2: Simulate Skew Using ADS (Transient Simulation)
Step-by-Step Operation
- Build schematic with Transient simulator.
- Import the same S‑parameters file.
- Set 50 Ω terminations and pulse sources.
- Run simulation and plot output waveforms v1, v2.
- Place markers at 50% amplitude for both edges.
- Read time difference = skew.
Result: Skew = 20 ps (matches PLTS; slight difference from software rounding)
ADS transient simulation schematic
ADS waveform with markers at 50% amplitude
ADS delta marker showing skew value
Other Measurement Methods
Skew can also be measured directly using:
- Network Analyzer (e.g., E5071C)
- Sampling Oscilloscope
Network analyzer skew measurement screen
Why Control Skew?
- Causes signal integrity degradation
- Increases EMI radiation
- Violates timing margins
Mitigation:
- PCB: length tuning
- Connectors/cables: structural optimization
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PLTS software interface – S‑parameter import settings
