## Short Transmission Line Model

I would like to know the electrical model of a trace in a PCB. That is, where is the inductance, the capacitor and the resistance.

Thank you.

Thanks for your interest in High-Speed Digital Design.

A good model for a SHORT transmission line is formed from four components:

Where the ground node really refers to the
nearest solid reference plane, regardless of
voltage.

Where the values of C1 and C2 are the same, and
equal to dly/(2*Z0), where dly is the line delay in
pico-seconds, Z0 is the characteristic impedance of
the line in ohms.

Where the value of L1 equals dly*z0

Where R1 is the resistance of the line (for short
lines on PCB's we usually assume 0.000 ohms)

### NOTE:

The Pi-model is accurate to about +/- two percent if the line delay is less than 1/6 of the signal risetime.

The Pi-model is accurate to about +/- twenty percent if the line delay is less than 1/3 of the signal risetime.

If the line delay stretches as far as 1/2 the signal risetime you can get huge unpredictable resonances and all bets are off. The model fails.

Best Regards,

Dr. Howard Johnson