Signal Integrity and Radiated Emission of High-Speed Digital Systems

Appendix B

Characteristic Impedance, Delay Time, and Attenuation of Microstrips and Striplines

This appendix provides closed-form expressions for calculating the characteristic impedance, delay time, and attenuation of traces having microstrip and stripline structures. A procedure for computing analytically the proximity-effect parameter K_p as defined in Chapter 7 is also outlined. Some results are compared with those found in the literature by using field-solver software.

B.1 Microstrip

Microstrip has the structure shown in Figure B.1 and is characterized by fields propagating in two different dielectrics: air and substrate with relative permittivity ε_r. This is particularly true if the trace is not covered by a soldermask to prevent corrosion. Usually, an effective dielectric constant, ε_re, is used for electric parameter calculations, which is given by

where

The per-unit-length (p.u.l.) propagation delay time t_pd is then given by

where μ₀ = 4π × 10⁻⁷ H/m and ε₀ = 8.854 × 10⁻¹² F/m.

Figure B.1 Microstrip structure

The microstrip characteristic impedance Z_0,ms is given in closed ...