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Lesson 10
A trace starts acting as a transmission line once its delay is more than a small fraction of the edge, about a sixth of the rise time. Then the driver only sees Z₀, every change of impedance sends part of the wave back, and the far end can ring past the rail. Commit to predictions first, then fire up the scope, watch the wave travel, and take it across a connector to a board you don't own.
Predict · 1 of 2 · the first peak
A 3.3 V CMOS driver with 25 Ω of output resistance sends a 1 ns edge down 15 cm of 50 Ω microstrip on Isola 370HR to a receiver: 3 pF and nothing else. CH1 is the driver end of the trace. What is the highest voltage the receiver pin reaches?
Result
Waiting on your call.
Make your mark, then lock it in. The answer, your error and the reason behind it land here, and the scope and every view below fire up.
Engine: in-browser chain-matrix (ABCD) solve of the telegrapher's equations for every segment, exact in the frequency domain and brought back to time with a 4,096-point FFT · Isola 370HR as a nominal Djordjevic–Sarkar fit to its datasheet (Dk within 0.09; Df within 0.007, but up to 39 % high around 1 GHz, as the fit follows the 2–10 GHz points), copper by Wheeler's rule with roughness · the solver checked against the exact bounce sum and the home page's coax solver; the trace formulas against a 2-D field solve (Z0 within 0.5 % at the four geometries checked) · the network arithmetic cross-checked against SPICE: six idealized circuits (lossless or constant-R lines), worst point 0.31 % of its swing (2.3 mV on the 0.75 V HCSL leg); largest error 8.6 mV (0.26 % of 3.3 V); that check does not cover the laminate or copper loss models · receivers are linear (no clamp diodes).