Henry Ott Consultants

Electromagnetic Compatibility Consulting and Training

Transmission Line Rules of Thumb

(Assuming FR-4 Epoxy Glass)

The following Rules of Thumb can be used to quickly estimate the configuration of various PCB transmission lines. They will get you "in the ball park" quickly, and are a good place to start a design. They are also useful as a "sanity check" on results obtained from other methods. I memorize them and always use them to get approximate answers quickly. A complete preliminary design can be done on a napkin or the back of an envelope using just these rules. A final design, however, should not be based upon them. Once you have obtained the initial design, you should use the closed form transmission line equations, or a simulation program for the final design.

Characteristic Impedance of Microstrip: 60-70 Ohms for w/h = 1, and 40-50 ohms for w/h = 2.	Average Propagation Delay: 2 nS / ft This is an average between microstrip and stripline.
Characteristic Impedance of Stripline: 0.6 times that of microstrip.	Typical Propagation Delay: 150 pS / in for Microstrip, or 180 pS / in for Stripline.
Characteristic Impedance of Asymmetric Stripline: 0.75 times that of microstrip.	FR-4 Epoxy Glass Dielectric Constant: Between 4.0 and 5.0 according to the composition, typically 4.5.
For < 2% Crosstalk:s = 4 h for Microstrip, and s = 2h for Stripline.	For Other Dielectric Materials: Characteristic impedance is proportional to one over the square root of the dielectric constant. Propagation delay is proportional to the square root of the dielectric constant.
Maximum Unterminated Stub Length: L(in) = tr (nS)	Dielectric Constant of Other Materials: FR-4 = 4.5, PTFE = 3.2, Polyester = 3.2.

Note: w = width of trace, h = height of trace above the reference plane, s = spacing between traces, tr = rise time.

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Henry Ott Consultants

48 Baker Road Livingston, NJ 07039

Phone: 973-992-1793, FAX: 973-533-1442

February 14, 2001