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# Reducing Crosstalk in Microstrip Lines

IP.com Disclosure Number: IPCOM000084249D
Original Publication Date: 1975-Oct-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 6 page(s) / 145K

IBM

## Related People

Rabbat, NB: AUTHOR

## Abstract

Transverse field distributions for two fundamental transverse electromagnetic (TEM) modes that can exist on a pair of parallel conducting strips between parallel ground planes are described.

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Reducing Crosstalk in Microstrip Lines

Transverse field distributions for two fundamental transverse electromagnetic (TEM) modes that can exist on a pair of parallel conducting strips between parallel ground planes are described.

If the strips are at the same potential and carry equal currents in the same direction, the electric field has an even symmetry about the vertical axis and this mode is called the "even-coupled strip mode". If the strips are at equal but opposite potentials and carry currents in opposite directions, due to the odd symmetry of the electric field, this mode is called the "odd coupled-strip mode".

In the case of the odd mode, the ground plane is the plane of symmetry. It can be seen from the field plots that the capacitance per strip to ground is less for the even case and more for the odd case than for a single isolated strip line of the same width. Consequently, the characteristic impedances of the two modes are unequal, being greater for the even than for the odd.

In a microstrip line, the medium is partly air and partly dielectric (i.e., inhomogeneous) and the TEM waves are distorted. However, it is usually assumed that the signal propagation is a surface printed wire above a ground plane is TEM.

The characteristic impedances for the odd and even modes can be calculated with the equation Z(o) = 1/V(r)C(o), where V(r) is the velocity of propagation down the line and C(o) is the capacitance per unit length. As mentioned above, the microstrip line has a coupled dielectric medium consisting of the board material and air. Kaupp (Ref. 1) has successfully used an effective dielectric constant (re) = 0.475 (r) + 0.67, where (r) is the relative dielectric constant.

For purpose of simplicity, various theoretical considerations will be simplified. It will be demonstrated how these theoretical considerations (simplified) will lead first to an accurate formula and second to the alternative scheme.

The capacitance C(o) will be considered as the summation of lumped capacitances representing the field distribution between the two microstrip lines. This is illustrated for the even-mode forward coupling and the odd-mode reverse coupling in Figs. 1 and 2. In Figs. 1 and 2, 1/2 C(F) represents the outer field distribution at the left of the equivalent circular conductor of the microstrip, and 1/2 C(F) represents the inner field distribution at the right of the equivalent circular conductor of the microstrip. For the even-mode and odd-mode coupling Z(oe) and Z(oo) can be written as shown in Tables 1 and 2. The accurate Z(oe) and Z(oo) expressions were obtained from an approximation and an assumption. The assumption considers the rectangular shaped microstrip as a circular line with an effective diameter d(e) (as shown in Tables 1 and 2). The comparison between the accurate formulas of Tables 1 and 2 and the experimental observations are illustrated in Tables 3 and 4.

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