How do I perform time domain reflectometry to characterize a high speed PCB interconnect?

How do I perform time domain reflectometry (TDR) to characterize a high speed PCB interconnect? TDR sends a fast step signal into the interconnect and measures the reflections over time, providing a direct impedance profile along the channel: (1) How TDR works: the instrument generates a fast step signal (rise time 20-50 ps for high-frequency characterization). The step propagates along the interconnect. At each impedance discontinuity: a portion of the step is reflected back. The instrument displays the reflected signal vs time. The time axis corresponds to position along the interconnect (distance = time × velocity / 2). The reflection amplitude directly indicates the impedance at that position: Z(t) = Z₀ × (1 + Γ(t)) / (1 - Γ(t)). (2) What TDR reveals: trace impedance: a flat line at 50 ohm indicates a well-controlled trace. Deviations show impedance variations due to trace width errors, dielectric thickness variation, or proximity to other features. Via transitions: a dip (low impedance) at the via indicates excess capacitance. A peak (high impedance) indicates excess inductance. Connector launch: the impedance of the launch structure. BGA breakout: impedance variations in the breakout region. Length matching: differential TDR shows the skew between P and N traces. (3) TDR instruments: oscilloscope-based TDR: Keysight DCA-X 86100D (sampling oscilloscope with TDR module). Rise time: 15-35 ps. Standard for SI characterization. VNA-based TDR: measure S11 on the VNA and apply inverse FFT (IFFT) to convert to time domain. The VNA frequency range determines the TDR resolution: rise time ≈ 0.35 / f_max. For f_max = 50 GHz: rise time ≈ 7 ps (higher resolution than oscilloscope TDR). (4) TDR measurement procedure: calibrate the TDR at the cable end (remove the cable effect). Connect to the DUT (PCB test coupon). Observe the impedance profile vs time. Compare to the target impedance (50/100 ohm). Measure the impedance at each feature (trace, via, connector, BGA). Use differential TDR for differential pairs (both traces driven simultaneously, shows the differential impedance).